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Precise pressure measurement in the range 0.1–500 torr
P r e c i s e p r e s s u r e m e a s u r e m e n t in t h e r a n g e 0.1 - 500 torr received 17 December 1970 J Gascoigne, Electron and Ion Diffusion Unit, Australian National University, Canberra, ACT, Australia
It is becoming increasingly important in several fields of work to make precision measurements of the pressure of ultra pure gases in the range O.1-500 torr and above. To cater for this need several commercial gauges have been developed over the last decade. Two of these gauges, a quartz high precision pressure gauge, and a capacitance manometer are described, and their performance and limitations discussed. A "dead-weight" primary pressure standard, which has been used as a sub-standard for checking the absolute accuracy of the instruments, is also described. It will be shown that, although the quartz manometer lacks high resolution at pressures less than 4 torr, it is in general stable over long periods of time. By contrast, the capacitance manometer possesses extremely high resolution at low pressures, but appears to lack the long term stability expected of this instrument. Introduction The measurement of pressure in the range from 0.1 torr to atmospheric pressure with high accuracy is, perhaps, an unusual requirement. In this laboratory, the need for precision pressure measurement in this range stems from the requirement to measure electron and ion transport coefficients with high absolute accuracy, and even higher relative accuracy, as the first stage in determining absolute cross sections for low energy electrons and ions. In all scattering experiments, whether of the single collision (beam) type, or of the swarm type used in this laboratory, the accuracy of the absolute determination depends on the accuracy with which the number density of the scattering centres can be determined. Usually this determination rests on an absolute pressure measurement. In beam experiments the pressures used are of the order of 10 -3 torr or less, and the measurement of pressure in this range to within 1 or 2 per cent is a task that is stdl under investigation in standards laboratories. In swarm experiments, on the other hand, the pressures used are in the range under discussion; the measurement of these pressures with an accuracy of 0.1 per cent is a more realizable objective. However, an additional requirement with swarm experiments is that high gas purity must be maintained throughout the duration of the experiment, which is carried out under static conditions. These experiments are often sensitive to a few parts per million of impurity, in fact in some instances impurity levels as low as I part in 10 8 or 10 a are critical. The two gauges described in this paper not only have the required sensitivity and resolution for measuring the pressures under discussion, but are also constructed of materials which enable the required gas purity to be maintained. For reasons that will become apparent later in this paper, the selection of an instrument which appears to have the required specification does not guarantee that the required accuracy will be achieved, and it is therefore necessary to have access to equipment for checking the calibration of the gauges. The first part of this paper therefore describes the method that has been developed for this purpose.
The double primary pressure standard When the precise measurement of pressure became one of the
major problems associated with the experimental work being undertaken, the dead-weight type of primary pressure standard appeared to be the simplest and most accurate method of producing standard pressures for cahbration purposes. The principle of this technique is very simple. An accurately made piston cylinder combination and weight set is used to produce the desired pressure. As shown in Figure l, if the effec-
PI REFERENCE PRESSURE
PISTON P2 PRESSURE PRODUCED
PRESSUREPRODUCED= r PISTON WEIGHT ] + REFERENCE BY PISTON P2 [.EFFECTIVEPISTON AREAJ PRESSUREP1 Figure l. The "dead-weight" principle used in the primary pressure standard. tlve area and the weight of the piston are known then it is a simple matter to deduce that the pressure Pz supporting the piston is equal to the reference pressure P1 plus the piston weight dwlded by the effective piston area. If the pressure P1 is atmospheric, then the pressure P2 is referred to as a gauge pressure. If P~ is zero, then P2 will be an absolute pressure. The main problem in manufacturing the piston-cylinder combinations is to produce the components to the required degree of accuracy and surface finish. In theory no gas leakage should occur past the piston, and there should be no friction. In practice both these factors are kept to a minimum by careful manufacture There is, of course, some gas leakage, but this
Vacuum/volume 21/numbers 112. Pergamon Press LtdlPrmted in Great Britain
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J Gascoigne : Precise pressure measurement in the range 0.1-500 torr
Figure 2. The piston of the primary pressure standard floating during a pressure cahbrat~on. helps reduce the friction between the piston a n d t h e cylinder by providing a gas bearing. Figure 2 shows t h e piston of the p r i m a r y pressure s t a n d a r d floating during a pressure calibration. W h e n the i n s t r u m e n t is being used, the piston is spun at a h u n d r e d or so r e v o l u t i o n s per m i n u t e so as t o ensure t h a t the p i s t o n is floating freely. T h e p i s t o n cylinder c o m b i n a t i o n s being used have been produced commercially a n d have a claimed overall accuracy of 0.015 per cent of t h e pressure produced. T h e accuracy of these c o m p o n e n t s is traceable t o t h e N a t i o n a l B u r e a u o f Standards, Washington. Due to the weight of the piston, the m i n i m u m pressure t h a t can be p r o d u c e d by this i n s t r u m e n t is 15.5 torr. As has already been stated the tolerance o n this pressure is 0.015 per cent or a little m o r e t h a n 2 ~, 10 -3 torr, a n d if a n absolute pressure is to be p r o d u c e d t h e n the reference pressure m u s t be m a i n t a i n e d below this figure, or the pressure k n o w n to this degree of accuracy. T h e m e t h o d of p r o d u c i n g pressures a b o v e 15.5 t o r r absolute is s h o w n In Figure 3. T h e reference v o l u m e is c o n t i n u a l l y
