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Sensor retention times of air pollution monitoring equipment
The Science of the Total Environment, 4 (1975) 271-278 © Elsevier ScientificPublishing Company,Amsterdam- Printed in Belgium
SENSOR RETENTION TIMES OF AIR POLLUTION MONITORING EQUIPMENT
G. DOWD, R. S. THOMAS and J. L. MONKMAN Chemistry Division, Technology Development, Air Pollution Control Directorate, Environmental Protection Service, Ottawa (Canada)
(Received February 15th, 1975)
ABSTRACT In order that air-pollution monitoring equipment may be tied directly into data processing systems, these instruments must have sensors capable of fast retention times so that analysis is almost continuous 1. It is therefore important to minimize the retention time of continuous monitoring equipment. This paper describes the retention times associated with NO, NO2, 03 and mercury detectors. It further describes how this factor is increased by loading, integrating and signal processing. The paper concludes with suggested circuitry designed to realize the best detector performance2.
INTRODUCTION The electrical retention time of a total instrument or component may be described in terms o f a resistance capacity integrating or differentiation networka in which the capacitance is responsible for the charge retention. Therefore, this paper will detail all measurements of retention time in terms of R x C (see) which is the time it takes for step response to attain 63.2% of its final value. This is confirmed by the expression v = V ( 1 - e - t / ' 0 , which states that the time scale is entirely determined by RC. These data may be corrected to outline upper limit frequency by employing calculations for step response4. Many modern electronic devices, such as field-effect input operational amplifiers s can be employed in order to limit instrumental retention times to that of the primary detector and that of its immediate circuitry, providing that a 1014 fl load does not affect the output 6. Although dressing of detectors contributes added capacity to the system, most retention, associated with total instrumentation, is due to interfacing, signal processing, integration and readout of the detector signal. This is often done by instrumental designers in order to present a steady output signal that can be demonstrated with ease on a strip chart recorder, or read off on a digital meter. 271
NO, NO2, 03, SO 2 and mercury detectors are current examples where signal process integration is used extensively in order to smooth out signal variations. Such devices are wanting when used as continuous monitors, for, at best, they display a highly integrated ambient output and ignore, or de-emphasize, large transient excursions. Some advantage could be gained by concentrating design on well-regulated and defined detectors, recording the total output in an analog data acquisition system, then signal processing these data. In this way, curves for ambient levels, peak levels and other information, even including detector noise, could be evaluated. As such acquisition equipment has been commercially available for a number of years, its use, with or in place of strip chart recorders, would be advantageous. RETENTION TIMES Figure 1 is a block diagram showing the three main areas where electrical signal retention is encountered in air-pollution monitoring equipment, along with the type of retention found. The interfacing of these areas is accomplished by resistance loading, whose effect must be accounted for in the evaluation of the overall instrumental performance. The detector was analyzed first for optical lag, which is its response to a step level change in optical intensity; second for its initial capacity, which is the circuit response to an anode voltage change; and third, for the effect of adding capacity, by way of cabling, to the detector output. For these tests, the detector used was an RCA 925 phototube, similar or equivalent to sensors found in NO, NO2, 0 3 and mercury analyzers. • 13V,~ I
I,
I
I I
UPAUITY~ OPIIUAL-- q
I : _be, ~
i
II~'
i
I
Fig. 1. Areas of signal retention.
