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Recording and analysis equipment for water demand in buildings
Build. Sci. ~¢ol.8, pp. 333-338. Pergamon Press 1973. Printed in Great Britain
II
I(E2j) 11
Recording and Analysis Equipment for Water Demand in Buildings E. M. McKAY*
Economic design of water supply systems in building.s' i,s o/?en handicap/wd by inadequate demand data. However bv makh?g full use ~?/ ach'ances it? data acquisition and analysLs" design value.s ./irm O' based on measurement are now practicabh'. ThLv paper &'scribes a monitoring system ach)pted to ,sttuh" coh/ water demand in university chemLvtr)' laboratories.
INTRODUCTION
the appropriate design criteria is given in a further paper (to be published).
M A N Y P R O B L E M S face the designer of complex water supply and distribution systems for buildings. In this paper we are concerned with those relating to variable demand and in particular with the relevant data required on which economic design may be based. Economic design in this context is based on probabilities of events (flowrates or volumes used over a given period). And as with probabilistic situations generally design criteria for water supply and distribution systems require very careful formulation if misunderstanding is not to result. This is quite apart from the step of actually choosing magnitudes for the various design probabilities. In order to deal with the problems arising from a variable demand we must therefore (1) establish our design criteria, (2) choose our design probabilities and (3) find by measurement and analysis the flowrates and volumes which satisfy (I) and (2). In what follows we will be concerned with design criteria and design values only insofar as they affect the demand data to be obtained. The instrumentation discussed in this paper relates to a study made of cold water demand in university chemistry laboratories, carried out for the Department of Education and Science. The results of this study will be published shortly. A measurements programme was adopted which entailed recording the time varying demand created by known populations of users. These records were then subjected to probability analyses both ot" flowrates and volumes in order to obtain the relevant design values. An account of these analysis techniques together with a discussion of
INSTRUMENTATION- -GENERAL REQUIREMENTS Before discussing the details of the instrumentation employed we will first list the essential characteristics required of the system. These were considered to be : (1) A capability for long term recording automatically controlled to monitor demand over selected parts of the day and requiring attention no more than once or twice a week. (2) Recorded data to be in a form which facilitate~ analysis by computer, and flowrate and volumetric demand together with real time information to be contained within a single record. This almost automatically require~ that the primary data be on magnetic or paper tape. (3) The cost per recording channel to be low and substantially independent of the number used per building, down to one only. (4) Facilities to provide for multiplexing flowmeter outputs to form one data channel and 1\~ channel data to be similarly combined in the analysis stages, so as to obtain records ol simultaneous demands before or after recording. (5) Chart records to be obtainable if required, to allow inspection of data before computer analysis of recordings. (6) The requirements set out in (2) may be met in a variety of ways. Two of the more practicable are (a) to obtain incremental volumes used over
*Building Research Establishment, Department of the Environment. 333
334
E. M. McKa)'
short intervals of time and (b) to sample at fixed frequency the instantaneous flowrate. Thus from (a) the incremental or cumulative volumes used over longer periods may be found simply by summation, and the instantaneous flowrate taken as that given approximately by the short-time average. F r o m (b) we may use the flowrate data directly and obtain the volumes by assuming the instantaneous flowrate to be the average over the sampling interval. As will be described later the first alternative was adopted here. THE F L O W M E T E R Having decided that flowrates as well as volumes were to be obtained and that these data were to be recorded on tape the choice of flowmeters was narrowed considerably. Cost was also of importance in view of the number of meters required. Those adopted were of the inferential type employing turbines for ½-in. and propellors for larger sizes as illustrated in figure 1. These meters have a reed
13.1
(b) Fig. 1. FIowmeters, showing different configurations (a) -} in. and (b) 1½ in.
switch output from which a pulse train is derived. Thus flowrate is obtained from the pulse repetition frequency (prf) and volumes b~, way of pulse counting. The meter has an accuracy which is nominally 1 per cent of the rated maxim'.nn flowrate and a flowrate range varying with size of 10:1--20:1 approximately. The nature of the signal derived from the flo~meter used has two important consequences liar this type of work. The first is that pulse trains are inherently more immune to interference than analogue signals. This has particular significance for measurements made in buildings with a nois> electrical environment and where the flowmeter may be some considerable distance from ihe recorder. Secondly signals in this tk~rm may readily be multiplexed to form a single data channeI: this point will be discussed in greater detail later.