In p r a c h c e it has been f o u n d t h a t it is possible to m a i n t a i n a reference pressure of a p p r o x i m a t e l y 2 1< 10-3 t o r r when 15 5 t o r r is being produced, a n d it is therefore necessary when calculating t h e actual a b s o l u t e pressure p r o d u c e d by the primary pressure s t a n d a r d to m a k e corrections n o t only for t e m p e r a t u r e , and the v a r i a t i o n of the gravitational acceleration f r o m the s t a n d a r d value, b u t also for the reference pressure. By using t h B c a l i b r a t i o n technique, it s h o u l d be possible to check gauges between 15.5 t o r r a n d 7 6 0 t o r r to a n accuracy of 0.015 per cent of the pressure but, because of m i n o r problems t h a t become evident in practice, it is considered t h a t 0 05 per cent o f the pressure p r o d u c e d is a m o r e reasonable, but conservative, tolerance t h a t may be claimed T o establish pressures below 15.5 torr, the c a l i b r a t i o n of the gauges to be described in this p a p e r relies o n the fact t h a t these i n s t r u m e n t s c a n be used as null m a n o m e t e r s , t h a t is, t h e reference side of the gauge can be held at pressures o t h e r t h a n zero pressure. It is therefore feasible to calibrate these i n s t r u m e n t s by holding the reference side of the null m a n o m e t e r at, say, 15.5 t o r r absolute a n d p r o d u c i n g pressures in excess of this m t h e positive pressure side, resulting in a pressure differential across the m a n o m e t e r . So t h a t stability w~th time can be m a i n t a i n e d it is necessary to use two p r i m a r y pressure s t a n d a r d s , one to m a i n t a i n the 15.5 t o r r reference pressure, a n d one to p r o d u c e the 15.5 t o r r plus the pressure increment required. This a r r a n g e m e n t is s h o w n in Figure 4 a n d has proved to be a m o s t reliable t e c h n i q u e T h e lmtial procedure consists of lntro-
REFERENCE GAUGE
REFERENCE GAUGE
POSITIVE PRESSURE
REFERENCEV A E PRESSURE V ./1
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STANDARD
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MANOMETERBEING CALIBRATED Figure 4. The method used to produce pressures of less than 15 5 torr I
I
CALIBRATING
PRESSURE Figure 3. Dmgram showing the method of producing pressures greater than 15.5 torr absolute. p u m p e d by a small 2-stage r o t a r y p u m p which has proved a d e q u a t e for this purpose. T h e p u m p i n g line incorporates a h q u t d m t r o g e n t r a p a n d t h e reference pressure is c o n t i n u a l l y m o n i t o r e d by a P i r a m gauge the p o r t of which is in the base of the bell j a r T h e zero of th~s gauge is checked f r o m time t o time by evacuating the bell j a r to a pressure of less t h a n 10 -4 t o r r using external pumping. 22
~SURE J PORT
dlfferentml. ducing 15.5 t o r r into b o t h sides of the null m a n o m e t e r a n d letting the whole a p p a r a t u s stabdlze. W h e n stability has been achieved, with b o t h pistons floating, the null of the i n s t r u m e n t is set at zero. This procedure nullifies any m i n o r v a r i a t i o n s m the two pressures p r o d u c e d by the two p r i m a r y pressure standards, and, because of this, t h e accuracy of the differential pressure subsequently p r o d u c e d does n o t d e p e n d o n the accuracy of these two pressures, providing they r e m a i n c o n s t a n t w i t h time. Once the zero has been determined, weights m a y be a d d e d to the positive pressure p r i m a r y pressure s t a n d a r d , thereby producing t h e desired dlfferentml pressure across t h e m a n o m e t e r . Because the m a n o m e t e r is set at zero when b o t h t h e p B t o n s of t h e p r i m a r y pressure s t a n d a r d s are floating, it ~s only necessary to look at the accuracy of the pressure increment a b o v e 15.5 t o r r to d e t e r m i n e the overall accuracy. This is constdered to be
d Gascolgne: Precise pressure measurement in the range 0.1-500 torr 0.05 per cent of the differential pressure, or 5 × 10 -4 torr, whichever is greater. By using this technique very consistent results have been readily obtained over a pressure range of 7.7 × l0 -~ torr to 15.5 torr, and extremely good cross checks have been obtained at 15.5 tort when the differential cahbration method has been compared with the absolute method.
The quartz high precision pressure gauge The first of the two manometers to be discussed is the quartz high precision pressure gauge. As the name implies, the pressure sensing element is of quartz, and it is the choice of this material which is the main reason for the high reproducibility and success of th~s instrument. The pressure sensing capsules used in this instrument are obtainable in numerous forms and pressure ranges, but this paper is confined to discussing the 0-250 torr and the 0-500 torr quartz capsules. These capsules comprise a quartz spiral Bourdon tube which is mounted on a fine quartz suspension and enclosed in a quartz envelope. The envelope has an aluminium mounting flange which positions it in the readout instrument. So that maximum stabdity can be achieved the capsule is maintained at a temperature higher than ambient using a proportionally controlled heater in the readout instrument. To enable the deflection of the Bourdon tube to be determined, a reflector is mounted on the lower end of the quartz spiral which rotates I00 ° for any pressure range. The position of the reflector is detected by an optical transducer null detector m the gauge readout unit which is capable of determimng the angular position of the reflector to within 2 units of the scale length of 300,000 units.
So that the capsule can be used with an ultra high vacuum system, it is, of course, necessary to connect it permanently to the vacuum system and bake it whenever the system is baked Unfortunately the manufacturer makes no prowsion for this operation to be carried out, and to overcome this problem it is necessary to make the modification shown in Figure 6. By boltIng an alumlnlum boss with an attached extension tube to the original mounting flange, it is possible to mount the capsule so that it can be permanently attached to the ultra high vacuum set. With the capsule mounted in place, it is comparatively simple to mount the readout instrument so that it can be raised into position and clamped to the capsule. This modification not only makes it possible to bake the capsule, but also provides protection to the capsule ports, as it incorporates a clamping device for the inter-connecting glass tubulatIon which can be seen in Figure 7. With this added safeguard, it is vtrtually impossible to break the capsule porting which is the most fragde part of this instrument. The overall repeatability of the instrument depends mainly on the repeatablhty of the capsule in use. However, it has been found that, when selected capsules have been used over long periods of t~me, even greater accuracy can be achieved than ~s claimed by the manufacturer provided several precautions are observed. In order to achieve the best accuracy, it is necessary to use the gauge at the pressure cahbration points. Although it is possible to interpolate between the calibration points, small variations m the calibration curve do occur, making it necessary to broaden the tolerance of the calibration between these points. These varmtions are thought to be due to anomalies in the
Figure 5. The double prH~ary pressure standard. 23
d Gascoigne : Precise pressure measurement in the range 0.1-500 torr
Figure 7. The clamp and modtfied porting used in the capsule modification.
