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Techniques for the automation of sampling and chemical analysis
Techniques for the Automation of Sampling and Chemical Analysis D. A. PATIENT Introduction
following sections by specific examples where they have been applied. (a) Repetitive measurement and delivery of liquid volumes as required for sampling and dispensing. (b) Performance of chemical techniques for the preparation or extraction of samples. (c) Automatic sequence control. (d) Determination of colour density by absorptiometric methods. (e) Detection of titration end-points, or related electrochemical properties. (f) Presentation of results in permanently recorded form. (g) Initiation of appropriate control action.
Wherever a chemical plant, or similar industrial installation, relies for successful operation on the results of analytical measurements at various stages in the process, the justification for making those measurements automatically may be established for a number of reasons: (a) The complexity of modern plant operation, which continues to increase, making it necessary to obtain more detailed information about conditions throughout the process. (b) The demand for higher levels of accuracy and repeatability, requiring skilled measurements to provide results adequate for control purposes. (c) The frequent desire to achieve control automatically from the results given by analytical measurements. (d) The removal of existing sources of inaccuracy arising through fatigue of operators. (e) The desire to use scientific staff most profitably, without occupying them on laborious routine test analyses.
Delivery of Liquid Samples and Reagents To make repeated measurements on the contents of a process stream it must be possible to withdraw samples, which must be truly representative of conditions existing within the process,
Requirements of reliability
Having accepted one or more of the above factors, satisfactory evidence must be produced to prove that the reliability of a proposed automatic installation will justify the operation of the process being dependent on the results given by the apparatus. According to the application, this will require that it shall perform with the minimum of attention for periods ranging from a few hours to several months at a time. From the designer's aspect such stringent requirements mean that all techniques of automation, and the components employed, have to be thoroughly examined for their expected trouble-free life, since the value of the apparatus to the user will be limited by the least reliable part. Since such equipment may be required to function unattended, provision must be included to indicate immediately should a fault condition arise. The examples quoted include such alarm circuits, and also systems by which the cause of a fault may be quickly located. Particular reference is given here to work involved in the design of automatic equipment for the measurement and control of the properties of liquid samples. A wide variety of such applications are found in industrial and medical work, and particularly in the chemical and oil industries. These include both major installations where completely automatic analysis is involved, and other instances where it is preferable to automate only individual stages of a procedure. The techniques to be automated It is intended that the instruments and devices described shall make possible the automation of analytical procedures which are at present repeatedly carried out by manual methods. The desired analytical results are obtained by using one or more of the following techniques, and these are illustrated in the
Figure I. Solenoid-operated diaphragm valves
and to treat these samples with any necessary reagents. This involves the control of the flow of liquids by reliable valves, the careful attainment of a contemporary sample, and the accurate measurement of the volumes taken. Liquid flow control
To be suitable for working with chemical process liquors the valve selected must be constructed of suitably corrosion-resistant materials and this, coupled with the requirement of longterm reliability, makes necessary the most careful design. A completely satisfactory valve for such duties has been produced and one form is shown in Figure I. A flexible diaphragm is normally spring-loaded to seal two ports in the valve block, and
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a 15 W solenoid, when energized, causes the diaphragm to be lifted clear of the ports to allow the liquid to flow. The valve action is extremely rapid since the movement of the armature is less than s\: in. (I ·19 mm), and this is essential to ensure continued operation without risk of early failure due to fatigue in the flexible diaphragm. The liquid meets only the material of the valve block and the diaphragm, and thus by suitable choice of materials for each the valves can handle even highly corrosive liquids. The diaphragms are normally of acid-resisting butyl rubber but, for the more corrosive applications, they are faced with polytetrafluoroethylene (PTFE). Table 1 lists various materials with typical liquids for which they are suited.
waste to prepare a contemporary sample at the apparatus. Such a system is shown in Figure 2, where eight sample lines
Table I Materials used for construction of valve blocks and vessels Material
Perspex acrylic resin U nplasticized
polyvinylchloride (PVC) Duralumin Stainless steel Polytetrafluoroethylene (PTFE)
Suitable for
Most dilute aqueous solutions. Not for organic solvents Most aqueous solutions. A limited range of organic solvents Neutral organic solvents. Not usually for aqueous solutions Organic solvents, certain alkalis and acids. Some aqueous solutions Practically all corrosive liquids where other materials are unsuitable
F(fjure 2. Multiple sample line flushing units
are divided between two similar flushing vessels for odd- and even-numbered sources, respectively. Measurement of liqUid volume
To perform measurements on successive samples to consistent accuracy it is necessary to take identical volumes of sample, and any reagents, on each occasion. Nevertheless, the variation that can be tolerated in the volume will vary from one application to another, and the most suitable sampling method must be selected. When conducting liquids are involved, a measuring vessel with two, or more, contact probes may be used with a simple electrical circuit (Figllre 3). This gives the necessary switching
The valve design follows one of two basic forms, having kin. (3·18 mm) bore ports normally for inlet flows, or ~. in. (9·53 mm) ports for efficient draining of vessels or pipelines. Alternative constructions use the solenoid, when energized, to close the valve. The solenoid can also be replaced by a toggle lever for manual operation, by a screw to form an adjustable constriction, or by an air hose connection for pneumatic operation. With -~ in. (3·18 mm) bore ports the valve closes reliably against pressures up to 15Ib./in. 2 (1·1 kg/cm2), with a safety factor at that pressure of approximately 1·6. Figure I illustrates the component parts of a valve assembly for connection by tubing to other parts of a liquid-handling system. However, because of the simple nature of the valve seating this can often be machined in the surface of a reaction vessel or other component, so that several valves, for inlet and drain purposes, may be mounted on a single unit.
Switch
6·8kfi
~~--~A-~--~--,
sov d.c. +
Procurement of contemporary sample
When such valves are connected in branch pipes for sampling from a process stream, it is essential for the stagnant volume of liquid, behind the valve since it was last opened, to be displaced to waste before a further sample is taken for measurement purposes. This is provided in the process instruments mentioned, but when samples are being drawn from only one source it is an advantage to bring a continuously flowing sample loop close to the input of the instrument. The sampling device is then arranged to fill and discharge to waste a sufficient number of times to remove the small stagnant volume before taking the sample for analysis. Where samples are being drawn from several sources in succession an alternative system is possible, for while one sample is being analysed the next sample line can be flushed to
r2·2
G
Level at which switCfiiiig OCcurs To sequence P control circuit
Figure 3. Electrical liq 11 id level probe circuit
action into the main sequence control circuits at the instant the liquid level rises to reach the upper of the two probes P. This circuit, which presents 20 V d.c. between the probes before they are covered, requires a minimum current of 300 ftA between them to operate the relay RI. The current passes between the probes for no longer than 20 msec, being limited by the closing of relay contact r2.1, so that polarization and electrolysis problems do not arise. When non-electrolytes, or liquids in flame hazard conditions,
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D. A. PATIENT
Table 2 Performance of automatic pipettes
are being used, a capacitance system may be adopted. A vessel designed for this purpose has been used with metal bands round the outside of the measuring tube!, these being connected to a capacitance-sensitive relay, which responds when the liquid meniscus reaches the level of a selected band. The volumetric accuracy of the above methods is limited chiefly by the rate of liquid inflow and the area of the liquid surface, but repeatabilities of the order of 2 per cent are possible. However, when highly repeatable volumes must be dispensed a further development gives accuracies considerably greater than those obtained by manual pipetting methods. The components of this automatic pipette are shown in Figure 4, and several can be seen mounted together in Figure 7.