Fig. 2. Instrumental set-up. The instrumental set-up used for this analysis is shown in Fig. 2. For this test, an incandescent lamp was used and intensity was controlled to obtain 1-5 #A of phototube current. Optical lag was measured by simulating an increase of 1% in the signal output. This was done by switching in a resistor across the anode-cathode junction. The initial capacity was measured by switching the anode voltage from 272
i
80 down to 7 V. A provision for adding capacity (cx) to the primary detector load enabled measurement of capacity effects. The amplifier used was a FET input 741 employed in the follower configuration, which exhibited an input impedance of better than 1013 i2, and had a rise time of less than 5 #sec. The output of the device was coupled into a storage oscilloscope. Measurements of the optical lag are shown in Fig. 3. Two curves are illustrated the first (solid lines) shows the circuit time constants associated with anode currents of 1-5 #A. Measurements were not made below 1/zA as there is appreciable loss of both level and stability. This effect is demonstrated by the second curve (dotted line) which shows a stable gain to 1/~A. Accompanying these data is a waveform obtained at the output of the detector. This form is clearly integrating.
g
~ o.I I
I/'/
b04
/
E
:
,ca WAVE FORM
_J
/
,02
o.~
_l
S - -
.o, ~
-E ~ [
ANODE CURRENT (pa)
'
;
:
I
ANODE CURRENT (pa)
Fig. 3. Optical retention. Fig. 4. Initial capacity.
Figure 4 is a graph of measurements made on the initial capacity by way of a step in anode voltage. The accompanying waveform was taken from the output of the detector. This form can be classified as differentiating having a measurable time constant. It will be noted that the R C values are slightly less than those determined in Fig. 3. When the primary detector is connected by a shielded cable to its amplifier, a capacity is introduced into the system that results in further integration. Figure 5 is a graph detailing this effect. Assuming that the cable has a nominal capacity of 35 pF/ft., measurements were made on simulated values to 100 ft. At the 100-ft. level, the RC time constant was 3 m sec or 30 times that of the detector level. The second area of retention as outlined in Fig. 1 is due to signal processing. For the majority of cases, this results in some form of integration. Three types are investigated in this paper. The first is time-lapse integration. This is a type wherein all transients are lost as to position and level. The total time signal is integrated and presented as an output with or without hold characteristics. Figure 6 demonstrates the result of this type of integration by plotting the transient-ambient factor against its 273
1o
AMBIENT= I OUT PUT = |
J i
_l
m
i
3 I so CABLE
&
1010 LENQTH
(feet)
ell
,'o INTECRATION
TIME
Fig. 5. Cabling retention. Fig. 6. Transient level loss (constant output).
own operating fraction of the integrating time. The curves were obtained on allowing excursions of level from an ambient 1 to a transient level of 11 (ambient+ 10 x 1). In all cases, the output is maintained at 2. This dearly shows that a transient of 10 times the ambient, that operates for 1/10 of the integrating time, will appear in the output at the same level as a transient of 1 times ambient (level 2) operating for the total integrating time. Figure 7 further illustrates transient loss by showing the resultant output of a 10-times ambient transient, having a level value of 11, that operates for various fractions of the integrating time. This graph shows a varying output for a constant level input that depends on the transient operating time. This results in level time integration whereby the output shows level 11 for a 10-times ambient signal operating for T duration; and level 2 for a 10-times ambient signal operating for 2"/10 duration.
'0
E Sill.
,~ ele
m'e I NTECBATION
I; TIME
Fig. 7. Transient level loss (constant level). Fig. 8. No-loss integrators.
274
,h
L
From Figs. 6 and 7, it may be seen that any analysis of data within time lapse integration is uncertain. This system may be classified as a loss type integration. The use of matched field-effect transistors, in conjunction with integrated circuit operational amplifiers, makes two other types of integration feasible. Figures 8 a and b are drawings of these devices. They are of the no-loss variety of integration, whereby the time and level of transients are retained along with a demonstration of ambient level. Both these circuits are stable and reliable; however, for long-term analysis, type " a " has the advantage that it will need little or no offset adjustment. Figure 8a is a standard operational amplifier follower circuit of the FET input variety. The output essentially follows the input voltage with practically no measurable signal loss. This circuit is embellished by a capacitor, charged through a 10a f2 resistor. The output level of this circuit is not lost in the amplifier, and is equal to the input at an integrating time constant of 108-C see. This device is independent of errors common in long time integrating circuits where the integration process causes a signal level loss that must be recovered through successive amplification. The response of this circuit to transients is demonstrated in Fig. 9. These curves were obtained by connecting a storage scope to the output of the operational amplifier. The bottom tracing shows an ambient level of 1. The three upper levels show the excursion of the signal due to transients of 3, 5 and 6.8 times the ambient, operating for 0.2 T (T being 108 C). The rise times are sharp due to the di/dt of the charging circuit. From these curves, we may reconstruct either electronically from voltages, or manually from graphs, the position and level of the transients. Figure 10
I
I
!