LONG TERM R E C O R D I N G As stated earlier it was decided that short period incremental volumes would be found initially; this decision being taken largely on the basis ef equipment commercially available at the time. The recorder chosen for this purpose is one which in its basic form has been widely used to monitor electricity demands. Four tracks are provided [\)1 demand data and a fifth one is used exclusively as a real-time reference. Pulses only are recordable, in the non-return to zero (N RZ) mode. Using a sprocket drive and 35 mm magnetic film very low transport speeds are possible, However in view of the dependence of maximum bit rate on fihn speed the latter has been increased by a factor of 60 from the standard to give 1/80 in/sec. This for film lengths of 150 ft gives a run time of 40 hr, which resulted in an interval between film changes of a week for research and a fortnight for undergraduate laboratories; the former were monitored typically on 4 days a week for 10 hr a day and the latter simply for times of occupation. e.g. three periods of six hours per week. Translation of the films was carried out by the recorder manufacturers, the output from this stage being a paper tape record of pulse counts over 30 sec periods (known as the integrating period and fixed by the manufacturers). Further thought is being given to a more flexible approach to film translation providing for a variable integrating period, magnetic tape output and possibly some preprocessing of data before computer analysis. Tests conducted at lhe chosen film speed of 1/80 in/sec showed that pulse rates of up to 8 Hz may be recorded with negligible error. However pulse rates considerably in excess of this may be
335
Recording and Analysis Equipment f o r Water D e m a n d in Buildings
generated by the flowmeters used (up to 140 Hz fl~r the l in meter) and pulse rate division was therefore necessary before recording. A simple binary scaled divider has been used for this purpose. the scale factor being preset.
I
cases. In these the pulses which otherwise would have been recorded on a single track were shared in a sequential manner, between two or four tracks. The translator reversed the process by outputting the combined pulse count as being of a single data channel. In this way the maximum recordable pulse rate may be increased by a factor of two or four respectively. However an upper limit of 255 pulses per integrating period exists in the manufacturer's translator which sets an independent limit to the highest bit rate per channel which ma~ be recorded. This is in consequence of the format adopted for the paper tape output. It should be pointed out that the translation Mcility has been designed originally to deal with electricity demand data. A photograph of a recorder (main board) installation is given in figure 2 and a block schematic diagram in figure 3. The recording of various periods which may be of any chosen length i.,, controlled by one or more time switches bringing the recorder into operation with different start times and duration for each set of days, e.g. Monday Wednesday and Friday 0930--1300, Tuesday and Thursday 1400-1800. These clocks will provide up to three separate periods on each sel of days. Data relating to an individual recording period are identified by means of time markers on the track provided; on the modified recorder used these are put on at the beginning of each recorded period. The tirne base throughout the period is then regained in translation by counting the sprocket holes of the film. A relinement has been provided lo allow for the tape slack to be taken up after switch-on. This lakes the form of a dela\ unit of about 30s which controb, the opening of the data tracks.
L~'. 2. Recorder in.~/allalion (Main hoard).
~n additional facility afforded by the translator .rod known as track sharing was employed in some
Tlme
switches
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336
t£ M. M c K a )
The multiplexing facility for combining flowmeter outputs is illustrated in figure 4; it is essentially an ac coupled diode gate. To a large extent this overcomes the effect of near coincident pulses from different flowmeters which might otherwise result in losses since the duty cycle of these pulses trains is of the order of 0.15. With the circuit shown the only pulses lost are those which have near-coincident trailing edges. It should be emphasised that flowmeters may be combined in this way only when they generate the same nominal number of pulses, unit volume. 5',/
~[he instrumentation described is considered t,, make for a reasonable compromise between the requirement for fine detail with regard to the variation in flowrate and the nmount of data recorded. It also meets the cost requirement previously set out by virtue of the relatively small number of channels per recorder thus avoiding much wasted capacity where only a few flowmeter,, are installed in one building, In principle the major weakness of the recorder used is the length of film although a later version is now available which will take a film of 250 ft. SHORT TERM RECORDING AND ANALYSIS
_
F(g. 4. Multiplexer for combining flowmeter signals.