Figure 6. The quartz capsule modification which allows the capsule to be baked with the vacuum system. capsule wall and to small variations m the gearing of the readout instrument. The main problem associated with this gauge is the fact that the Bourdon tube is very susceptible to vibration at low pressures, say below 5 torr. If this condition is present, the null signal can be substantially m error, resulting m incorrect pressure measurement. This becomes a problem at low pressures, particularly when setting zero, because there is virtually no damping of the Bourdon tube once the gas pressure has been reduced. If th~s condition is suspected, it can readily be determined by connecting an oscdloscope to the output terminal of the null detector. By doing this while the gauge is at null it will be found that the fundamental frequency of the Bourdon tube ts in the vicinity of 18-20 cycles, and if the peak to peak voltage exceeds a certain m i n i m u m value then the null point can be in error. Because there are variations in the performance of each instrument, it is necessary to determine the m a x i m u m permissible peak voltage for each capsule and readout instrument Once this has been determined it is a simple matter to check for Bourdon tube vibration, which can be a problem particularly when mechanical pumps are used on the vacuum system. Providing the quartz high precision pressure gauge is manually nulled, and the cahbration points used excluswely, it ~s possible to obtain very good repeatabihty over long periods of
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t~me. To be more specific, ff a selected 0-250 torr capsule is used in a readout gauge having a scale length of 300,000 umts, it is possable to retain an accuracy of 0.25 per cent at 1 torr, and 0.1 per cent at pressures greater than 4 torr, throughout several years of servace.
The capacitance manometer The second type of gauge to be d~scussed is the capacitance manometer. There are several manufacturers marketing th~s instrument, but only one or two produce heads which are bakeable and hence suitable for vacuum w o r k where m a x i m u m gas purity is essential. All appear to have basically the same operating principle, and consmt of a tightly stretched metal diaphragm separating the two sides of the null manometer. The position of the diaphragm ~s determined by measuring the capacity between it and electrodes mounted on both sides of the diaphragm, using an A C bridge technique In selecting this instrument, a close scrutiny was made of the specifications of all gauges on the market, and a capacitance m a n o m e t e r having a 0-3 torr, all welded, bakeable head was chosen. The head, which was specmlly selected for ~ts stabdtty, ~s normally operated at a temperature h~gher than ambient to ensure m a x i m u m stabd~ty The readout of th~s instrument ~s most convement as ~t reads d~rectly m pressure and only requires minor corrections to determine the actual pressure. The cla~med reproduc~bdlty and accuracy of th~s instrument ~s
d Gascoigne: Precise pressure measurement in the range 0.1-500 torr 0.066 per cent at 3 torr, 0.1 per cent at 1 torr, a n d 5 per cent at 10 -~ torr. T h e m a n u f a c t u r e r o f this i n s t r u m e n t provides n o t only a pressure c a l i b r a t i o n but also a n electrostatic calibration. T h e electrostatic c a l i b r a t i o n consists o f t h e a p p l i c a t i o n of a n electrostatic force to o n e side o f the d i a p h r a g m , using o n e o f the capacitance probes, a n d t h e m e a s u r e m e n t o f its deflection using the o t h e r probe. This p r o c e d u r e is intended to provide a crosscheck on the sensltwity of the i n s t r u m e n t w h e n e v e r a pressure s t a n d a r d is n o t available. O n checking the c a l i b r a t i o n o n receipt of the i n s t r u m e n t , it was f o u n d t h a t the electrostatic c a l i b r a t i o n was in e r r o r by 3 per cent, a n d the pressure c a l i b r a t i o n in e r r o r by 4 per cent at 2-1/2 torr, a n d 5 per cent at 3/4 torr. T h a t is, t h e pressure eahb r a t l o n was in e r r o r by forty times the claimed accuracy, a n d the two different c a h b r a t i o n techniques disagreed w i t h one a n o t h e r by at least ten times the claimed repeatability. It was clearly necessary to r e c a h b r a t e the i n s t r u m e n t before it could be p u t into service. D u r i n g the ensuing twenty m o n t h period, the c a l i b r a t i o n of the m a n o m e t e r r e m a i n e d generally consistent, a l t h o u g h there were occasions w h e n changes o f 0.3 per cent, or three times the claimed accuracy, o c c u r r e d over s h o r t periods of time Shifts in c a h b r a t l o n up to 0 9 per cent have also been observed w h e n the m a n o m e t e r has been m o v e d f r o m one v a c u u m system to a n o t h e r . In these circumstances it c a n take one or two weeks for the m a n o m e t e r to r e t u r n t o the original calibration. N o e x p l a n a t i o n has as yet been f o u n d for this behaviour. A n o t h e r source of e r r o r t h a t has b e c o m e a p p a r e n t while working with this particular i n s t r u m e n t is due to the fact t h a t the h e a d is very t e m p e r a t u r e sensitive w h e n e v e r a pressure differential is applied across the m a n o m e t e r . However, with zero differential pressure, n o t e m p e r a t u r e l n s t a b l h t y is observable.
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Figure $. A recorder trace showing the temperature sensltwity of the bakeable capacitance manometer head at a differential pressure a r d its stability at zero differential pressure.