I
/
A
1111111NT LEIFEL
TINE~mmm.-
TIME
3D
V2 : TIME
I
'/1
1- • t/|O
Fig. 9. Transients in follower circuit. Fig. 10. Reconstruction of transients from follower output.
demonstrates this technique. Positions A and B define the start and stop of the transient. Knowing vl, the magnitude of 1/2 may be calculated 7. The tracing returning to the ambient level may be processed in the same manner. The calculated results appear on the right of the figure showing a step response for the time duration AB. The resolution available is limited by the frequency response of the recording device. Figure 8b is another example of no-loss integration requiring little satellite circuitry. Setting the killer switch (S) for an integral T, results in an output wave as 275
seen in Fig. 1la. When this wave is differentiated, the output appears as in Fig. 1 lb. Both these tracings were obtained for a level 1 ambient. Here the slope of the w a v e is proportional to the signal level. The output of the operational amplifier is o f low enough resistance to make the value of " b " a true derivative.
It
I'---T --4
i d_V
i i
J
J
Y V I
dt
TIME
i T I M E
v
Fig. 11. Slope integration. Fig. 12. Transient slope integration.
Figure 12 is a copy of tracings from an active circuit in which an a m b i e n t level of I was subjected to transient-ambient factors of 3 and 6-8. The bottom line is f r o m a previous ambient tracing. The differentiated value, obtained from this tracing, is shown in Fig. 13. Again, the resolution obtainable in this type of analysis is d e p e n d e n t on the frequency characteristics of the recorded signal. However, excursions o f up to ten times ambient are readily reconstructed.
Ux
|lli|B
TIME
[
v
Fig. 13. Reconstruction of transients from slope integration.
The third area of retention (Fig. 1) lies in the recording signal system. T h i s is a factor of the recording method used, the initial response being that of the r e c o r d e r itself. 276
The advantages of high input and low source impedance is outlined by Donaldson s. Let us refer to the low pass filter turning over at 1
\R + r/ where r is the source impedance, R is the recording impedance or load and C is the shunt capacity. Wc is high if r, R and C are small. As r is fixed, and R must be greater than r to ensure no signal loss, C must be as small as possible. A FET-input operational amplifier in the follower mode will solve this problem. In a practical sense, r of 3 K and r Y 1 megohm result in a signal loss of about 0.3%. Now, if excessive C exists, and R is blocked, then the value of RC represents a sizable loading time constant. Some instruments have large capacities across R, placed there to smooth out instrumental fluctuations. These capacities are very often of the electrolytic type and they can produce errors due to variable ohmic degrading of R. In addition to this, they can exhibit voltage levels on their own that will confuse low-level outputs. Figure 14 is a graph o f residual voltage levels found in a 1000 mF electrolytic condenser
v
lm I
=
i
o.,
I
o.,
I
o.e
o'.8
~o
R ( me|ohms )
Fig. 14. Residual voltaic levels.
subjected to various loads. All these levels are time and temperature varying. It will be noted that the residual voltaic level is large enough to cause destructive interference in recorder outputs o f most air-pollution monitoring equipment. Figure 15 is a block diagram of a data acquisition system for air-pollution monitoring equipment. It is optimized for low signal retention. The primary detector feeds directly into a field-effect operational amplifier in the follower mode. This transmits the signal to low impedance without destructive attenuation, loading or integration. The o u t p u t of this device feeds into two sub-systems. The first is an ambient recorder preceded by an adjustable integrator. The integrator may be time varied in order to produce a noise-free curve on the output. The second is a data-logging 277
FM tape recorder. Recorded tapes may be analyzed at a later date in a frequency spectrum analyzer to determine transients, ambients and electrical noise. The two subsystems do not interfere with each other in any way as the R value can be made l07 times the MSF 741 output impedance.