The composite pulse train is then shaped and inverted by a monostable before application to the divider chain. The effect of contact bounce from the flowmeter reed switch is also suppressed at this stage. The divider chain provides the facility for prf division by a factor of up to 16; higher values could have been used if required. Where track sharing is required three further bistables are employed as shown in figure 3(b). This has been done on a two track basis only but the principle may be extended to the use of all four data tracks for one channel. It will be noted that further division results from this operation. With regard to fall off in translation accuracy with decreasing pulse counts it may be shown that this is no worse than that for the flowmeter. Thus let n be the number of pulses recorded over a fixed time z when the average flowrate is q. Now the error in n, + 6n say, is given by 6n = N/IO0
As a supplement to the long term recording system described another based on two more conventional ½ in four track recorders has been used, which also accept pulses only. The ability of these short-term recorders to monitor instantaneous flowrates has enabled an assessment to be made of the 30 sec integrating period, by studying the variation within it. Also with the aid of a demand analyser it has been possible to obtain a direct indication of the probabilities of chosen flowrates within selected periods. Finally it has been possible to assess the possibility of using short term recordings only for design flowrate data. The signals for the short term recorders may be derived, using an appropriate interface, either from the main boards or directly from the flowmeters. Even at their slowest speed they will accept on one track without division the simultaneous outputs from a number of flowmeters wflh a limit set by the packing density of the tape and tape speed. Typically the maximum pulse rate at a speed of 1{- i n s e t will be in the range I-1.5 kHz. The interface unit referred to above is illustrated m figure 5. It comprises four monostabtes, three
..... E'
I ~-- ........
4-
°o~"-4---
-
where N corresponds to the rated maximum flowrate for the meter. This follows since the error in flowmeter pulse rate is + 1 per cent of its rated maximum. The relative error, ~, is given by 8
-~
6n n
--
N 100
1 l
Now the resolution of the recorded data is also proportional to 1In since it is simply plus or minus one count in n.
diP,
I]85 - B
Fig. 5. Multiplex interface.
Recording and Anah'sis Equipment for Water Demand in Building,s ~.ith single inputs and the fourth employing a five input diode gate of the type used on the main boards. It serves a number of purposes but basically it may be looked on as a switchable multiplexer with a facility for changing the pulse amplitude. The second feature enables short-term records to be obtained from the low voltage (5V) inputs to the main board, its multiplexing facility enables a variety of track channel combinations to be dealt wilh on recording and also on playback. The dernand analyser through which these recordings are played back has been described more fully elsewhere, see McKay [l]. Its purpose is to provide a direct indication on electromechanical registers of the fractions of time that
337
point flowrates to be found from one run only; a typical plot is shown in figure 7. Simultaneously with the probability analysis it is possible to obtain a chart record of flowrate vs time from a low impedance voltage output specially provided. This has proved a useful check facility especially in the early stages when there was little knowledge of the range of flowrates to be expected. CALIBRATION OF FLOWMETERS AND A DIRECT INDICATION OF FLOWRATES AND VOLUME
A final piece of equipment substantiall~ similar to the first part of the demand analyser (up to Enablmq
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Registers 1 - 8
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Ft]~,. 6. Demand analy.ver schematic.