MaximulIl capacity of pipetle
Weight of ten deliveries at a given setting (Water at 20'C)
Maximum result Minimum result Range over 10 Small sample standard deviation
1 ml
1 ml
5ml
IOml
20ml
0·088g 0·087 0·088 0·088 0·088 0·088 0·088 0·086 0·087 0·089 0·089 0·086 0·003
0·512g 0·513 0·512 0·511 0·513 0·511 0·512 0·510 0·511 0·512 0·513 0·510 0·003
3·989g 3-988 3·990 3-989 3-989 3-989 3-989 3·988 3-989 3·989 3·990 3·988 0·002
9·935g 9·931 9·933 9·934 9·934 9·933 9·933 9·934 9·932 9·935 9·935 9·931 0·004
19·92lg 19·921 19·920 19·921 19·922 19·923 19·922 19·921 19·926 19·926 19·926 19·920 0·006
0·0008
0·0011
0·0006
0·0012
0·0021
been shown that after several months operation at a frequency of 100~200 operations/day the volume delivered was still within the limits quoted. The volume accuracy is approximately the same for all sizes, being ±0·003 ml or better. Thus the error, which is of the order of ±O·l per cent for a 1 ml delivery, is correspondingly lower for larger volumes. The polythene gallery cemented at the top of the syringe barrel (Figure 4) provides an annular volume of a few ml which is maintained partly filled with water or other suitable liquid. This serves to lubricate the top of the syringe piston and prevents the formation of small crystals there when liquids such as sodium bicarbonate are being handled. F(ljure 4. Automatic pipette
Reagent dispensing
Two of the valves (see 'Liquid flow control') are fitted to a common valve block to provide inlet and outlet taps to a modified hypodermic syringe also mounted on the block. This syringe fills, when the inlet valve is opened, by upward displacement of the piston under the hydrostatic pressure of the liquid supply to which the inlet is connected. Two insulated assemblies (Figure 4) are fitted in a slot in the panel above the valve block. Each of these has a pivoted arm carrying a silver contact which makes electrical connection with an adjacent fixed contact when the cap of the syringe piston strikes the arm. The top assembly is positioned in the guide slot so that it determines the extent to which the pipette fills, and a graduated scale with vernier may be fitted for calibration purposes. When the top pair of contacts are closed, as the piston rises, they operate into the sequence control system causing the inlet valve to close and the outlet valve to open. The internally loaded piston then falls under its own weight to expel the measured volume of liquid into the receiving vessel. Syringes are available from 1 to 100 ml maximum capacity, and may be interchanged if required to dispense a volume beyond the range of the syringe first fitted. The very high order of repeatability achieved with pipettes of this form is shown by the weighings recorded in Table 2, which were obtained with several sizes of pipette. It has also
The syringe pipette finds uses, not only in fully automatic instruments, but also in smaller equipment such as a multiple dispensing unit (Figure 5). This delivers accurately repeatable Scales - Stop SWitches with verniers ~_-
Pipettes
Pipette jet
F(ljure 5. Multiple dispenSing uflit
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TECHNIQUES FOR THE AUTOMATION OF SAMPLING AND CHEMICAL ANALYSIS
volumes of reagent (within the limits of Table 2) into separate vessels carried in suitable racks below the three outlet jets. Approximately 30 vessels may be treated per min, and this rate of working and accuracy can be maintained over long periods 2 • The racks may be advanced through the apparatus either by manual control or by an automatic mechanical traverse. This consistent delivery of liquid volumes would be beyond the capability of an operator even though precautions were taken to minimize distractions and fatigue.
Direct reaction technique
A vessel as seen at the bottom of Figure 7, and constructed from one of the materials of Table 1, is used when sample
Proportional dispensing
When it is more important for the ratio of the volumes of successive liquids dispensed to be held constant, rather than that the actual volume of the additions should be set to a given precise value, the peristaltic pump is used (Figure 6). This is
F(f{ure 7. Automatic titrator panel
preparation can be done by mixing appropriate volumes of sample and one or more reagents. It is fitted with inlet jets from the pipettes, a drain valve, and either mechanical, magnetic, or gas stirring, and thus represents the 'beaker' of a normal manual analysis. 11 is often possible to make the required measurement, while the mixture is in this vessel, by fitting suitable electrodes for electrochemical tests, or windows for colorimetric work. [n other cases it is preferable to pass the prepared specimen from such a reaction vessel to subsequent measurement stages, as in the absorptiometric system described under 'Measurement of Concentration by Colour Density'.
F(f{ul'e 6. Peristaltic pump
applicable to the preparation of blood samples for analysis, and for similar applications. The flexible tube of the pump is compressed by three eccentric rollers on a rotor which makes an exact number of revolutions between I and 20 according to the setting of a selector switch. Thus to add say five volumes of reagent to two of sample, the pump is first caused to make two revolutions with the intake jet immersed in the sample container, followed by five revolutions with it in the reagent. This ejects the sample and some of the reagent into the receiving vessel, and the reagent still i~ the flexible tube is then delivered by rotating the pump four tImes to clear the tube, after the reagent container has been removed from the intake jet. Removal of the tube for sterilization or replacement is readily possible for changing quickly from one set of liquids to another. Although both the dispensing devices described are only semi-automatic, they nevertheless relieve the operator of the very functions where inaccuracies could otherwise arise. Preparation of a Sample from a Process Stream A sample flow must often be treated in some way before making a measurement to determine the concentration of some constituent. Two methods of preparation are described below.
Extraction technique
The automation of liquid-liquid extraction to obtain the required constituent from a sample stre~m can be illustra~ed by its current use by the U.K. AtomiC Energy AuthOrIty, and Figure 8 shows the unit in which a uranium sample is separated from an aqueous stream by solvent extraction. In the vertical column Cl' tri-butylphosphate, entering through the lower side tube S, extracts the uranium and emerges at the point E, from which it passes through the limb D into the second column C 2 • Here it meets a downward flow of ethylenediaminetetraacetic acid (EDTA), which enters at Band backwashes the uranium from the solvent. The solvent, for reclamation, emerges at R, while the uranium-bearing EDT A coming from the lower end of column C2 is complexed by the continuous addition of ammonium thioglycollate at T to produce the colour whose density is then measured absorptiometrically. The flow rate of sample through the columns is of the order of 3 ml/min, and is controlled together with the flow of the other reagents by the four injectors J, which deliver these liquids in correct proportions at approximately one drop per
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D. A. PATIENT
sec. Highly efficient extraction is achieved in the two columns by thoroughly intermixing the two constituents by cylindrical
rhombs it describes a cylindrical locus on which the necessary absorption cells are disposed, so that the light beam passes through each in turn. The composite photomultiplier signal is switched by a commutator on the same rotor to separate the To ",corder
Motor
SOIJrce Photomultlplier
Figure 9. Schematic diagram of absorptiometer unit
Figure 8. Liquid-liquid solvent extraction unit
stirrer rods which are rotated on the axis of each column by the stirrer motors M. Automatic Sequence Control
components corresponding to the different cells so that the required ratio may be obtained electrically. This absorptiometer has been used for uranium estimations by different techniques. One uses the colour complex with ammonium thiocyanate\ measuring the absorption produced at 365 mfl, while the other uses the thioglycollate complex (see Extraction technique) and measured at 425 mfl. In the latter case the colourless reference solution is obtained from the sample stream after the characteristic colour has been developed and the sample has passed through one cell, by decolourizing the emergent flow with excess carbon dioxide. This gas flow also transfers the sample to the reference cell. The absorption cell assembly is seen in Figure 10, with the
In a fully automatic apparatus which is required to carry out a series of analytical stages without the intervention of an operator it is necessary for each operation to be initiated when the preceding one has been satisfactorily completed. While such automatic programming may be achieved by electronic control circuits, it is preferable to use electromechanical devices including relays, uniselectors, and synchronous motordriven timers for this purpose 3 • Sequence circuits of this type are more readily understood by maintenance staff than are the equivalent electronic circuits, and the techniques required for servicing are similar to those already practised by plant instrument engineers. Measurement of Concentration by Colour Density To compensate for changing background turbidity of the sample stream when making a continuous measurement, and to avoid errors due to changes in the light source intensity and photo-detector sensitivity, a special absorptiometer unit (Figure 9) is necessary. This enables a number of different optical cells to be examined, and the absorption results compared, using a common light beam and photomultiplier. By passing an untreated sample flow through one cell, and the same sample, after development of the characteristic colour, through another cell, the required compensation is obtained. The ratio of the absorption in the two cells gives a result related to sample concentration only, independent of the other factors mentioned. Two glass rhombs are rotated about the optical axis causing the light path to be deflected so that while passing between the
F(f{ure 10. Absorption cell assemb~y
carbon dioxide inlet and lift vessel at the extreme left, and the two cells each side of the space in which the rhomb rotor fits, the right-hand cell being for reference purposes. An installation at one of the U.K. Atomic Energy Authority's plants has five individual liquid-liquid extraction units, each as Figure 8, and makes a measurement on five different sample streams, taking 3 min for each. Two identical absorptiometerrecorder units allow anyone sample stream to be monitored continuously, while the other four are monitored in turn. Under normal conditions all five streams are monitored in one unit, while the other is available as a stand-by, or so that periodic servicing work may be done without interrupting the function of the instrument.