This p o i n t is illustrated in Figure 8 which shows recorder traces t a k e n f r o m t h e o u t p u t o f the capacitance m o n o m e t e r . A t point A, where the trace begins, a pressure of 15.5 t o r r absolute is applied to b o t h sides of the null m a n o m e t e r , resulting in zero differential pressure across the gauge. T h e first part o f the trace was o b t a i n e d with virtually n o air circulation in the l a b o r a t o r y T h e alrconditioner, which n o r m a l l y m a i n t a i n s a m o d e r a t e air flow t h r o u g h the l a b o r a t o r y , was t u r n e d o n at point B. As can be seen, n o shift in the o u t p u t of the gauge is detectable. In the t o p half o f Figure 8, a second trace is s h o w n where the pressure in the positive side o f the null m a n o m e t e r was increased to 17.980 t o r r absolute, resulting in a differential pressure of 2.480 torr. In this case, when the air c o n d i t i o n e r was t u r n e d on, at p o i n t C, the very s u d d e n shift of the indicated pressure is quite a p p a r e n t , the gauge reading 8 × 10 -~ t o r r higher within 4 mln, t h a t is, the gauge r e a d o u t h a d shifted by 0.3 per cent, or three times the claimed repeatability, at the p o i n t where the recorder went off scale. Because of the slope of the recorder trace, it is obvious t h a t this was n o t t h e m a x i m u m deviation experienced u n d e r these conditions. This shift, a l t h o u g h n o t s h o w n in its entirety, c o n t i n u e s until the t e m p e r a t u r e t r a n s i e n t in the gauge head ceases t o exist. Once the gauge has reached the new stable temperature, t h e indicated pressure will have r e t u r n e d to the original reading. In o t h e r words, if this recording h a d been contlnued, the indicated pressure would have r e t u r n e d to the original value once t h e m a n o m e t e r h a d reached the new stable temperature. T h e c h a n g e in h e a d t e m p e r a t u r e which causes this c h a n g e in pressure r e a d o u t seems to be extremely small. Once the airc o n d i t i o n e r is switched o n it is some time before the incoming air is appreciably cooled even t h o u g h the air circulation is immediately started. In fact, a n i n d e p e n d e n t t h e r m o c o u p l e indicates t h a t the t e m p e r a t u r e c h a n g e o f the head during a c o m p l e t e a l r c o n d i t l o n e r cooling cycle is less t h a n 0.1°C In order t o o v e r c o m e this p r o b l e m it was f o u n d necessary to t h e r m a l l y insulate t h e null m a n o m e t e r gauge head. U n f o r t u n ately this m a k e s the h e a d r a t h e r b u l k y b u t it does solve the problem. However, the p o i n t which needs stressing m o s t regarding the t e m p e r a t u r e instability of this i n s t r u m e n t is t h e fact t h a t , usually, t h e only test t h a t the user can m a k e is t o test the zero stability o f the i n s t r u m e n t As has been shown, n o a b n o r m a l i t y is observable at zero differential pressure a n d the user will r e m a i n u n a w a r e o f t h e t e m p e r a t u r e instabihty of the i n s t r u m e n t at differential pressures, t h a t is, d u r i n g pressure m e a s u r e m e n t , unless a pressure s t a n d a r d h a v i n g sufficient sensitivity is available t o check this specific p r o b l e m . U n f o r t u n a t e l y , it seems t h a t a l t h o u g h t h e all-welded a n d b a k e a b l e m a n o m e t e r possesses a d e q u a t e sensitivity, o u r instrum e n t , w h i c h m a y or m a y n o t be typical, exhibits occasional shifts in c a l i b r a t i o n of 3-9 times t h e claimed reproducibility, even t h o u g h the head was specially selected for its stability. Because of this it is necessary to m a i n t a i n a continual check o n t h e c a l i b r a t i o n of this i n s t r u m e n t if the pressure r e a d o u t is to r e m a i n meaningful. It is relevant to a d d t h a t two non-bakeable heads, a 1 torr, a n d a 30 t o r r head, were briefly checked over a ten-day period, in the l a b o r a t o r y u n d e r exactly the same conditions, a n d n o t e m p e r a t u r e instability was evident in either of these instruments. This is possibly due to the slightly different design used in these heads in which the actual m a n o m e t e r is housed in a v a c u u m vessel. In this design t h e v a c u u m space s u r r o u n d i n g the 25
J Gascoigne: Precise pressure measurement in the range 0.1-500 torr sensing head probably provides the necessary thermal insulation to maintain the required stability within the manometer. Unfortunately this instrument is sealed using Vlton gaskets, and is therefore not suitable where maximum gas purity is reqmred. The calibration of the 30 torr head was within its claimed specification when compared with the primary pressure standard already described.
Conclusions To summarize this paper, by using a dead-weight primary pressure standard, it is possible to calibrate pressure gauges by direct comparison above 15.5 tort absolute to an accuracy of better than 0.05 per cent of the pressure. Furthermore, the technique of using a double primary pressure standard to calibrate null manometers has proved to be very successful. The accuracy achieved by this method is considered to be 0.05 per cent of the differential pressure or 5/, 10 -~ torr, whichever is greater The quartz high precision pressure gauge is capable of maretaming extremely good reproducibility over long periods of time and, prowdmg the necessary precautions are taken, an accuracy of 0.1 per cent is obtainable at pressures greater than 4 tort. On the other hand, although a non-bakeable capacitance
26
manometer has performed within its claimed specification, the all-welded and bakeable manometer marketed by the same manufacturer does not perform as specified. Because of the occasional shifts in calibration, and the temperature sensitivity at differential pressures, exhibited by this instrument it seems that more developmental work must be carried out on the design of the all-welded head before the present stated accuracy can be achieved. It is therefore necessary either to broaden considerably the specifications claimed for this instrument, or alternatively, maintain a continual check of the cahbratlon of this manometer using a pressure standard.
Acknowledgements The author is indebted to Drs R W Crompton and M T Elford, and to other members of this research group, for the help and discussion given throughout a number of years of developmental work which has provided the background for this paper, and also for their help and criticism given during the preparation of this paper. Thanks also go to Mr M A Whittington of this Unit not only for the overall layout design, construction and testing of the double primary pressure standard, but also for drafting the diagrams used in this paper.