,,zi~en flowrates have been exceeded. Illustrated in figure 6 the principle of the analyser may be briefly described as follows : Initially the pulses are conditioned for acceptance by the following pulse rate divider chain which for simplicity of operation is controlled by two switches. The first allows for various record/replay speed ratios and the second for various ranges of the original pulse rate. Following the dividers a conventional monostable is used with filtered output serving as a p r f - d . c , converter. The d.c. output which is proportional to prf and therefore a measure of flowrate then feeds a number of comparators in parallel each of which may be set to trigger at a different level. When any comparator has been switched it gates a pulse train of a known fixed frequency, ./i to its appropriate register. Then if the total gate-open time is t in an analysis lime of T we have the fraction of time P(q > q,.,,r)given by
P(q > q~r) =
d" 77./"
n =
N
Where/V is the maximum possible count registered. It will be remembered that the analysis time T will be a fraction of real time determined by the replay/record speed ratio. The eight registers provided have generally been sufficient to enable the required percentage ~)
005 t
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f
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998 t 999 99 9
9
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Flowrate ql, I.sec-1 FI~. 7. Prohahilit.v plol q/ instantaneou,s flo.'rale
1 6:
338
E . M . McKa.v
the prf--d.c, converter) has also been employed for onsite checking, and calibration of flowmeters. An additional facility provides for the counting of ftowmeter pulses either internally on an electromechanical register or externally using an electronic counter. Thus a given flowrate through a flowmeter may be set up approximately with the aid of the metered d.c. output using the nominal figure of pulses/unit volume. Then over a given period of time the generated pulses may be counted and simultaneously the volume passed is obtained by weighing. The d,c. output may also be used to drive a chart recorder when required. CONCLUDING REMARKS Whilst significant advances have been made in data logging and analysis it would be wrong to suggest that the task of monitoring water demand is now a simple one. All of the instrumentation described in this paper has either been specially designed or where bought in has required considerable modification. There is still much room for improvement but from the experience gained on this work the major requirement is considered to be an inexpensive flowmeter suitable for recording purposes and having good reliability, particularly when subjected to long periods of inactivity. Positive displacement meters with a reed switch output and an acceptably low pressure drop might well provide the answer if they were commercially available. If cost could be significantly reduced the electromagnetic flowmeter would also be attractive, but its signal would of course require d.c.---prf conversion if the recorders described in this paper were to be used. In general the long-term recording equipment has performed satisfactorily but much time has been wasted identifying the real time markers from those generated by mains borne interference. Minor modifications show that this could now be
overcome. The root of this and of other problems lies really in the use of a translator built for another purpose. A purpose-built translator is now being considered which would enable a different form of timing data to be employed and more significantly have a magnetic tape output thereby reducing read-in time to the computer. It could also carry out some of the simpler analysis procedures thereby reducing the volume of data fed to the computer. A technique which has not as yet been fully explored is that of directly simulating the coincident demand of larger populations of a given type or of composite populations. Using the short-term recordings this involves multiplexing the data of separate tracks on playback before feeding the combined signal to the demand analyser, When all tracks have been recorded simultaneously the common time base ensures that coincident demand is being analysed. It is admissable also to combine data obtained from the same user population but t\~r different days so long as the start-times of the recordings are known, A variety of possibilities exist for simulating demands of larger user groups depending on the number of original records obtained and the number of recorder/playback units available, Practical limitations are set by (1) the highest prf which the demand analyser will accept and (2) the packing density and tape speeds if re-recording is involved. Further work is being done on this kind of simulation (which applies equally to the 30 sec averaged flowrates). In this way it is hoped to test the theoretical relations between design flowrate and user population.
Acknowledgement--Thework described has been carried out as part of the research programme of the Building Research Establishment of the Department of the Environment and th is paper is published by permission of the Director,
REFERENCE E. M. McKAY, Pulse rate analyser for studying water demand in buildings, lnstrum Pract 749 (1970). La conception 6conomique de syst~mes d'alimentation d'eau dans les batiments est souvent ralentie par l'insuttisance de donn6es. Cependant, en faisant bon emploi des m6thodes avanc6es d'acquisition et d'analyse de donn6es, des valeurs de design fermement fond6es sur des mesures sont maintenant possibles. Ce texte d6crit un syst~me de surveillance adopts pour &udier les besoins en eau froide dans les laboratoires de chimie d'universit6s. Wirtschaftlicher Bau yon Wasserversorgungssystemen in GeNiuden wird oft durch unzureichende Angaben der Erfordernisse benachteiligt, Jetzt sind allerdings unter eingehender Benutzung der Fortschritte in Kenndatensammlung und Analyse Baueigenschaften f~r Umsetzung in die Praxis erh/iltlich, die sich streng auf Messung grtinden. Dieser Bericht beschreibt eine Oberwachungseinrichtung, welche der Untersuchung des Kaltwasserbedarfs in chemisehen Laboratorien yon UniversitiSten angepasst ist.