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TECHNIQUES FOR THE AUTOMATION OF SAMPLING AND CHEMICAL ANALYSIS
Fully Automatic Titration Titration is a most significant analytical method, and has frequently been carried out automatically to the extent of detecting the end-point. However, the ful.lest advantage can only be gained when all stages, from the tak1l1g of the sa mpl~ t~ the recording and use of the result, are rendered automat l~ . This applies whether the end-point is be~g detecte~ pote.ntlOmetrically, or at a given pH value; by a dead-stop CircUIt for iodometric work, or colorimetrically. Five pipettes (see page 298) for sampling and reagent a~dition are shown on the titration panel of Figure 7, together With the reaction and titration vessel at the bottom below the burette and its valve block. Adjustlllent for titrant normality
A titration result is obtained first as the volume of titrant used and for instance with a 50 ml burette, this would be quot~d to the nearest 0·05 ml, ] /1,000th of the burette capacity. To achieve this discrimination automatically, and also to present a record in the units required and not merely as volume G L
meter for analogue output corresponding to the volume of titrant used, are driven by the same motor that rotates the leadscrew L (Figure 11). The photoelectric .system mou~ted ?n C, moves vertically in the rails G when L IS rotated, bemg hnked by the arm A to the leadscrew nut.N. Thi.s arm pivots at P on the nut, and carries a roller R beanng agamst the straight edge of a bar E, which is itself pivoted at F. While the bar is parallel with the rails the photoelectric system C moves directly with the m~t. However, if ~he ~crew S is turned, the nut M moves to the nght or left and mchnes the bar a few degrees from the vertical, so that C is caused to move up to ] 0 per cent more or less than any corresponding movement of the nut N. Since any movement of N is directly .trans~itt~d to the recording unit, the adjustment allows a given titratIOn result to be recorded as a figure within the range ± 10 per cent of the value indicated when the edge E is vertical. The recording range of the figure wheels is conveniently 0-:1,000, and it would be possible to calibrate the apparatus with a standard sample and titrant of carefully prepared normality. so that t~e result of the titration would be indicated exactly 111 the Units required. . The adjustment described removes the tedIOus and r~current necessity for such carefu l preparation of titrant stock, smce the normality need only be to within ± J0 per cent of the calculated value, final setting being achieved using the screw S. Alternative to conventional burette
N
Axis of burette
E
M
s
Fig ure 11. Schematic diagram of normality compensation device
of titrant used, the digital recorder at the top of Figure 7 is used with the photoelectric meniscus-finding mechanism seen there and schematica ll y in Figure 11 . The recorder figure wheels, and also a precision potentio-
When compactness and also economy are more essen~ial than such features as the adjustment descnbed 111 the precedmg section, an alternative titration device has been used. Based on the syringe pipettes described on page .298, this ~as the syringe piston coupled to a nut on a motor-dnven fine pitch leadscrew, the movement of the piston being linked to the pen of a chart recorder. This device has been used in an instrument to determine the concentration of dissolved oxygen in water by an automated Winkler technique, to within 0·1 p .p.m. il: the range 0- 10 p.p. m. This follows work. by Bri~gs et al.", but uses a 'dead-stop' electrometric end-pomt, a nd IS SUited for ~he automatic monitoring of pollution due to effluents entenng river waters. Recording of Results This may be done, normall y on paper chart, in. a number of ways as illustrated in Figure 12, either as contllluous IIlked PM 2 ·1 5 PM 2 ' 1 9 PM 2 '2 3
·2 6 ·30 ·3 4 ·3 7 ·4 1
6 13 • 6 18 •
PM 2 PM 2 PM2 PM2 PM 2
·45
612 • 6 13 •
'48
. 52
' 56 . 59
PM3 3 PM3 7 PM3 ·1 0 PM3 · 14 PM3 ·1 8
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615 • 6 17 •
2 2 2 2 2
PM PM PM PM PM
Figure 12. Alternative chart records
6 13 •
6 14
6 19 • 6 17 •
619 6 14 •
6 12 •
6 14 • 6 18 • 612
6 14 • 6 17 •
D. A. PATIENT
traces, individual results, or in digital form. Whichever system is most suitable, a means of identification is provided when more than one sample stream is involved, either by a code, sample number, or time print.
When more complex control action is desired, electrical analogue signals from one or more potentiometers in the measuring unit would be used to direct the necessary computer, which in turn delivers the control signals to the plant.
Control Action
References 1
Signals for control purposes can be provided from the measuring units described, either in electrical or pneumatic analogue form, or encoded digitally. If only simple on-off action is required in order to control the process when the measured variable exceeds predetermined limits, this may often be achieved directly from the measuring units, for instance from switches operated by the leadscrew mechanism of an auto-titrator (page 301).
2
3
4 5
BISBY, H., BROWN, L. H. and CHAPMAN, D. R. J. sci. lnstrum. 33 (1956) 467 GREEN, N. C. and MONK, P. R. Chem. and lnd. No. 39 (1959) 1210 PATIENT, D. A. Automation Progress I (1956) 25 PATIENT, D. A. Proc. Congr. Modern Anal. Chem., June, 1957. Cambridge; Heffer BRIGGS, R., KNOWLES, G. and SCRAGG, L J. Analyst 79 (1954) 744
Summary This paper describes techniques and devices used to achieve automation of analytical test procedures on which depend the satisfactory and economic operation of laboratory or plant processes, with emphasis on those of a chemical nature involving liquid samples. The subject includes: Ca) sensitive control of the flow of liquids by the design of suitable electromagnetic or pneumatic valves, (b) automatic measurement of liquid volumes by various methods, one in particular giving accuracy greater than that obtainable by manual methods and having exceedingly high long-term repeatability, and Cc) methods applied to the automatic measurement of the variable on whose value the operation of the process depends-for
instance by absorptiometric colour measurements or titrimetric endpoints. The devices used for the automatic control of such equipment are described together with details of construction, with particular reference to considerations of reliability and ease of routine maintenance. The presentation of recorded results is discussed together with the methods by which such results may be translated into plant control. Examples of instrumentation including these techniques are quoted to illustrate their significance, both in cases where full automation has been applied, and also where certain phases only of a process have been rendered automatic.