It is becoming increasingly important in several fields of work to make precision measurements of the pressure of ultra pure gases in the range O.1-500 torr and above. To cater for this need several commercial gauges have been developed over the last decade. Two of these gauges, a quartz high precision pressure gauge, and a capacitance manometer are described, and their performance and limitations discussed. A "dead-weight" primary pressure standard, which has been used as a sub-standard for checking the absolute accuracy of the instruments, is also described. It will be shown that, although the quartz manometer lacks high resolution at pressures less than 4 torr, it is in general stable over long periods of time. By contrast, the capacitance manometer possesses extremely high resolution at low pressures, but appears to lack the long term stability expected of this instrument. Introduction The measurement of pressure in the range from 0.1 torr to atmospheric pressure with high accuracy is, perhaps, an unusual requirement. In this laboratory, the need for precision pressure measurement in this range stems from the requirement to measure electron and ion transport coefficients with high absolute accuracy, and even higher relative accuracy, as the first stage in determining absolute cross sections for low energy electrons and ions. In all scattering experiments, whether of the single collision (beam) type, or of the swarm type used in this laboratory, the accuracy of the absolute determination depends on the accuracy with which the number density of the scattering centres can be determined. Usually this determination rests on an absolute pressure measurement. In beam experiments the pressures used are of the order of 10 -3 torr or less, and the measurement of pressure in this range to within 1 or 2 per cent is a task that is stdl under investigation in standards laboratories. In swarm experiments, on the other hand, the pressures used are in the range under discussion; the measurement of these pressures with an accuracy of 0.1 per cent is a more realizable objective. However, an additional requirement with swarm experiments is that high gas purity must be maintained throughout the duration of the experiment, which is carried out under static conditions. These experiments are often sensitive to a few parts per million of impurity, in fact in some instances impurity levels as low as I part in 10 8 or 10 a are critical. The two gauges described in this paper not only have the required sensitivity and resolution for measuring the pressures under discussion, but are also constructed of materials which enable the required gas purity to be maintained. For reasons that will become apparent later in this paper, the selection of an instrument which appears to have the required specification does not guarantee that the required accuracy will be achieved, and it is therefore necessary to have access to equipment for checking the calibration of the gauges. The first part of this paper therefore describes the method that has been developed for this purpose.
The double primary pressure standard When the precise measurement of pressure became one of the
major problems associated with the experimental work being undertaken, the dead-weight type of primary pressure standard appeared to be the simplest and most accurate method of producing standard pressures for cahbration purposes. The principle of this technique is very simple. An accurately made piston cylinder combination and weight set is used to produce the desired pressure. As shown in Figure l, if the effec-
PI REFERENCE PRESSURE
PISTON P2 PRESSURE PRODUCED
PRESSUREPRODUCED= r PISTON WEIGHT ] + REFERENCE BY PISTON P2 [.EFFECTIVEPISTON AREAJ PRESSUREP1 Figure l. The "dead-weight" principle used in the primary pressure standard. tlve area and the weight of the piston are known then it is a simple matter to deduce that the pressure Pz supporting the piston is equal to the reference pressure P1 plus the piston weight dwlded by the effective piston area. If the pressure P1 is atmospheric, then the pressure P2 is referred to as a gauge pressure. If P~ is zero, then P2 will be an absolute pressure. The main problem in manufacturing the piston-cylinder combinations is to produce the components to the required degree of accuracy and surface finish. In theory no gas leakage should occur past the piston, and there should be no friction. In practice both these factors are kept to a minimum by careful manufacture There is, of course, some gas leakage, but this
Vacuum/volume 21/numbers 112. Pergamon Press LtdlPrmted in Great Britain
21
J Gascoigne : Precise pressure measurement in the range 0.1-500 torr
Figure 2. The piston of the primary pressure standard floating during a pressure cahbrat~on. helps reduce the friction between the piston a n d t h e cylinder by providing a gas bearing. Figure 2 shows t h e piston of the p r i m a r y pressure s t a n d a r d floating during a pressure calibration. W h e n the i n s t r u m e n t is being used, the piston is spun at a h u n d r e d or so r e v o l u t i o n s per m i n u t e so as t o ensure t h a t the p i s t o n is floating freely. T h e p i s t o n cylinder c o m b i n a t i o n s being used have been produced commercially a n d have a claimed overall accuracy of 0.015 per cent of t h e pressure produced. T h e accuracy of these c o m p o n e n t s is traceable t o t h e N a t i o n a l B u r e a u o f Standards, Washington. Due to the weight of the piston, the m i n i m u m pressure t h a t can be p r o d u c e d by this i n s t r u m e n t is 15.5 torr. As has already been stated the tolerance o n this pressure is 0.015 per cent or a little m o r e t h a n 2 ~, 10 -3 torr, a n d if a n absolute pressure is to be p r o d u c e d t h e n the reference pressure m u s t be m a i n t a i n e d below this figure, or the pressure k n o w n to this degree of accuracy. T h e m e t h o d of p r o d u c i n g pressures a b o v e 15.5 t o r r absolute is s h o w n In Figure 3. T h e reference v o l u m e is c o n t i n u a l l y