II
I(E2j) 11
Recording and Analysis Equipment for Water Demand in Buildings E. M. McKAY*
Economic design of water supply systems in building.s' i,s o/?en handicap/wd by inadequate demand data. However bv makh?g full use ~?/ ach'ances it? data acquisition and analysLs" design value.s ./irm O' based on measurement are now practicabh'. ThLv paper &'scribes a monitoring system ach)pted to ,sttuh" coh/ water demand in university chemLvtr)' laboratories.
INTRODUCTION
the appropriate design criteria is given in a further paper (to be published).
M A N Y P R O B L E M S face the designer of complex water supply and distribution systems for buildings. In this paper we are concerned with those relating to variable demand and in particular with the relevant data required on which economic design may be based. Economic design in this context is based on probabilities of events (flowrates or volumes used over a given period). And as with probabilistic situations generally design criteria for water supply and distribution systems require very careful formulation if misunderstanding is not to result. This is quite apart from the step of actually choosing magnitudes for the various design probabilities. In order to deal with the problems arising from a variable demand we must therefore (1) establish our design criteria, (2) choose our design probabilities and (3) find by measurement and analysis the flowrates and volumes which satisfy (I) and (2). In what follows we will be concerned with design criteria and design values only insofar as they affect the demand data to be obtained. The instrumentation discussed in this paper relates to a study made of cold water demand in university chemistry laboratories, carried out for the Department of Education and Science. The results of this study will be published shortly. A measurements programme was adopted which entailed recording the time varying demand created by known populations of users. These records were then subjected to probability analyses both ot" flowrates and volumes in order to obtain the relevant design values. An account of these analysis techniques together with a discussion of
INSTRUMENTATION- -GENERAL REQUIREMENTS Before discussing the details of the instrumentation employed we will first list the essential characteristics required of the system. These were considered to be : (1) A capability for long term recording automatically controlled to monitor demand over selected parts of the day and requiring attention no more than once or twice a week. (2) Recorded data to be in a form which facilitate~ analysis by computer, and flowrate and volumetric demand together with real time information to be contained within a single record. This almost automatically require~ that the primary data be on magnetic or paper tape. (3) The cost per recording channel to be low and substantially independent of the number used per building, down to one only. (4) Facilities to provide for multiplexing flowmeter outputs to form one data channel and 1\~ channel data to be similarly combined in the analysis stages, so as to obtain records ol simultaneous demands before or after recording. (5) Chart records to be obtainable if required, to allow inspection of data before computer analysis of recordings. (6) The requirements set out in (2) may be met in a variety of ways. Two of the more practicable are (a) to obtain incremental volumes used over
*Building Research Establishment, Department of the Environment. 333
334
E. M. McKa)'
short intervals of time and (b) to sample at fixed frequency the instantaneous flowrate. Thus from (a) the incremental or cumulative volumes used over longer periods may be found simply by summation, and the instantaneous flowrate taken as that given approximately by the short-time average. F r o m (b) we may use the flowrate data directly and obtain the volumes by assuming the instantaneous flowrate to be the average over the sampling interval. As will be described later the first alternative was adopted here. THE F L O W M E T E R Having decided that flowrates as well as volumes were to be obtained and that these data were to be recorded on tape the choice of flowmeters was narrowed considerably. Cost was also of importance in view of the number of meters required. Those adopted were of the inferential type employing turbines for ½-in. and propellors for larger sizes as illustrated in figure 1. These meters have a reed
13.1
(b) Fig. 1. FIowmeters, showing different configurations (a) -} in. and (b) 1½ in.
switch output from which a pulse train is derived. Thus flowrate is obtained from the pulse repetition frequency (prf) and volumes b~, way of pulse counting. The meter has an accuracy which is nominally 1 per cent of the rated maxim'.nn flowrate and a flowrate range varying with size of 10:1--20:1 approximately. The nature of the signal derived from the flo~meter used has two important consequences liar this type of work. The first is that pulse trains are inherently more immune to interference than analogue signals. This has particular significance for measurements made in buildings with a nois> electrical environment and where the flowmeter may be some considerable distance from ihe recorder. Secondly signals in this tk~rm may readily be multiplexed to form a single data channeI: this point will be discussed in greater detail later.