Sommaire Ce rapport decrit des techniques et des dispositifs utilises pour accomplir I"automatisation des procedes d'essais analytiques dont depend le fonctionnement satisfaisant et economique du laboratoire ou de I"usine de processus, en mettant I"accent sur ceux de nature chimique, comportant des echantillons liquides. Le sujet comprend: (a) commande instantanee des debits de liquides avec realisation de vannes convenables electromagnetiques ou pneumatiques, (b) mesure automatique des volumes de liquides par differentes methodes, dont I'une, en particulier, donne une meilleure precision que celle que I"on peut obtenir par les methodes manuelles, et ayant une extraordinaire reproductibilite it long terme, et (c) methodes appliquees it la mesure automatique de la variable dont la valeur conditionne le fonctionnement du processus par exemple par
les mesures colorimetriques d'absorption ou de points limites titrimetriques. Les dispositifs utilises pour la commande automatique d' un equipement de ce genre sont decrits en meme temps que les details de construction, avec reference speciale aux considerations de 'fiabilite' et de facilite it maintenance. On discute la presentation des resultats releves ainsi que les methodes par lesquelles de tels resultats peuvent etre traduits en commande automatique dans I"usine. On cite des exemples d' instrumentation incorporanl ces techniques, afin d'illustrer leur signification, aussi bien dans des cas oll I"on applique I"automatisation integrale, que dans ceux oll certaines phases seulement d'un processus ont ete rendues automatiques.
Zusammenfassung Der Aufsatz beschreibt Verfahren und Geriite zur Automatisierung chemischer Analysen, von denen die Qualitiit und die Wirtschaftlichkeit von Laboratoriums- oder Produktionsverfahren abhangen, wobei hauptsiichlich auf die Entnahme von Pro ben bei der chemischen Untersuchung von Fliissigkeiten eingegangen wird. Der Aufsatz behandelt insbesondere (a) eine genaue Regelung des F1iissigkeitsstroms durch die Konstruktion geeigneter eJektromagnetischer oder pneumatischer Ventile, (b) automatische Messung von Fliissigkeitsvolumen durch verschiedene Methoden , wobei eine Methode eine grof3ere Genauigkeit als manuelle Methoden ermoglicht und eine sehr gute Reproduzierbarkeit iiber lange Zeiten aufweist, sowie (c) Methoden zur automatischen Messung der Veriinderlichen, von deren Wert der Ablauf des Verfahrens abhiingt- zum
Beispiel durch Bestimmung der absorptiol1letrischen Farbanderungen oder der Endpunkte der Titration . Die fUr die Regelung derartiger Anlagen benut zten Gerate sowie ihre charakteristischen Ein zelteile werden beschrieben. Bei der Konstruktion kOl1ll1lt es besonders auf zuverlassigkeit und einfachen Betrieb an. Die Darstellung der aufgezeichneten Mef3ergebnisse wird zusal1lmen mit den Methoden besprochen, mit deren Hilfe derartige Ergebnisse zur Regelung von Produktionsanlagen eingesetzt werden konnen . Es werden Beispiele der fnstrumentierung nach den dargelegten Prinzipien angefUhrt, an denen ihre Bedeutung sowohl bei vollstiindiger als auch teilweiser Autol1latisierung gezeigt werden soIl.
DISCUSSION The following questions and answers arose Discussion:
during
A. The presence of solid matter or dirt in a liquid sample or reagent is quite likely to cause difficulties with such instruments, though this is common to many other types of equipment, and adequate precautions will always be made when such conditions are anticipated . It would be normal to fit filters externally to the apparatus in such cases, and more
the
Q . Process waste water is often polluted with solid particles: will the 'Analmatic' instruments perform correctly under such conditions? 302
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TECHNIQUES FOR THE AUTOMATION OF SAMPLING AND CHEMICAL ANALYSIS
than one filter would be fitted in each flow line, so that while one filter is in use, the other(s) can be automatically cleaned by back-flushing, or manually replaced, without interrupting the operation of the apparatus. Q. What errors are introduced by transfer of water from the polythene gallery around the top of the syringe barrel into the liquid being delivered from the pipette? A. Careful tests have shown that although liquid is transferred up into the water in the gallery, the amount of water lubricant being transferred by the syringe piston into the volume of liquid delivered is completely negligible as far as its effect on the stated accuracy of the pipette is concerned. Q. What error is introduced by the unknown volume of liquid that is enclosed by the distended valve diaphragm, and which is expelled when the valve closes? A. The volume involved which is forced into the delivery jet is approximately 0 ·03 ml, which is less than the volume of a freely-formed drop of water. In fact, the volume which is expelled in this way produces a benefit rather than a disadvantage. The result is that the drop of liquid, which remains on the jet of the majority of liquid dispensing devices, e.g. burette or pipette, and which inevitably introduces an uncertain error, is in this case always displaced so that this error cannot arise. Q. What is the material of the tubing used in the peristaltic pump? A. Silicone rubber tubing is used for this purpose, as this provides the desired degree of flexibility together with the maximum resistance to chemicals. Unless liquids which attack the tube are being used, the tubing will last a considerable time, and many weeks of use should be obtained from it. A feature of the instrument is that all stress on the tube may be released when the instrument is not in use, by unscrewing the knurled screw seen at the top of Figure 6 in the paper. This raises the upper part of the curved pump surface, and the tube is no longer compressed. This, in fact, is the condition shown in Figure 6. When corrosive liquids, which may attack the tube over a period of days, are being used, it is a matter of perhaps 60 sec to replace the tube by a new length, and this can be planned as a regular maintenance operation at whatever frequency is found necessary. Q. What is the accuracy of delivery from the peristaltic pump? A. It will be appreciated that for purposes for which the proportioning pipette is intended, it is not necessary to know the exact volume which is delivered per rotation of the pump, but it is essential that the volume delivered should be consistent for every rotation . In fact, the delivery per rotation is approximately 0·3 ml, and this has been shown to be repeated to ±0'003 ml, or ± I per cent. Q. How is a fresh sample obtained at the jet of an automatic pipette when this is being used in a fully automatic instrument, to avoid errors due to liquid remaining stagnant in the bores of the valve block between one delivery and the next? A. No problem is caused by this 'hold-up' volume when the pipette is being used to dispense a reagent from a standard stock reservoir, but when it is being used to deliver samples
drawn at intervals from a process pipeline adequate precautions have to be taken. The length of connecting tube between the process pipe and the instrument is itself flushed through with fresh sample before each measurement, and then the pipette in question is arranged to fill and empty a sufficient number of times (this may be determined and preset), to flush out all traces of the previous sample before the sample for measurement is taken. Q. Can your method of automation of chemical analyses compete with other methods of automatic analysis, e.g. X-ray spectrophotometry etc.? Can you give your opinion of the accuracy, speed, and technical applications of X-ray methods of chemical analysis? A. No such direct comparison can easily be made, for the 'Analmatic' automatic instruments in question are designed to carry out all stages of an analysis completely automatically, from the taking of a sample when required, to the printing of the result and initiation of any necessary control action . In general, the majority of other automatic measuring instruments of a chemical nature require the preparation, and insertion, of a sample for measurement, by manual methods. No significant comments can be offered on the latter half of the question as this subject is outside the field of our present work . Q. What method of determining the end point of titration is considered to be the most accurate and desirable: Conducti metric, potentiometric, colorimetric, impedance measurement, or high frequency oscillation changes? What is the main line of development of your automatic titrators? A. Of the alternatives mentioned, chiefly potentiometric and colorimetric end points have been investigated, with a smaller section of the work devoted to conductimetric determinations. It has been found that the largest number of industrial applications for this sort of analysis can be satisfied by determinations by electrometric means, and for this reason the greater part of development lies in the production of instruments in this category. Q . Are pneumatic controls ever employed, bearing in mind their advantage in non-flameproof atmospheres? A. Pneumatic operation of certain components, in particular the diaphragm valves, has been achieved, and the advantages of such systems are realized and employed where appropriate. Pneumatic systems for remote indication of results, alarm, or control purposes, are also used. Q. Can the instruments operate on samples supplied at low pressures? A. These instruments are completely satisfactory providing a minimum pressure equivalent to approximately I m height of liquid can be provided . Q. The question of filtering has already been mentioned. Can the instruments include means for cooling or heating sample liquids if these are initially at inconvenient temperatures? A. Such auxiliary devices can certainly be provided, but they would normally be fitted externally to the instrument, and would be individually designed to suit each particular application.