In p r a c h c e it has been f o u n d t h a t it is possible to m a i n t a i n a reference pressure of a p p r o x i m a t e l y 2 1< 10-3 t o r r when 15 5 t o r r is being produced, a n d it is therefore necessary when calculating t h e actual a b s o l u t e pressure p r o d u c e d by the primary pressure s t a n d a r d to m a k e corrections n o t only for t e m p e r a t u r e , and the v a r i a t i o n of the gravitational acceleration f r o m the s t a n d a r d value, b u t also for the reference pressure. By using t h B c a l i b r a t i o n technique, it s h o u l d be possible to check gauges between 15.5 t o r r a n d 7 6 0 t o r r to a n accuracy of 0.015 per cent of the pressure but, because of m i n o r problems t h a t become evident in practice, it is considered t h a t 0 05 per cent o f the pressure p r o d u c e d is a m o r e reasonable, but conservative, tolerance t h a t may be claimed T o establish pressures below 15.5 torr, the c a l i b r a t i o n of the gauges to be described in this p a p e r relies o n the fact t h a t these i n s t r u m e n t s c a n be used as null m a n o m e t e r s , t h a t is, t h e reference side of the gauge can be held at pressures o t h e r t h a n zero pressure. It is therefore feasible to calibrate these i n s t r u m e n t s by holding the reference side of the null m a n o m e t e r at, say, 15.5 t o r r absolute a n d p r o d u c i n g pressures in excess of this m t h e positive pressure side, resulting in a pressure differential across the m a n o m e t e r . So t h a t stability w~th time can be m a i n t a i n e d it is necessary to use two p r i m a r y pressure s t a n d a r d s , one to m a i n t a i n the 15.5 t o r r reference pressure, a n d one to p r o d u c e the 15.5 t o r r plus the pressure increment required. This a r r a n g e m e n t is s h o w n in Figure 4 a n d has proved to be a m o s t reliable t e c h n i q u e T h e lmtial procedure consists of lntro-
REFERENCE GAUGE
REFERENCE GAUGE
POSITIVE PRESSURE
REFERENCEV A E PRESSURE V ./1
STANDARD
STANDARD
~ . ~ GREFERENCE"
PRESSURE
REFERE~ PORT J
AUGE
VACUUM PUMP I
MANOMETERBEING CALIBRATED Figure 4. The method used to produce pressures of less than 15 5 torr I
I
CALIBRATING
PRESSURE Figure 3. Dmgram showing the method of producing pressures greater than 15.5 torr absolute. p u m p e d by a small 2-stage r o t a r y p u m p which has proved a d e q u a t e for this purpose. T h e p u m p i n g line incorporates a h q u t d m t r o g e n t r a p a n d t h e reference pressure is c o n t i n u a l l y m o n i t o r e d by a P i r a m gauge the p o r t of which is in the base of the bell j a r T h e zero of th~s gauge is checked f r o m time t o time by evacuating the bell j a r to a pressure of less t h a n 10 -4 t o r r using external pumping. 22
~SURE J PORT
dlfferentml. ducing 15.5 t o r r into b o t h sides of the null m a n o m e t e r a n d letting the whole a p p a r a t u s stabdlze. W h e n stability has been achieved, with b o t h pistons floating, the null of the i n s t r u m e n t is set at zero. This procedure nullifies any m i n o r v a r i a t i o n s m the two pressures p r o d u c e d by the two p r i m a r y pressure standards, and, because of this, t h e accuracy of the differential pressure subsequently p r o d u c e d does n o t d e p e n d o n the accuracy of these two pressures, providing they r e m a i n c o n s t a n t w i t h time. Once the zero has been determined, weights m a y be a d d e d to the positive pressure p r i m a r y pressure s t a n d a r d , thereby producing t h e desired dlfferentml pressure across t h e m a n o m e t e r . Because the m a n o m e t e r is set at zero when b o t h t h e p B t o n s of t h e p r i m a r y pressure s t a n d a r d s are floating, it ~s only necessary to look at the accuracy of the pressure increment a b o v e 15.5 t o r r to d e t e r m i n e the overall accuracy. This is constdered to be
d Gascolgne: Precise pressure measurement in the range 0.1-500 torr 0.05 per cent of the differential pressure, or 5 × 10 -4 torr, whichever is greater. By using this technique very consistent results have been readily obtained over a pressure range of 7.7 × l0 -~ torr to 15.5 torr, and extremely good cross checks have been obtained at 15.5 tort when the differential cahbration method has been compared with the absolute method.
The quartz high precision pressure gauge The first of the two manometers to be discussed is the quartz high precision pressure gauge. As the name implies, the pressure sensing element is of quartz, and it is the choice of this material which is the main reason for the high reproducibility and success of th~s instrument. The pressure sensing capsules used in this instrument are obtainable in numerous forms and pressure ranges, but this paper is confined to discussing the 0-250 torr and the 0-500 torr quartz capsules. These capsules comprise a quartz spiral Bourdon tube which is mounted on a fine quartz suspension and enclosed in a quartz envelope. The envelope has an aluminium mounting flange which positions it in the readout instrument. So that maximum stabdity can be achieved the capsule is maintained at a temperature higher than ambient using a proportionally controlled heater in the readout instrument. To enable the deflection of the Bourdon tube to be determined, a reflector is mounted on the lower end of the quartz spiral which rotates I00 ° for any pressure range. The position of the reflector is detected by an optical transducer null detector m the gauge readout unit which is capable of determimng the angular position of the reflector to within 2 units of the scale length of 300,000 units.
So that the capsule can be used with an ultra high vacuum system, it is, of course, necessary to connect it permanently to the vacuum system and bake it whenever the system is baked Unfortunately the manufacturer makes no prowsion for this operation to be carried out, and to overcome this problem it is necessary to make the modification shown in Figure 6. By boltIng an alumlnlum boss with an attached extension tube to the original mounting flange, it is possible to mount the capsule so that it can be permanently attached to the ultra high vacuum set. With the capsule mounted in place, it is comparatively simple to mount the readout instrument so that it can be raised into position and clamped to the capsule. This modification not only makes it possible to bake the capsule, but also provides protection to the capsule ports, as it incorporates a clamping device for the inter-connecting glass tubulatIon which can be seen in Figure 7. With this added safeguard, it is vtrtually impossible to break the capsule porting which is the most fragde part of this instrument. The overall repeatability of the instrument depends mainly on the repeatablhty of the capsule in use. However, it has been found that, when selected capsules have been used over long periods of t~me, even greater accuracy can be achieved than ~s claimed by the manufacturer provided several precautions are observed. In order to achieve the best accuracy, it is necessary to use the gauge at the pressure cahbration points. Although it is possible to interpolate between the calibration points, small variations m the calibration curve do occur, making it necessary to broaden the tolerance of the calibration between these points. These varmtions are thought to be due to anomalies in the
Figure 5. The double prH~ary pressure standard. 23
d Gascoigne : Precise pressure measurement in the range 0.1-500 torr
Figure 7. The clamp and modtfied porting used in the capsule modification.