LONG TERM R E C O R D I N G As stated earlier it was decided that short period incremental volumes would be found initially; this decision being taken largely on the basis ef equipment commercially available at the time. The recorder chosen for this purpose is one which in its basic form has been widely used to monitor electricity demands. Four tracks are provided [\)1 demand data and a fifth one is used exclusively as a real-time reference. Pulses only are recordable, in the non-return to zero (N RZ) mode. Using a sprocket drive and 35 mm magnetic film very low transport speeds are possible, However in view of the dependence of maximum bit rate on fihn speed the latter has been increased by a factor of 60 from the standard to give 1/80 in/sec. This for film lengths of 150 ft gives a run time of 40 hr, which resulted in an interval between film changes of a week for research and a fortnight for undergraduate laboratories; the former were monitored typically on 4 days a week for 10 hr a day and the latter simply for times of occupation. e.g. three periods of six hours per week. Translation of the films was carried out by the recorder manufacturers, the output from this stage being a paper tape record of pulse counts over 30 sec periods (known as the integrating period and fixed by the manufacturers). Further thought is being given to a more flexible approach to film translation providing for a variable integrating period, magnetic tape output and possibly some preprocessing of data before computer analysis. Tests conducted at lhe chosen film speed of 1/80 in/sec showed that pulse rates of up to 8 Hz may be recorded with negligible error. However pulse rates considerably in excess of this may be
335
Recording and Analysis Equipment f o r Water D e m a n d in Buildings
generated by the flowmeters used (up to 140 Hz fl~r the l in meter) and pulse rate division was therefore necessary before recording. A simple binary scaled divider has been used for this purpose. the scale factor being preset.
I
cases. In these the pulses which otherwise would have been recorded on a single track were shared in a sequential manner, between two or four tracks. The translator reversed the process by outputting the combined pulse count as being of a single data channel. In this way the maximum recordable pulse rate may be increased by a factor of two or four respectively. However an upper limit of 255 pulses per integrating period exists in the manufacturer's translator which sets an independent limit to the highest bit rate per channel which ma~ be recorded. This is in consequence of the format adopted for the paper tape output. It should be pointed out that the translation Mcility has been designed originally to deal with electricity demand data. A photograph of a recorder (main board) installation is given in figure 2 and a block schematic diagram in figure 3. The recording of various periods which may be of any chosen length i.,, controlled by one or more time switches bringing the recorder into operation with different start times and duration for each set of days, e.g. Monday Wednesday and Friday 0930--1300, Tuesday and Thursday 1400-1800. These clocks will provide up to three separate periods on each sel of days. Data relating to an individual recording period are identified by means of time markers on the track provided; on the modified recorder used these are put on at the beginning of each recorded period. The tirne base throughout the period is then regained in translation by counting the sprocket holes of the film. A relinement has been provided lo allow for the tape slack to be taken up after switch-on. This lakes the form of a dela\ unit of about 30s which controb, the opening of the data tracks.
L~'. 2. Recorder in.~/allalion (Main hoard).
~n additional facility afforded by the translator .rod known as track sharing was employed in some
Tlme
switches
Powe[
I
'%
, ~ i5 v
I/~S;
r12 V
_
p,of . . . . . . % : : ~ g ' ~ " l
:
,'....
],,,>.~
;
:
Tlme
.,,c.,t
I
~
i
Ihou~
r "3'
{
!
...... 4
oMte~t; adle
l
/
Record heads B]I
I
r~
M u l h p l e x i Monoslable ! PLdSe rate dlvislorl
i
Track sharing
1
r
i
;
Othe, t ra(;ks
(bJ
1-7~,. 3. (a) Recorder .~1k~'1~'171schematic, (h) Sin~,le channel (with prori.ffon Jbr multiplexing and track .~hari1¢~,L
336
t£ M. M c K a )
The multiplexing facility for combining flowmeter outputs is illustrated in figure 4; it is essentially an ac coupled diode gate. To a large extent this overcomes the effect of near coincident pulses from different flowmeters which might otherwise result in losses since the duty cycle of these pulses trains is of the order of 0.15. With the circuit shown the only pulses lost are those which have near-coincident trailing edges. It should be emphasised that flowmeters may be combined in this way only when they generate the same nominal number of pulses, unit volume. 5',/
~[he instrumentation described is considered t,, make for a reasonable compromise between the requirement for fine detail with regard to the variation in flowrate and the nmount of data recorded. It also meets the cost requirement previously set out by virtue of the relatively small number of channels per recorder thus avoiding much wasted capacity where only a few flowmeter,, are installed in one building, In principle the major weakness of the recorder used is the length of film although a later version is now available which will take a film of 250 ft. SHORT TERM RECORDING AND ANALYSIS
_
F(g. 4. Multiplexer for combining flowmeter signals.