F.M. tNtt0a lilHOil
Fig. 15. Low signal-retention system. CONCLUSION
This paper reports on analyses of retention times associated with photometric sensors used in air-pollution monitoring equipment. Values of 0.1-0.2 msec, or 0.06-0.18 msec are reported for load resistors of 1 Mr2 and anode currents of 1.0-5.0/IA, depending on whether the step response is integrating or differentiating. Cabling capacity or by-pass condensers are shown to increase the retention times to 30 times the sensor level. The paper analyzes three types of signal process integration, two of them using field-effect input operational amplifiers. The effect of residual voltages on large electrolytic condensers is measured in relation to loading. A circuit component layout for optimizing upper frequency response and minimizing retention time completes the presentation. REFERENCES 1 J. Segraves, Report on ISA, Instrument Technology, December 1967, p. 30. 2 P. K. Stein, Concept of a System Operating Surface, Symposium on Advances in Instruments and Measurements, Joint Session of lEEE and ASTM, June 1-3, 1971, Ottawa. 3 F. J. M. Farley, Elements of Pulse Circuits, Methuen, London, 1958, Ch. 1. 4 G. F. Dowd and M. Crevier, Can. J. Biochem. Physiol., 38 (1960) 992. 5 Mini-Systems Inc., Specifications of FET.Input Operational Amplifier Type MSF-741. 6 H. Ashworth, J. Can. Elect. Eng., 16 (1972) 33. 7 Sears-Zemansky, University Physics, Addison-Wesley, 1971, 4th ed., Ch. 29, No. 7. 8 J. K. Donaldson, Electrical Apparatusfor Biological Research, Butterworths, London, 1958, Ch. 11.
278
SENSOR RETENTION TIMES OF AIR POLLUTION MONITORING EQUIPMENT
G. DOWD, R. S. THOMAS and J. L. MONKMAN Chemistry Division, Technology Development, Air Pollution Control Directorate, Environmental Protection Service, Ottawa (Canada)
(Received February 15th, 1975)
ABSTRACT In order that air-pollution monitoring equipment may be tied directly into data processing systems, these instruments must have sensors capable of fast retention times so that analysis is almost continuous 1. It is therefore important to minimize the retention time of continuous monitoring equipment. This paper describes the retention times associated with NO, NO2, 03 and mercury detectors. It further describes how this factor is increased by loading, integrating and signal processing. The paper concludes with suggested circuitry designed to realize the best detector performance2.