Figure 6. The quartz capsule modification which allows the capsule to be baked with the vacuum system. capsule wall and to small variations m the gearing of the readout instrument. The main problem associated with this gauge is the fact that the Bourdon tube is very susceptible to vibration at low pressures, say below 5 torr. If this condition is present, the null signal can be substantially m error, resulting m incorrect pressure measurement. This becomes a problem at low pressures, particularly when setting zero, because there is virtually no damping of the Bourdon tube once the gas pressure has been reduced. If th~s condition is suspected, it can readily be determined by connecting an oscdloscope to the output terminal of the null detector. By doing this while the gauge is at null it will be found that the fundamental frequency of the Bourdon tube ts in the vicinity of 18-20 cycles, and if the peak to peak voltage exceeds a certain m i n i m u m value then the null point can be in error. Because there are variations in the performance of each instrument, it is necessary to determine the m a x i m u m permissible peak voltage for each capsule and readout instrument Once this has been determined it is a simple matter to check for Bourdon tube vibration, which can be a problem particularly when mechanical pumps are used on the vacuum system. Providing the quartz high precision pressure gauge is manually nulled, and the cahbration points used excluswely, it ~s possible to obtain very good repeatabihty over long periods of
24
t~me. To be more specific, ff a selected 0-250 torr capsule is used in a readout gauge having a scale length of 300,000 umts, it is possable to retain an accuracy of 0.25 per cent at 1 torr, and 0.1 per cent at pressures greater than 4 torr, throughout several years of servace.
The capacitance manometer The second type of gauge to be d~scussed is the capacitance manometer. There are several manufacturers marketing th~s instrument, but only one or two produce heads which are bakeable and hence suitable for vacuum w o r k where m a x i m u m gas purity is essential. All appear to have basically the same operating principle, and consmt of a tightly stretched metal diaphragm separating the two sides of the null manometer. The position of the diaphragm ~s determined by measuring the capacity between it and electrodes mounted on both sides of the diaphragm, using an A C bridge technique In selecting this instrument, a close scrutiny was made of the specifications of all gauges on the market, and a capacitance m a n o m e t e r having a 0-3 torr, all welded, bakeable head was chosen. The head, which was specmlly selected for ~ts stabdtty, ~s normally operated at a temperature h~gher than ambient to ensure m a x i m u m stabd~ty The readout of th~s instrument ~s most convement as ~t reads d~rectly m pressure and only requires minor corrections to determine the actual pressure. The cla~med reproduc~bdlty and accuracy of th~s instrument ~s
d Gascoigne: Precise pressure measurement in the range 0.1-500 torr 0.066 per cent at 3 torr, 0.1 per cent at 1 torr, a n d 5 per cent at 10 -~ torr. T h e m a n u f a c t u r e r o f this i n s t r u m e n t provides n o t only a pressure c a l i b r a t i o n but also a n electrostatic calibration. T h e electrostatic c a l i b r a t i o n consists o f t h e a p p l i c a t i o n of a n electrostatic force to o n e side o f the d i a p h r a g m , using o n e o f the capacitance probes, a n d t h e m e a s u r e m e n t o f its deflection using the o t h e r probe. This p r o c e d u r e is intended to provide a crosscheck on the sensltwity of the i n s t r u m e n t w h e n e v e r a pressure s t a n d a r d is n o t available. O n checking the c a l i b r a t i o n o n receipt of the i n s t r u m e n t , it was f o u n d t h a t the electrostatic c a l i b r a t i o n was in e r r o r by 3 per cent, a n d the pressure c a l i b r a t i o n in e r r o r by 4 per cent at 2-1/2 torr, a n d 5 per cent at 3/4 torr. T h a t is, t h e pressure eahb r a t l o n was in e r r o r by forty times the claimed accuracy, a n d the two different c a h b r a t i o n techniques disagreed w i t h one a n o t h e r by at least ten times the claimed repeatability. It was clearly necessary to r e c a h b r a t e the i n s t r u m e n t before it could be p u t into service. D u r i n g the ensuing twenty m o n t h period, the c a l i b r a t i o n of the m a n o m e t e r r e m a i n e d generally consistent, a l t h o u g h there were occasions w h e n changes o f 0.3 per cent, or three times the claimed accuracy, o c c u r r e d over s h o r t periods of time Shifts in c a h b r a t l o n up to 0 9 per cent have also been observed w h e n the m a n o m e t e r has been m o v e d f r o m one v a c u u m system to a n o t h e r . In these circumstances it c a n take one or two weeks for the m a n o m e t e r to r e t u r n t o the original calibration. N o e x p l a n a t i o n has as yet been f o u n d for this behaviour. A n o t h e r source of e r r o r t h a t has b e c o m e a p p a r e n t while working with this particular i n s t r u m e n t is due to the fact t h a t the h e a d is very t e m p e r a t u r e sensitive w h e n e v e r a pressure differential is applied across the m a n o m e t e r . However, with zero differential pressure, n o t e m p e r a t u r e l n s t a b l h t y is observable.
;
.
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Figure $. A recorder trace showing the temperature sensltwity of the bakeable capacitance manometer head at a differential pressure a r d its stability at zero differential pressure.