The composite pulse train is then shaped and inverted by a monostable before application to the divider chain. The effect of contact bounce from the flowmeter reed switch is also suppressed at this stage. The divider chain provides the facility for prf division by a factor of up to 16; higher values could have been used if required. Where track sharing is required three further bistables are employed as shown in figure 3(b). This has been done on a two track basis only but the principle may be extended to the use of all four data tracks for one channel. It will be noted that further division results from this operation. With regard to fall off in translation accuracy with decreasing pulse counts it may be shown that this is no worse than that for the flowmeter. Thus let n be the number of pulses recorded over a fixed time z when the average flowrate is q. Now the error in n, + 6n say, is given by 6n = N/IO0
As a supplement to the long term recording system described another based on two more conventional ½ in four track recorders has been used, which also accept pulses only. The ability of these short-term recorders to monitor instantaneous flowrates has enabled an assessment to be made of the 30 sec integrating period, by studying the variation within it. Also with the aid of a demand analyser it has been possible to obtain a direct indication of the probabilities of chosen flowrates within selected periods. Finally it has been possible to assess the possibility of using short term recordings only for design flowrate data. The signals for the short term recorders may be derived, using an appropriate interface, either from the main boards or directly from the flowmeters. Even at their slowest speed they will accept on one track without division the simultaneous outputs from a number of flowmeters wflh a limit set by the packing density of the tape and tape speed. Typically the maximum pulse rate at a speed of 1{- i n s e t will be in the range I-1.5 kHz. The interface unit referred to above is illustrated m figure 5. It comprises four monostabtes, three
..... E'
I ~-- ........
4-
°o~"-4---
-
where N corresponds to the rated maximum flowrate for the meter. This follows since the error in flowmeter pulse rate is + 1 per cent of its rated maximum. The relative error, ~, is given by 8
-~
6n n
--
N 100
1 l
Now the resolution of the recorded data is also proportional to 1In since it is simply plus or minus one count in n.
diP,
I]85 - B
Fig. 5. Multiplex interface.
Recording and Anah'sis Equipment for Water Demand in Building,s ~.ith single inputs and the fourth employing a five input diode gate of the type used on the main boards. It serves a number of purposes but basically it may be looked on as a switchable multiplexer with a facility for changing the pulse amplitude. The second feature enables short-term records to be obtained from the low voltage (5V) inputs to the main board, its multiplexing facility enables a variety of track channel combinations to be dealt wilh on recording and also on playback. The dernand analyser through which these recordings are played back has been described more fully elsewhere, see McKay [l]. Its purpose is to provide a direct indication on electromechanical registers of the fractions of time that
337
point flowrates to be found from one run only; a typical plot is shown in figure 7. Simultaneously with the probability analysis it is possible to obtain a chart record of flowrate vs time from a low impedance voltage output specially provided. This has proved a useful check facility especially in the early stages when there was little knowledge of the range of flowrates to be expected. CALIBRATION OF FLOWMETERS AND A DIRECT INDICATION OF FLOWRATES AND VOLUME
A final piece of equipment substantiall~ similar to the first part of the demand analyser (up to Enablmq
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,,zi~en flowrates have been exceeded. Illustrated in figure 6 the principle of the analyser may be briefly described as follows : Initially the pulses are conditioned for acceptance by the following pulse rate divider chain which for simplicity of operation is controlled by two switches. The first allows for various record/replay speed ratios and the second for various ranges of the original pulse rate. Following the dividers a conventional monostable is used with filtered output serving as a p r f - d . c , converter. The d.c. output which is proportional to prf and therefore a measure of flowrate then feeds a number of comparators in parallel each of which may be set to trigger at a different level. When any comparator has been switched it gates a pulse train of a known fixed frequency, ./i to its appropriate register. Then if the total gate-open time is t in an analysis lime of T we have the fraction of time P(q > q,.,,r)given by
P(q > q~r) =
d" 77./"
n =
N