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following sections by specific examples where they have been applied. (a) Repetitive measurement and delivery of liquid volumes as required for sampling and dispensing. (b) Performance of chemical techniques for the preparation or extraction of samples. (c) Automatic sequence control. (d) Determination of colour density by absorptiometric methods. (e) Detection of titration end-points, or related electrochemical properties. (f) Presentation of results in permanently recorded form. (g) Initiation of appropriate control action.
Wherever a chemical plant, or similar industrial installation, relies for successful operation on the results of analytical measurements at various stages in the process, the justification for making those measurements automatically may be established for a number of reasons: (a) The complexity of modern plant operation, which continues to increase, making it necessary to obtain more detailed information about conditions throughout the process. (b) The demand for higher levels of accuracy and repeatability, requiring skilled measurements to provide results adequate for control purposes. (c) The frequent desire to achieve control automatically from the results given by analytical measurements. (d) The removal of existing sources of inaccuracy arising through fatigue of operators. (e) The desire to use scientific staff most profitably, without occupying them on laborious routine test analyses.
Delivery of Liquid Samples and Reagents To make repeated measurements on the contents of a process stream it must be possible to withdraw samples, which must be truly representative of conditions existing within the process,
Requirements of reliability
Having accepted one or more of the above factors, satisfactory evidence must be produced to prove that the reliability of a proposed automatic installation will justify the operation of the process being dependent on the results given by the apparatus. According to the application, this will require that it shall perform with the minimum of attention for periods ranging from a few hours to several months at a time. From the designer's aspect such stringent requirements mean that all techniques of automation, and the components employed, have to be thoroughly examined for their expected trouble-free life, since the value of the apparatus to the user will be limited by the least reliable part. Since such equipment may be required to function unattended, provision must be included to indicate immediately should a fault condition arise. The examples quoted include such alarm circuits, and also systems by which the cause of a fault may be quickly located. Particular reference is given here to work involved in the design of automatic equipment for the measurement and control of the properties of liquid samples. A wide variety of such applications are found in industrial and medical work, and particularly in the chemical and oil industries. These include both major installations where completely automatic analysis is involved, and other instances where it is preferable to automate only individual stages of a procedure. The techniques to be automated It is intended that the instruments and devices described shall make possible the automation of analytical procedures which are at present repeatedly carried out by manual methods. The desired analytical results are obtained by using one or more of the following techniques, and these are illustrated in the
Figure I. Solenoid-operated diaphragm valves
and to treat these samples with any necessary reagents. This involves the control of the flow of liquids by reliable valves, the careful attainment of a contemporary sample, and the accurate measurement of the volumes taken. Liquid flow control
To be suitable for working with chemical process liquors the valve selected must be constructed of suitably corrosion-resistant materials and this, coupled with the requirement of longterm reliability, makes necessary the most careful design. A completely satisfactory valve for such duties has been produced and one form is shown in Figure I. A flexible diaphragm is normally spring-loaded to seal two ports in the valve block, and
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TECHNIQUES FOR THE AUTOMATION OF SAMPLING AND CHEMICAL ANALYSIS
a 15 W solenoid, when energized, causes the diaphragm to be lifted clear of the ports to allow the liquid to flow. The valve action is extremely rapid since the movement of the armature is less than s\: in. (I ·19 mm), and this is essential to ensure continued operation without risk of early failure due to fatigue in the flexible diaphragm. The liquid meets only the material of the valve block and the diaphragm, and thus by suitable choice of materials for each the valves can handle even highly corrosive liquids. The diaphragms are normally of acid-resisting butyl rubber but, for the more corrosive applications, they are faced with polytetrafluoroethylene (PTFE). Table 1 lists various materials with typical liquids for which they are suited.
waste to prepare a contemporary sample at the apparatus. Such a system is shown in Figure 2, where eight sample lines
Table I Materials used for construction of valve blocks and vessels Material
Perspex acrylic resin U nplasticized
polyvinylchloride (PVC) Duralumin Stainless steel Polytetrafluoroethylene (PTFE)
Suitable for
Most dilute aqueous solutions. Not for organic solvents Most aqueous solutions. A limited range of organic solvents Neutral organic solvents. Not usually for aqueous solutions Organic solvents, certain alkalis and acids. Some aqueous solutions Practically all corrosive liquids where other materials are unsuitable
F(fjure 2. Multiple sample line flushing units
are divided between two similar flushing vessels for odd- and even-numbered sources, respectively. Measurement of liqUid volume
To perform measurements on successive samples to consistent accuracy it is necessary to take identical volumes of sample, and any reagents, on each occasion. Nevertheless, the variation that can be tolerated in the volume will vary from one application to another, and the most suitable sampling method must be selected. When conducting liquids are involved, a measuring vessel with two, or more, contact probes may be used with a simple electrical circuit (Figllre 3). This gives the necessary switching
The valve design follows one of two basic forms, having kin. (3·18 mm) bore ports normally for inlet flows, or ~. in. (9·53 mm) ports for efficient draining of vessels or pipelines. Alternative constructions use the solenoid, when energized, to close the valve. The solenoid can also be replaced by a toggle lever for manual operation, by a screw to form an adjustable constriction, or by an air hose connection for pneumatic operation. With -~ in. (3·18 mm) bore ports the valve closes reliably against pressures up to 15Ib./in. 2 (1·1 kg/cm2), with a safety factor at that pressure of approximately 1·6. Figure I illustrates the component parts of a valve assembly for connection by tubing to other parts of a liquid-handling system. However, because of the simple nature of the valve seating this can often be machined in the surface of a reaction vessel or other component, so that several valves, for inlet and drain purposes, may be mounted on a single unit.
Switch
6·8kfi
~~--~A-~--~--,
sov d.c. +
Procurement of contemporary sample
When such valves are connected in branch pipes for sampling from a process stream, it is essential for the stagnant volume of liquid, behind the valve since it was last opened, to be displaced to waste before a further sample is taken for measurement purposes. This is provided in the process instruments mentioned, but when samples are being drawn from only one source it is an advantage to bring a continuously flowing sample loop close to the input of the instrument. The sampling device is then arranged to fill and discharge to waste a sufficient number of times to remove the small stagnant volume before taking the sample for analysis. Where samples are being drawn from several sources in succession an alternative system is possible, for while one sample is being analysed the next sample line can be flushed to
r2·2
G
Level at which switCfiiiig OCcurs To sequence P control circuit
Figure 3. Electrical liq 11 id level probe circuit
action into the main sequence control circuits at the instant the liquid level rises to reach the upper of the two probes P. This circuit, which presents 20 V d.c. between the probes before they are covered, requires a minimum current of 300 ftA between them to operate the relay RI. The current passes between the probes for no longer than 20 msec, being limited by the closing of relay contact r2.1, so that polarization and electrolysis problems do not arise. When non-electrolytes, or liquids in flame hazard conditions,
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D. A. PATIENT
Table 2 Performance of automatic pipettes
are being used, a capacitance system may be adopted. A vessel designed for this purpose has been used with metal bands round the outside of the measuring tube!, these being connected to a capacitance-sensitive relay, which responds when the liquid meniscus reaches the level of a selected band. The volumetric accuracy of the above methods is limited chiefly by the rate of liquid inflow and the area of the liquid surface, but repeatabilities of the order of 2 per cent are possible. However, when highly repeatable volumes must be dispensed a further development gives accuracies considerably greater than those obtained by manual pipetting methods. The components of this automatic pipette are shown in Figure 4, and several can be seen mounted together in Figure 7.