INTRODUCTION The electrical retention time of a total instrument or component may be described in terms o f a resistance capacity integrating or differentiation networka in which the capacitance is responsible for the charge retention. Therefore, this paper will detail all measurements of retention time in terms of R x C (see) which is the time it takes for step response to attain 63.2% of its final value. This is confirmed by the expression v = V ( 1 - e - t / ' 0 , which states that the time scale is entirely determined by RC. These data may be corrected to outline upper limit frequency by employing calculations for step response4. Many modern electronic devices, such as field-effect input operational amplifiers s can be employed in order to limit instrumental retention times to that of the primary detector and that of its immediate circuitry, providing that a 1014 fl load does not affect the output 6. Although dressing of detectors contributes added capacity to the system, most retention, associated with total instrumentation, is due to interfacing, signal processing, integration and readout of the detector signal. This is often done by instrumental designers in order to present a steady output signal that can be demonstrated with ease on a strip chart recorder, or read off on a digital meter. 271
NO, NO2, 03, SO 2 and mercury detectors are current examples where signal process integration is used extensively in order to smooth out signal variations. Such devices are wanting when used as continuous monitors, for, at best, they display a highly integrated ambient output and ignore, or de-emphasize, large transient excursions. Some advantage could be gained by concentrating design on well-regulated and defined detectors, recording the total output in an analog data acquisition system, then signal processing these data. In this way, curves for ambient levels, peak levels and other information, even including detector noise, could be evaluated. As such acquisition equipment has been commercially available for a number of years, its use, with or in place of strip chart recorders, would be advantageous. RETENTION TIMES Figure 1 is a block diagram showing the three main areas where electrical signal retention is encountered in air-pollution monitoring equipment, along with the type of retention found. The interfacing of these areas is accomplished by resistance loading, whose effect must be accounted for in the evaluation of the overall instrumental performance. The detector was analyzed first for optical lag, which is its response to a step level change in optical intensity; second for its initial capacity, which is the circuit response to an anode voltage change; and third, for the effect of adding capacity, by way of cabling, to the detector output. For these tests, the detector used was an RCA 925 phototube, similar or equivalent to sensors found in NO, NO2, 0 3 and mercury analyzers. • 13V,~ I
I,
I
I I
UPAUITY~ OPIIUAL-- q
I : _be, ~
i
II~'
i
I
Fig. 1. Areas of signal retention.
Fig. 2. Instrumental set-up. The instrumental set-up used for this analysis is shown in Fig. 2. For this test, an incandescent lamp was used and intensity was controlled to obtain 1-5 #A of phototube current. Optical lag was measured by simulating an increase of 1% in the signal output. This was done by switching in a resistor across the anode-cathode junction. The initial capacity was measured by switching the anode voltage from 272
i
80 down to 7 V. A provision for adding capacity (cx) to the primary detector load enabled measurement of capacity effects. The amplifier used was a FET input 741 employed in the follower configuration, which exhibited an input impedance of better than 1013 i2, and had a rise time of less than 5 #sec. The output of the device was coupled into a storage oscilloscope. Measurements of the optical lag are shown in Fig. 3. Two curves are illustrated the first (solid lines) shows the circuit time constants associated with anode currents of 1-5 #A. Measurements were not made below 1/zA as there is appreciable loss of both level and stability. This effect is demonstrated by the second curve (dotted line) which shows a stable gain to 1/~A. Accompanying these data is a waveform obtained at the output of the detector. This form is clearly integrating.
g
~ o.I I
I/'/
b04
/
E
:
,ca WAVE FORM
_J
/
,02
o.~
_l
S - -
.o, ~
-E ~ [
ANODE CURRENT (pa)
'
;
:
I
ANODE CURRENT (pa)
Fig. 3. Optical retention. Fig. 4. Initial capacity.
Figure 4 is a graph of measurements made on the initial capacity by way of a step in anode voltage. The accompanying waveform was taken from the output of the detector. This form can be classified as differentiating having a measurable time constant. It will be noted that the R C values are slightly less than those determined in Fig. 3. When the primary detector is connected by a shielded cable to its amplifier, a capacity is introduced into the system that results in further integration. Figure 5 is a graph detailing this effect. Assuming that the cable has a nominal capacity of 35 pF/ft., measurements were made on simulated values to 100 ft. At the 100-ft. level, the RC time constant was 3 m sec or 30 times that of the detector level. The second area of retention as outlined in Fig. 1 is due to signal processing. For the majority of cases, this results in some form of integration. Three types are investigated in this paper. The first is time-lapse integration. This is a type wherein all transients are lost as to position and level. The total time signal is integrated and presented as an output with or without hold characteristics. Figure 6 demonstrates the result of this type of integration by plotting the transient-ambient factor against its 273
1o
AMBIENT= I OUT PUT = |
J i
_l
m
i
3 I so CABLE
&
1010 LENQTH
(feet)
ell
,'o INTECRATION
TIME
Fig. 5. Cabling retention. Fig. 6. Transient level loss (constant output).