Where/V is the maximum possible count registered. It will be remembered that the analysis time T will be a fraction of real time determined by the replay/record speed ratio. The eight registers provided have generally been sufficient to enable the required percentage ~)
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338
E . M . McKa.v
the prf--d.c, converter) has also been employed for onsite checking, and calibration of flowmeters. An additional facility provides for the counting of ftowmeter pulses either internally on an electromechanical register or externally using an electronic counter. Thus a given flowrate through a flowmeter may be set up approximately with the aid of the metered d.c. output using the nominal figure of pulses/unit volume. Then over a given period of time the generated pulses may be counted and simultaneously the volume passed is obtained by weighing. The d,c. output may also be used to drive a chart recorder when required. CONCLUDING REMARKS Whilst significant advances have been made in data logging and analysis it would be wrong to suggest that the task of monitoring water demand is now a simple one. All of the instrumentation described in this paper has either been specially designed or where bought in has required considerable modification. There is still much room for improvement but from the experience gained on this work the major requirement is considered to be an inexpensive flowmeter suitable for recording purposes and having good reliability, particularly when subjected to long periods of inactivity. Positive displacement meters with a reed switch output and an acceptably low pressure drop might well provide the answer if they were commercially available. If cost could be significantly reduced the electromagnetic flowmeter would also be attractive, but its signal would of course require d.c.---prf conversion if the recorders described in this paper were to be used. In general the long-term recording equipment has performed satisfactorily but much time has been wasted identifying the real time markers from those generated by mains borne interference. Minor modifications show that this could now be
overcome. The root of this and of other problems lies really in the use of a translator built for another purpose. A purpose-built translator is now being considered which would enable a different form of timing data to be employed and more significantly have a magnetic tape output thereby reducing read-in time to the computer. It could also carry out some of the simpler analysis procedures thereby reducing the volume of data fed to the computer. A technique which has not as yet been fully explored is that of directly simulating the coincident demand of larger populations of a given type or of composite populations. Using the short-term recordings this involves multiplexing the data of separate tracks on playback before feeding the combined signal to the demand analyser, When all tracks have been recorded simultaneously the common time base ensures that coincident demand is being analysed. It is admissable also to combine data obtained from the same user population but t\~r different days so long as the start-times of the recordings are known, A variety of possibilities exist for simulating demands of larger user groups depending on the number of original records obtained and the number of recorder/playback units available, Practical limitations are set by (1) the highest prf which the demand analyser will accept and (2) the packing density and tape speeds if re-recording is involved. Further work is being done on this kind of simulation (which applies equally to the 30 sec averaged flowrates). In this way it is hoped to test the theoretical relations between design flowrate and user population.
Acknowledgement--Thework described has been carried out as part of the research programme of the Building Research Establishment of the Department of the Environment and th is paper is published by permission of the Director,
REFERENCE E. M. McKAY, Pulse rate analyser for studying water demand in buildings, lnstrum Pract 749 (1970). La conception 6conomique de syst~mes d'alimentation d'eau dans les batiments est souvent ralentie par l'insuttisance de donn6es. Cependant, en faisant bon emploi des m6thodes avanc6es d'acquisition et d'analyse de donn6es, des valeurs de design fermement fond6es sur des mesures sont maintenant possibles. Ce texte d6crit un syst~me de surveillance adopts pour &udier les besoins en eau froide dans les laboratoires de chimie d'universit6s. Wirtschaftlicher Bau yon Wasserversorgungssystemen in GeNiuden wird oft durch unzureichende Angaben der Erfordernisse benachteiligt, Jetzt sind allerdings unter eingehender Benutzung der Fortschritte in Kenndatensammlung und Analyse Baueigenschaften f~r Umsetzung in die Praxis erh/iltlich, die sich streng auf Messung grtinden. Dieser Bericht beschreibt eine Oberwachungseinrichtung, welche der Untersuchung des Kaltwasserbedarfs in chemisehen Laboratorien yon UniversitiSten angepasst ist.