This p o i n t is illustrated in Figure 8 which shows recorder traces t a k e n f r o m t h e o u t p u t o f the capacitance m o n o m e t e r . A t point A, where the trace begins, a pressure of 15.5 t o r r absolute is applied to b o t h sides of the null m a n o m e t e r , resulting in zero differential pressure across the gauge. T h e first part o f the trace was o b t a i n e d with virtually n o air circulation in the l a b o r a t o r y T h e alrconditioner, which n o r m a l l y m a i n t a i n s a m o d e r a t e air flow t h r o u g h the l a b o r a t o r y , was t u r n e d o n at point B. As can be seen, n o shift in the o u t p u t of the gauge is detectable. In the t o p half o f Figure 8, a second trace is s h o w n where the pressure in the positive side o f the null m a n o m e t e r was increased to 17.980 t o r r absolute, resulting in a differential pressure of 2.480 torr. In this case, when the air c o n d i t i o n e r was t u r n e d on, at p o i n t C, the very s u d d e n shift of the indicated pressure is quite a p p a r e n t , the gauge reading 8 × 10 -~ t o r r higher within 4 mln, t h a t is, the gauge r e a d o u t h a d shifted by 0.3 per cent, or three times the claimed repeatability, at the p o i n t where the recorder went off scale. Because of the slope of the recorder trace, it is obvious t h a t this was n o t t h e m a x i m u m deviation experienced u n d e r these conditions. This shift, a l t h o u g h n o t s h o w n in its entirety, c o n t i n u e s until the t e m p e r a t u r e t r a n s i e n t in the gauge head ceases t o exist. Once the gauge has reached the new stable temperature, t h e indicated pressure will have r e t u r n e d to the original reading. In o t h e r words, if this recording h a d been contlnued, the indicated pressure would have r e t u r n e d to the original value once t h e m a n o m e t e r h a d reached the new stable temperature. T h e c h a n g e in h e a d t e m p e r a t u r e which causes this c h a n g e in pressure r e a d o u t seems to be extremely small. Once the airc o n d i t i o n e r is switched o n it is some time before the incoming air is appreciably cooled even t h o u g h the air circulation is immediately started. In fact, a n i n d e p e n d e n t t h e r m o c o u p l e indicates t h a t the t e m p e r a t u r e c h a n g e o f the head during a c o m p l e t e a l r c o n d i t l o n e r cooling cycle is less t h a n 0.1°C In order t o o v e r c o m e this p r o b l e m it was f o u n d necessary to t h e r m a l l y insulate t h e null m a n o m e t e r gauge head. U n f o r t u n ately this m a k e s the h e a d r a t h e r b u l k y b u t it does solve the problem. However, the p o i n t which needs stressing m o s t regarding the t e m p e r a t u r e instability of this i n s t r u m e n t is t h e fact t h a t , usually, t h e only test t h a t the user can m a k e is t o test the zero stability o f the i n s t r u m e n t As has been shown, n o a b n o r m a l i t y is observable at zero differential pressure a n d the user will r e m a i n u n a w a r e o f t h e t e m p e r a t u r e instabihty of the i n s t r u m e n t at differential pressures, t h a t is, d u r i n g pressure m e a s u r e m e n t , unless a pressure s t a n d a r d h a v i n g sufficient sensitivity is available t o check this specific p r o b l e m . U n f o r t u n a t e l y , it seems t h a t a l t h o u g h t h e all-welded a n d b a k e a b l e m a n o m e t e r possesses a d e q u a t e sensitivity, o u r instrum e n t , w h i c h m a y or m a y n o t be typical, exhibits occasional shifts in c a l i b r a t i o n of 3-9 times t h e claimed reproducibility, even t h o u g h the head was specially selected for its stability. Because of this it is necessary to m a i n t a i n a continual check o n t h e c a l i b r a t i o n of this i n s t r u m e n t if the pressure r e a d o u t is to r e m a i n meaningful. It is relevant to a d d t h a t two non-bakeable heads, a 1 torr, a n d a 30 t o r r head, were briefly checked over a ten-day period, in the l a b o r a t o r y u n d e r exactly the same conditions, a n d n o t e m p e r a t u r e instability was evident in either of these instruments. This is possibly due to the slightly different design used in these heads in which the actual m a n o m e t e r is housed in a v a c u u m vessel. In this design t h e v a c u u m space s u r r o u n d i n g the 25
J Gascoigne: Precise pressure measurement in the range 0.1-500 torr sensing head probably provides the necessary thermal insulation to maintain the required stability within the manometer. Unfortunately this instrument is sealed using Vlton gaskets, and is therefore not suitable where maximum gas purity is reqmred. The calibration of the 30 torr head was within its claimed specification when compared with the primary pressure standard already described.
Conclusions To summarize this paper, by using a dead-weight primary pressure standard, it is possible to calibrate pressure gauges by direct comparison above 15.5 tort absolute to an accuracy of better than 0.05 per cent of the pressure. Furthermore, the technique of using a double primary pressure standard to calibrate null manometers has proved to be very successful. The accuracy achieved by this method is considered to be 0.05 per cent of the differential pressure or 5/, 10 -~ torr, whichever is greater The quartz high precision pressure gauge is capable of maretaming extremely good reproducibility over long periods of time and, prowdmg the necessary precautions are taken, an accuracy of 0.1 per cent is obtainable at pressures greater than 4 tort. On the other hand, although a non-bakeable capacitance
26
manometer has performed within its claimed specification, the all-welded and bakeable manometer marketed by the same manufacturer does not perform as specified. Because of the occasional shifts in calibration, and the temperature sensitivity at differential pressures, exhibited by this instrument it seems that more developmental work must be carried out on the design of the all-welded head before the present stated accuracy can be achieved. It is therefore necessary either to broaden considerably the specifications claimed for this instrument, or alternatively, maintain a continual check of the cahbratlon of this manometer using a pressure standard.
Acknowledgements The author is indebted to Drs R W Crompton and M T Elford, and to other members of this research group, for the help and discussion given throughout a number of years of developmental work which has provided the background for this paper, and also for their help and criticism given during the preparation of this paper. Thanks also go to Mr M A Whittington of this Unit not only for the overall layout design, construction and testing of the double primary pressure standard, but also for drafting the diagrams used in this paper.