own operating fraction of the integrating time. The curves were obtained on allowing excursions of level from an ambient 1 to a transient level of 11 (ambient+ 10 x 1). In all cases, the output is maintained at 2. This dearly shows that a transient of 10 times the ambient, that operates for 1/10 of the integrating time, will appear in the output at the same level as a transient of 1 times ambient (level 2) operating for the total integrating time. Figure 7 further illustrates transient loss by showing the resultant output of a 10-times ambient transient, having a level value of 11, that operates for various fractions of the integrating time. This graph shows a varying output for a constant level input that depends on the transient operating time. This results in level time integration whereby the output shows level 11 for a 10-times ambient signal operating for T duration; and level 2 for a 10-times ambient signal operating for 2"/10 duration.
'0
E Sill.
,~ ele
m'e I NTECBATION
I; TIME
Fig. 7. Transient level loss (constant level). Fig. 8. No-loss integrators.
274
,h
L
From Figs. 6 and 7, it may be seen that any analysis of data within time lapse integration is uncertain. This system may be classified as a loss type integration. The use of matched field-effect transistors, in conjunction with integrated circuit operational amplifiers, makes two other types of integration feasible. Figures 8 a and b are drawings of these devices. They are of the no-loss variety of integration, whereby the time and level of transients are retained along with a demonstration of ambient level. Both these circuits are stable and reliable; however, for long-term analysis, type " a " has the advantage that it will need little or no offset adjustment. Figure 8a is a standard operational amplifier follower circuit of the FET input variety. The output essentially follows the input voltage with practically no measurable signal loss. This circuit is embellished by a capacitor, charged through a 10a f2 resistor. The output level of this circuit is not lost in the amplifier, and is equal to the input at an integrating time constant of 108-C see. This device is independent of errors common in long time integrating circuits where the integration process causes a signal level loss that must be recovered through successive amplification. The response of this circuit to transients is demonstrated in Fig. 9. These curves were obtained by connecting a storage scope to the output of the operational amplifier. The bottom tracing shows an ambient level of 1. The three upper levels show the excursion of the signal due to transients of 3, 5 and 6.8 times the ambient, operating for 0.2 T (T being 108 C). The rise times are sharp due to the di/dt of the charging circuit. From these curves, we may reconstruct either electronically from voltages, or manually from graphs, the position and level of the transients. Figure 10
I
I
!
I
/
A
1111111NT LEIFEL
TINE~mmm.-
TIME
3D
V2 : TIME
I
'/1
1- • t/|O
Fig. 9. Transients in follower circuit. Fig. 10. Reconstruction of transients from follower output.
demonstrates this technique. Positions A and B define the start and stop of the transient. Knowing vl, the magnitude of 1/2 may be calculated 7. The tracing returning to the ambient level may be processed in the same manner. The calculated results appear on the right of the figure showing a step response for the time duration AB. The resolution available is limited by the frequency response of the recording device. Figure 8b is another example of no-loss integration requiring little satellite circuitry. Setting the killer switch (S) for an integral T, results in an output wave as 275
seen in Fig. 1la. When this wave is differentiated, the output appears as in Fig. 1 lb. Both these tracings were obtained for a level 1 ambient. Here the slope of the w a v e is proportional to the signal level. The output of the operational amplifier is o f low enough resistance to make the value of " b " a true derivative.
It
I'---T --4
i d_V
i i
J
J
Y V I
dt
TIME
i T I M E
v
Fig. 11. Slope integration. Fig. 12. Transient slope integration.
Figure 12 is a copy of tracings from an active circuit in which an a m b i e n t level of I was subjected to transient-ambient factors of 3 and 6-8. The bottom line is f r o m a previous ambient tracing. The differentiated value, obtained from this tracing, is shown in Fig. 13. Again, the resolution obtainable in this type of analysis is d e p e n d e n t on the frequency characteristics of the recorded signal. However, excursions o f up to ten times ambient are readily reconstructed.