MaximulIl capacity of pipetle
Weight of ten deliveries at a given setting (Water at 20'C)
Maximum result Minimum result Range over 10 Small sample standard deviation
1 ml
1 ml
5ml
IOml
20ml
0·088g 0·087 0·088 0·088 0·088 0·088 0·088 0·086 0·087 0·089 0·089 0·086 0·003
0·512g 0·513 0·512 0·511 0·513 0·511 0·512 0·510 0·511 0·512 0·513 0·510 0·003
3·989g 3-988 3·990 3-989 3-989 3-989 3-989 3·988 3-989 3·989 3·990 3·988 0·002
9·935g 9·931 9·933 9·934 9·934 9·933 9·933 9·934 9·932 9·935 9·935 9·931 0·004
19·92lg 19·921 19·920 19·921 19·922 19·923 19·922 19·921 19·926 19·926 19·926 19·920 0·006
0·0008
0·0011
0·0006
0·0012
0·0021
been shown that after several months operation at a frequency of 100~200 operations/day the volume delivered was still within the limits quoted. The volume accuracy is approximately the same for all sizes, being ±0·003 ml or better. Thus the error, which is of the order of ±O·l per cent for a 1 ml delivery, is correspondingly lower for larger volumes. The polythene gallery cemented at the top of the syringe barrel (Figure 4) provides an annular volume of a few ml which is maintained partly filled with water or other suitable liquid. This serves to lubricate the top of the syringe piston and prevents the formation of small crystals there when liquids such as sodium bicarbonate are being handled. F(ljure 4. Automatic pipette
Reagent dispensing
Two of the valves (see 'Liquid flow control') are fitted to a common valve block to provide inlet and outlet taps to a modified hypodermic syringe also mounted on the block. This syringe fills, when the inlet valve is opened, by upward displacement of the piston under the hydrostatic pressure of the liquid supply to which the inlet is connected. Two insulated assemblies (Figure 4) are fitted in a slot in the panel above the valve block. Each of these has a pivoted arm carrying a silver contact which makes electrical connection with an adjacent fixed contact when the cap of the syringe piston strikes the arm. The top assembly is positioned in the guide slot so that it determines the extent to which the pipette fills, and a graduated scale with vernier may be fitted for calibration purposes. When the top pair of contacts are closed, as the piston rises, they operate into the sequence control system causing the inlet valve to close and the outlet valve to open. The internally loaded piston then falls under its own weight to expel the measured volume of liquid into the receiving vessel. Syringes are available from 1 to 100 ml maximum capacity, and may be interchanged if required to dispense a volume beyond the range of the syringe first fitted. The very high order of repeatability achieved with pipettes of this form is shown by the weighings recorded in Table 2, which were obtained with several sizes of pipette. It has also
The syringe pipette finds uses, not only in fully automatic instruments, but also in smaller equipment such as a multiple dispensing unit (Figure 5). This delivers accurately repeatable Scales - Stop SWitches with verniers ~_-
Pipettes
Pipette jet
F(ljure 5. Multiple dispenSing uflit
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TECHNIQUES FOR THE AUTOMATION OF SAMPLING AND CHEMICAL ANALYSIS
volumes of reagent (within the limits of Table 2) into separate vessels carried in suitable racks below the three outlet jets. Approximately 30 vessels may be treated per min, and this rate of working and accuracy can be maintained over long periods 2 • The racks may be advanced through the apparatus either by manual control or by an automatic mechanical traverse. This consistent delivery of liquid volumes would be beyond the capability of an operator even though precautions were taken to minimize distractions and fatigue.
Direct reaction technique
A vessel as seen at the bottom of Figure 7, and constructed from one of the materials of Table 1, is used when sample
Proportional dispensing
When it is more important for the ratio of the volumes of successive liquids dispensed to be held constant, rather than that the actual volume of the additions should be set to a given precise value, the peristaltic pump is used (Figure 6). This is
F(f{ure 7. Automatic titrator panel
preparation can be done by mixing appropriate volumes of sample and one or more reagents. It is fitted with inlet jets from the pipettes, a drain valve, and either mechanical, magnetic, or gas stirring, and thus represents the 'beaker' of a normal manual analysis. 11 is often possible to make the required measurement, while the mixture is in this vessel, by fitting suitable electrodes for electrochemical tests, or windows for colorimetric work. [n other cases it is preferable to pass the prepared specimen from such a reaction vessel to subsequent measurement stages, as in the absorptiometric system described under 'Measurement of Concentration by Colour Density'.
F(f{ul'e 6. Peristaltic pump
applicable to the preparation of blood samples for analysis, and for similar applications. The flexible tube of the pump is compressed by three eccentric rollers on a rotor which makes an exact number of revolutions between I and 20 according to the setting of a selector switch. Thus to add say five volumes of reagent to two of sample, the pump is first caused to make two revolutions with the intake jet immersed in the sample container, followed by five revolutions with it in the reagent. This ejects the sample and some of the reagent into the receiving vessel, and the reagent still i~ the flexible tube is then delivered by rotating the pump four tImes to clear the tube, after the reagent container has been removed from the intake jet. Removal of the tube for sterilization or replacement is readily possible for changing quickly from one set of liquids to another. Although both the dispensing devices described are only semi-automatic, they nevertheless relieve the operator of the very functions where inaccuracies could otherwise arise. Preparation of a Sample from a Process Stream A sample flow must often be treated in some way before making a measurement to determine the concentration of some constituent. Two methods of preparation are described below.
Extraction technique
The automation of liquid-liquid extraction to obtain the required constituent from a sample stre~m can be illustra~ed by its current use by the U.K. AtomiC Energy AuthOrIty, and Figure 8 shows the unit in which a uranium sample is separated from an aqueous stream by solvent extraction. In the vertical column Cl' tri-butylphosphate, entering through the lower side tube S, extracts the uranium and emerges at the point E, from which it passes through the limb D into the second column C 2 • Here it meets a downward flow of ethylenediaminetetraacetic acid (EDTA), which enters at Band backwashes the uranium from the solvent. The solvent, for reclamation, emerges at R, while the uranium-bearing EDT A coming from the lower end of column C2 is complexed by the continuous addition of ammonium thioglycollate at T to produce the colour whose density is then measured absorptiometrically. The flow rate of sample through the columns is of the order of 3 ml/min, and is controlled together with the flow of the other reagents by the four injectors J, which deliver these liquids in correct proportions at approximately one drop per
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D. A. PATIENT
sec. Highly efficient extraction is achieved in the two columns by thoroughly intermixing the two constituents by cylindrical
rhombs it describes a cylindrical locus on which the necessary absorption cells are disposed, so that the light beam passes through each in turn. The composite photomultiplier signal is switched by a commutator on the same rotor to separate the To ",corder
Motor
SOIJrce Photomultlplier
Figure 9. Schematic diagram of absorptiometer unit
Figure 8. Liquid-liquid solvent extraction unit
stirrer rods which are rotated on the axis of each column by the stirrer motors M. Automatic Sequence Control
components corresponding to the different cells so that the required ratio may be obtained electrically. This absorptiometer has been used for uranium estimations by different techniques. One uses the colour complex with ammonium thiocyanate\ measuring the absorption produced at 365 mfl, while the other uses the thioglycollate complex (see Extraction technique) and measured at 425 mfl. In the latter case the colourless reference solution is obtained from the sample stream after the characteristic colour has been developed and the sample has passed through one cell, by decolourizing the emergent flow with excess carbon dioxide. This gas flow also transfers the sample to the reference cell. The absorption cell assembly is seen in Figure 10, with the
In a fully automatic apparatus which is required to carry out a series of analytical stages without the intervention of an operator it is necessary for each operation to be initiated when the preceding one has been satisfactorily completed. While such automatic programming may be achieved by electronic control circuits, it is preferable to use electromechanical devices including relays, uniselectors, and synchronous motordriven timers for this purpose 3 • Sequence circuits of this type are more readily understood by maintenance staff than are the equivalent electronic circuits, and the techniques required for servicing are similar to those already practised by plant instrument engineers. Measurement of Concentration by Colour Density To compensate for changing background turbidity of the sample stream when making a continuous measurement, and to avoid errors due to changes in the light source intensity and photo-detector sensitivity, a special absorptiometer unit (Figure 9) is necessary. This enables a number of different optical cells to be examined, and the absorption results compared, using a common light beam and photomultiplier. By passing an untreated sample flow through one cell, and the same sample, after development of the characteristic colour, through another cell, the required compensation is obtained. The ratio of the absorption in the two cells gives a result related to sample concentration only, independent of the other factors mentioned. Two glass rhombs are rotated about the optical axis causing the light path to be deflected so that while passing between the
F(f{ure 10. Absorption cell assemb~y
carbon dioxide inlet and lift vessel at the extreme left, and the two cells each side of the space in which the rhomb rotor fits, the right-hand cell being for reference purposes. An installation at one of the U.K. Atomic Energy Authority's plants has five individual liquid-liquid extraction units, each as Figure 8, and makes a measurement on five different sample streams, taking 3 min for each. Two identical absorptiometerrecorder units allow anyone sample stream to be monitored continuously, while the other four are monitored in turn. Under normal conditions all five streams are monitored in one unit, while the other is available as a stand-by, or so that periodic servicing work may be done without interrupting the function of the instrument.
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TECHNIQUES FOR THE AUTOMATION OF SAMPLING AND CHEMICAL ANALYSIS
Fully Automatic Titration Titration is a most significant analytical method, and has frequently been carried out automatically to the extent of detecting the end-point. However, the ful.lest advantage can only be gained when all stages, from the tak1l1g of the sa mpl~ t~ the recording and use of the result, are rendered automat l~ . This applies whether the end-point is be~g detecte~ pote.ntlOmetrically, or at a given pH value; by a dead-stop CircUIt for iodometric work, or colorimetrically. Five pipettes (see page 298) for sampling and reagent a~dition are shown on the titration panel of Figure 7, together With the reaction and titration vessel at the bottom below the burette and its valve block. Adjustlllent for titrant normality
A titration result is obtained first as the volume of titrant used and for instance with a 50 ml burette, this would be quot~d to the nearest 0·05 ml, ] /1,000th of the burette capacity. To achieve this discrimination automatically, and also to present a record in the units required and not merely as volume G L
meter for analogue output corresponding to the volume of titrant used, are driven by the same motor that rotates the leadscrew L (Figure 11). The photoelectric .system mou~ted ?n C, moves vertically in the rails G when L IS rotated, bemg hnked by the arm A to the leadscrew nut.N. Thi.s arm pivots at P on the nut, and carries a roller R beanng agamst the straight edge of a bar E, which is itself pivoted at F. While the bar is parallel with the rails the photoelectric system C moves directly with the m~t. However, if ~he ~crew S is turned, the nut M moves to the nght or left and mchnes the bar a few degrees from the vertical, so that C is caused to move up to ] 0 per cent more or less than any corresponding movement of the nut N. Since any movement of N is directly .trans~itt~d to the recording unit, the adjustment allows a given titratIOn result to be recorded as a figure within the range ± 10 per cent of the value indicated when the edge E is vertical. The recording range of the figure wheels is conveniently 0-:1,000, and it would be possible to calibrate the apparatus with a standard sample and titrant of carefully prepared normality. so that t~e result of the titration would be indicated exactly 111 the Units required. . The adjustment described removes the tedIOus and r~current necessity for such carefu l preparation of titrant stock, smce the normality need only be to within ± J0 per cent of the calculated value, final setting being achieved using the screw S. Alternative to conventional burette
N
Axis of burette
E
M
s
Fig ure 11. Schematic diagram of normality compensation device
of titrant used, the digital recorder at the top of Figure 7 is used with the photoelectric meniscus-finding mechanism seen there and schematica ll y in Figure 11 . The recorder figure wheels, and also a precision potentio-
When compactness and also economy are more essen~ial than such features as the adjustment descnbed 111 the precedmg section, an alternative titration device has been used. Based on the syringe pipettes described on page .298, this ~as the syringe piston coupled to a nut on a motor-dnven fine pitch leadscrew, the movement of the piston being linked to the pen of a chart recorder. This device has been used in an instrument to determine the concentration of dissolved oxygen in water by an automated Winkler technique, to within 0·1 p .p.m. il: the range 0- 10 p.p. m. This follows work. by Bri~gs et al.", but uses a 'dead-stop' electrometric end-pomt, a nd IS SUited for ~he automatic monitoring of pollution due to effluents entenng river waters. Recording of Results This may be done, normall y on paper chart, in. a number of ways as illustrated in Figure 12, either as contllluous IIlked PM 2 ·1 5 PM 2 ' 1 9 PM 2 '2 3
·2 6 ·30 ·3 4 ·3 7 ·4 1
6 13 • 6 18 •
PM 2 PM 2 PM2 PM2 PM 2
·45
612 • 6 13 •
'48
. 52
' 56 . 59
PM3 3 PM3 7 PM3 ·1 0 PM3 · 14 PM3 ·1 8
301
1402
615 • 6 17 •
2 2 2 2 2
PM PM PM PM PM
Figure 12. Alternative chart records
6 13 •
6 14
6 19 • 6 17 •
619 6 14 •
6 12 •
6 14 • 6 18 • 612
6 14 • 6 17 •
D. A. PATIENT
traces, individual results, or in digital form. Whichever system is most suitable, a means of identification is provided when more than one sample stream is involved, either by a code, sample number, or time print.
When more complex control action is desired, electrical analogue signals from one or more potentiometers in the measuring unit would be used to direct the necessary computer, which in turn delivers the control signals to the plant.
Control Action
References 1
Signals for control purposes can be provided from the measuring units described, either in electrical or pneumatic analogue form, or encoded digitally. If only simple on-off action is required in order to control the process when the measured variable exceeds predetermined limits, this may often be achieved directly from the measuring units, for instance from switches operated by the leadscrew mechanism of an auto-titrator (page 301).
2
3
4 5
BISBY, H., BROWN, L. H. and CHAPMAN, D. R. J. sci. lnstrum. 33 (1956) 467 GREEN, N. C. and MONK, P. R. Chem. and lnd. No. 39 (1959) 1210 PATIENT, D. A. Automation Progress I (1956) 25 PATIENT, D. A. Proc. Congr. Modern Anal. Chem., June, 1957. Cambridge; Heffer BRIGGS, R., KNOWLES, G. and SCRAGG, L J. Analyst 79 (1954) 744
Summary This paper describes techniques and devices used to achieve automation of analytical test procedures on which depend the satisfactory and economic operation of laboratory or plant processes, with emphasis on those of a chemical nature involving liquid samples. The subject includes: Ca) sensitive control of the flow of liquids by the design of suitable electromagnetic or pneumatic valves, (b) automatic measurement of liquid volumes by various methods, one in particular giving accuracy greater than that obtainable by manual methods and having exceedingly high long-term repeatability, and Cc) methods applied to the automatic measurement of the variable on whose value the operation of the process depends-for
instance by absorptiometric colour measurements or titrimetric endpoints. The devices used for the automatic control of such equipment are described together with details of construction, with particular reference to considerations of reliability and ease of routine maintenance. The presentation of recorded results is discussed together with the methods by which such results may be translated into plant control. Examples of instrumentation including these techniques are quoted to illustrate their significance, both in cases where full automation has been applied, and also where certain phases only of a process have been rendered automatic.