Ux
|lli|B
TIME
[
v
Fig. 13. Reconstruction of transients from slope integration.
The third area of retention (Fig. 1) lies in the recording signal system. T h i s is a factor of the recording method used, the initial response being that of the r e c o r d e r itself. 276
The advantages of high input and low source impedance is outlined by Donaldson s. Let us refer to the low pass filter turning over at 1
\R + r/ where r is the source impedance, R is the recording impedance or load and C is the shunt capacity. Wc is high if r, R and C are small. As r is fixed, and R must be greater than r to ensure no signal loss, C must be as small as possible. A FET-input operational amplifier in the follower mode will solve this problem. In a practical sense, r of 3 K and r Y 1 megohm result in a signal loss of about 0.3%. Now, if excessive C exists, and R is blocked, then the value of RC represents a sizable loading time constant. Some instruments have large capacities across R, placed there to smooth out instrumental fluctuations. These capacities are very often of the electrolytic type and they can produce errors due to variable ohmic degrading of R. In addition to this, they can exhibit voltage levels on their own that will confuse low-level outputs. Figure 14 is a graph o f residual voltage levels found in a 1000 mF electrolytic condenser
v
lm I
=
i
o.,
I
o.,
I
o.e
o'.8
~o
R ( me|ohms )
Fig. 14. Residual voltaic levels.
subjected to various loads. All these levels are time and temperature varying. It will be noted that the residual voltaic level is large enough to cause destructive interference in recorder outputs o f most air-pollution monitoring equipment. Figure 15 is a block diagram of a data acquisition system for air-pollution monitoring equipment. It is optimized for low signal retention. The primary detector feeds directly into a field-effect operational amplifier in the follower mode. This transmits the signal to low impedance without destructive attenuation, loading or integration. The o u t p u t of this device feeds into two sub-systems. The first is an ambient recorder preceded by an adjustable integrator. The integrator may be time varied in order to produce a noise-free curve on the output. The second is a data-logging 277
FM tape recorder. Recorded tapes may be analyzed at a later date in a frequency spectrum analyzer to determine transients, ambients and electrical noise. The two subsystems do not interfere with each other in any way as the R value can be made l07 times the MSF 741 output impedance.
F.M. tNtt0a lilHOil
Fig. 15. Low signal-retention system. CONCLUSION
This paper reports on analyses of retention times associated with photometric sensors used in air-pollution monitoring equipment. Values of 0.1-0.2 msec, or 0.06-0.18 msec are reported for load resistors of 1 Mr2 and anode currents of 1.0-5.0/IA, depending on whether the step response is integrating or differentiating. Cabling capacity or by-pass condensers are shown to increase the retention times to 30 times the sensor level. The paper analyzes three types of signal process integration, two of them using field-effect input operational amplifiers. The effect of residual voltages on large electrolytic condensers is measured in relation to loading. A circuit component layout for optimizing upper frequency response and minimizing retention time completes the presentation. REFERENCES 1 J. Segraves, Report on ISA, Instrument Technology, December 1967, p. 30. 2 P. K. Stein, Concept of a System Operating Surface, Symposium on Advances in Instruments and Measurements, Joint Session of lEEE and ASTM, June 1-3, 1971, Ottawa. 3 F. J. M. Farley, Elements of Pulse Circuits, Methuen, London, 1958, Ch. 1. 4 G. F. Dowd and M. Crevier, Can. J. Biochem. Physiol., 38 (1960) 992. 5 Mini-Systems Inc., Specifications of FET.Input Operational Amplifier Type MSF-741. 6 H. Ashworth, J. Can. Elect. Eng., 16 (1972) 33. 7 Sears-Zemansky, University Physics, Addison-Wesley, 1971, 4th ed., Ch. 29, No. 7. 8 J. K. Donaldson, Electrical Apparatusfor Biological Research, Butterworths, London, 1958, Ch. 11.
278