Sommaire Ce rapport decrit des techniques et des dispositifs utilises pour accomplir I"automatisation des procedes d'essais analytiques dont depend le fonctionnement satisfaisant et economique du laboratoire ou de I"usine de processus, en mettant I"accent sur ceux de nature chimique, comportant des echantillons liquides. Le sujet comprend: (a) commande instantanee des debits de liquides avec realisation de vannes convenables electromagnetiques ou pneumatiques, (b) mesure automatique des volumes de liquides par differentes methodes, dont I'une, en particulier, donne une meilleure precision que celle que I"on peut obtenir par les methodes manuelles, et ayant une extraordinaire reproductibilite it long terme, et (c) methodes appliquees it la mesure automatique de la variable dont la valeur conditionne le fonctionnement du processus par exemple par
les mesures colorimetriques d'absorption ou de points limites titrimetriques. Les dispositifs utilises pour la commande automatique d' un equipement de ce genre sont decrits en meme temps que les details de construction, avec reference speciale aux considerations de 'fiabilite' et de facilite it maintenance. On discute la presentation des resultats releves ainsi que les methodes par lesquelles de tels resultats peuvent etre traduits en commande automatique dans I"usine. On cite des exemples d' instrumentation incorporanl ces techniques, afin d'illustrer leur signification, aussi bien dans des cas oll I"on applique I"automatisation integrale, que dans ceux oll certaines phases seulement d'un processus ont ete rendues automatiques.
Zusammenfassung Der Aufsatz beschreibt Verfahren und Geriite zur Automatisierung chemischer Analysen, von denen die Qualitiit und die Wirtschaftlichkeit von Laboratoriums- oder Produktionsverfahren abhangen, wobei hauptsiichlich auf die Entnahme von Pro ben bei der chemischen Untersuchung von Fliissigkeiten eingegangen wird. Der Aufsatz behandelt insbesondere (a) eine genaue Regelung des F1iissigkeitsstroms durch die Konstruktion geeigneter eJektromagnetischer oder pneumatischer Ventile, (b) automatische Messung von Fliissigkeitsvolumen durch verschiedene Methoden , wobei eine Methode eine grof3ere Genauigkeit als manuelle Methoden ermoglicht und eine sehr gute Reproduzierbarkeit iiber lange Zeiten aufweist, sowie (c) Methoden zur automatischen Messung der Veriinderlichen, von deren Wert der Ablauf des Verfahrens abhiingt- zum
Beispiel durch Bestimmung der absorptiol1letrischen Farbanderungen oder der Endpunkte der Titration . Die fUr die Regelung derartiger Anlagen benut zten Gerate sowie ihre charakteristischen Ein zelteile werden beschrieben. Bei der Konstruktion kOl1ll1lt es besonders auf zuverlassigkeit und einfachen Betrieb an. Die Darstellung der aufgezeichneten Mef3ergebnisse wird zusal1lmen mit den Methoden besprochen, mit deren Hilfe derartige Ergebnisse zur Regelung von Produktionsanlagen eingesetzt werden konnen . Es werden Beispiele der fnstrumentierung nach den dargelegten Prinzipien angefUhrt, an denen ihre Bedeutung sowohl bei vollstiindiger als auch teilweiser Autol1latisierung gezeigt werden soIl.
DISCUSSION The following questions and answers arose Discussion:
during
A. The presence of solid matter or dirt in a liquid sample or reagent is quite likely to cause difficulties with such instruments, though this is common to many other types of equipment, and adequate precautions will always be made when such conditions are anticipated . It would be normal to fit filters externally to the apparatus in such cases, and more
the
Q . Process waste water is often polluted with solid particles: will the 'Analmatic' instruments perform correctly under such conditions? 302
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than one filter would be fitted in each flow line, so that while one filter is in use, the other(s) can be automatically cleaned by back-flushing, or manually replaced, without interrupting the operation of the apparatus. Q. What errors are introduced by transfer of water from the polythene gallery around the top of the syringe barrel into the liquid being delivered from the pipette? A. Careful tests have shown that although liquid is transferred up into the water in the gallery, the amount of water lubricant being transferred by the syringe piston into the volume of liquid delivered is completely negligible as far as its effect on the stated accuracy of the pipette is concerned. Q. What error is introduced by the unknown volume of liquid that is enclosed by the distended valve diaphragm, and which is expelled when the valve closes? A. The volume involved which is forced into the delivery jet is approximately 0 ·03 ml, which is less than the volume of a freely-formed drop of water. In fact, the volume which is expelled in this way produces a benefit rather than a disadvantage. The result is that the drop of liquid, which remains on the jet of the majority of liquid dispensing devices, e.g. burette or pipette, and which inevitably introduces an uncertain error, is in this case always displaced so that this error cannot arise. Q. What is the material of the tubing used in the peristaltic pump? A. Silicone rubber tubing is used for this purpose, as this provides the desired degree of flexibility together with the maximum resistance to chemicals. Unless liquids which attack the tube are being used, the tubing will last a considerable time, and many weeks of use should be obtained from it. A feature of the instrument is that all stress on the tube may be released when the instrument is not in use, by unscrewing the knurled screw seen at the top of Figure 6 in the paper. This raises the upper part of the curved pump surface, and the tube is no longer compressed. This, in fact, is the condition shown in Figure 6. When corrosive liquids, which may attack the tube over a period of days, are being used, it is a matter of perhaps 60 sec to replace the tube by a new length, and this can be planned as a regular maintenance operation at whatever frequency is found necessary. Q. What is the accuracy of delivery from the peristaltic pump? A. It will be appreciated that for purposes for which the proportioning pipette is intended, it is not necessary to know the exact volume which is delivered per rotation of the pump, but it is essential that the volume delivered should be consistent for every rotation . In fact, the delivery per rotation is approximately 0·3 ml, and this has been shown to be repeated to ±0'003 ml, or ± I per cent. Q. How is a fresh sample obtained at the jet of an automatic pipette when this is being used in a fully automatic instrument, to avoid errors due to liquid remaining stagnant in the bores of the valve block between one delivery and the next? A. No problem is caused by this 'hold-up' volume when the pipette is being used to dispense a reagent from a standard stock reservoir, but when it is being used to deliver samples
drawn at intervals from a process pipeline adequate precautions have to be taken. The length of connecting tube between the process pipe and the instrument is itself flushed through with fresh sample before each measurement, and then the pipette in question is arranged to fill and empty a sufficient number of times (this may be determined and preset), to flush out all traces of the previous sample before the sample for measurement is taken. Q. Can your method of automation of chemical analyses compete with other methods of automatic analysis, e.g. X-ray spectrophotometry etc.? Can you give your opinion of the accuracy, speed, and technical applications of X-ray methods of chemical analysis? A. No such direct comparison can easily be made, for the 'Analmatic' automatic instruments in question are designed to carry out all stages of an analysis completely automatically, from the taking of a sample when required, to the printing of the result and initiation of any necessary control action . In general, the majority of other automatic measuring instruments of a chemical nature require the preparation, and insertion, of a sample for measurement, by manual methods. No significant comments can be offered on the latter half of the question as this subject is outside the field of our present work . Q. What method of determining the end point of titration is considered to be the most accurate and desirable: Conducti metric, potentiometric, colorimetric, impedance measurement, or high frequency oscillation changes? What is the main line of development of your automatic titrators? A. Of the alternatives mentioned, chiefly potentiometric and colorimetric end points have been investigated, with a smaller section of the work devoted to conductimetric determinations. It has been found that the largest number of industrial applications for this sort of analysis can be satisfied by determinations by electrometric means, and for this reason the greater part of development lies in the production of instruments in this category. Q . Are pneumatic controls ever employed, bearing in mind their advantage in non-flameproof atmospheres? A. Pneumatic operation of certain components, in particular the diaphragm valves, has been achieved, and the advantages of such systems are realized and employed where appropriate. Pneumatic systems for remote indication of results, alarm, or control purposes, are also used. Q. Can the instruments operate on samples supplied at low pressures? A. These instruments are completely satisfactory providing a minimum pressure equivalent to approximately I m height of liquid can be provided . Q. The question of filtering has already been mentioned. Can the instruments include means for cooling or heating sample liquids if these are initially at inconvenient temperatures? A. Such auxiliary devices can certainly be provided, but they would normally be fitted externally to the instrument, and would be individually designed to suit each particular application.
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