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Detailed evaluation of three chemiluminescent ozone monitors
The Science of the Total Environment, 4 (1975) 135-154 © Elsevier Scientific Publishing Company, Amsterdam - Printed in Belgium
DETAILED EVALUATION OF T H R E E C H E M I L U M I N E S C E N T O Z O N E MONITORS
W. J. FINDLAY, G. DOWD and N. QUICKERT Instrument Evaluation Section, Chemistry Division, Technology Development Branch, Air Pollution Control Directorate, Ottawa (Canada)
(Received January 30th, 1975)
ABSTRACT A detailed evaluation of three gas phase chemiluminescent ozone analyzers was undertaken. Experiments were carried out to study the characteristics o f the gas flow systems and significant differences were found in certain areas. The instrument response was measured as a function of sample and ethylene flow-rates and the dependences were found to be different for each monitor. Errors introduced by using particulate filters on the input stream were also investigated. Response times were measured and compared with those given by the manufacturers. An analysis of the output signals was performed using a standard strip chart recorder and a storage oscilloscope. The latter provided an indication of the noise content at various frequencies; in some cases significant high frequency noise was found. The study also included a comparison of operating characteristics over an extended period. The instruments sampled ambient air for one month and flow-rate changes and zero- and span-drifts were noted. Measurement data from each monitor were compared and correlation coefficients calculated. The correlation between each pair was better than 0.97.
INTRODUCTION Both Canada and the United States have chosen the chemiluminescent reaction of ozone with ethylene as the reference method for ozone analysis, with the result that a number of instruments based on this reaction have become commercially available in the past few years. This poses a problem for anyone wishing to purchase ozone monitoring equipment since a choice must be made from among a number of quite similar instruments. Some information has been published recently comparing three different chemiluminescent ozone monitors 1, including a study of response characteristics, calibration stability and measurement correlations. This work I concluded that the three monitors agreed closely in their operating characteristics. 135
It is also stated that differences in engineering and design cause minor differences in response but this point is not elaborated. This paper describes an evaluation which takes a somewhat different a p p r o a c h . Rather than restricting the evaluation to overall performance characteristics, a m o r e detailed study was carried out. This included a study of the response as a f u n c t i o n of a number of instrument variables, such as reagent and sample flow-rates. A l s o included is an analysis of the electronics, with emphasis on the noise content o f the output at various frequencies. Overall design criteria have been investigated and are related to performance characteristics. EVALUATION
The instruments chosen for the study were a R E M ozone monitor, M o d e l 612 from R E M Incorporated, Santa Monica, California, a Bendix ozone m o n i t o r , Series 8000 from Bendix Corporation, Ronceverte, West Virginia and a M E C o z o n e meter, Model 1100 from McMillan Electronics, Houston, Texas. A detailed description of these instruments can be obtained from the manufacturers or their i n s t r u m e n t manuals. A brief general description is given below. Certain details which are i m p o r tant to performance characteristics will be described in the appropriate sections o f the evaluation. The R E M monitor was obtained in February, 1972 and was one of t h e first chemiluminescent ozone monitors available commercially. The reaction cell is m a d e o f glass and neither the reactor nor photomultiplier are temperature-controlled. The sample flow is driven by an electronic diaphragm pump with adjustable p u m p i n g speed. The sample and ethylene flow-rates are indicated by front panel rotameters. The measurement ranges are 0-0.2 and 0-2 ppm and the output is shown on a digital display. An analogue output (0-10 V) is available at the rear of the instrument. The Bendix monitor was obtained in August 1972 and has a built-in z e r o gas and calibration source. Thermoelectric coolers are placed at the reaction c e l l - p h o t o multiplier interface; the reaction cell is made of aluminum. The sample flow is indicated by a rotameter and driven by a mechanical diaphragm pump. A needle valve controls the flow. The ethylene flow-rate can be controlled at the instrument by a pressure regulator but the actual flow is not indicated. Seven measurement r a n g e s are available, from 0--0.01 to 0-1 ppm. The output is indicated by a front panel m e t e r and two analogue outputs are available: 0-10 mV at the front and 0-1 V at the r e a r o f the instrument. The MEC monitor was acquired in March 1973. It has a glass reactor, which together with the photomultiplier and electrometer are contained in an insulated unit which is thermoelectrically cooled. Both ethylene and sample flow-rates are indicated by rotameters on the front panel. The sample flow is adjustable by a needle valve and is driven by a mechanical diaphragm pump. Four measurement ranges from 0-0.05 to 0-1 ppm are available. A front panel meter indicates the o u t p u t ; analogue outputs (0-0.1 V and 0-1 V) are available at the rear of the instrument. Most of the detailed evaluation was carried out over a period o f several 136
months in 1973. The REM had been operating continuously for 1½ years before the tests and the Bendix about one year. The MEC had been in operation for only several months.
Gas flow systems Ozone-ethylene chemiluminescent monitors require a fixed flow-rate of sample and ethylene to be mixed at constant pressure and temperature. Only under these conditions is the chemiluminescent intensity proportional to the ozone concentration. Further information on this point can be obtained from Cole 2, who has given an analysis of the dependence of the response on flow-rate, temperature and pressure, and Steffenson3, who has expanded the output dependence to the kinetic parameters of the reaction. In order to evaluate the different properties of the flow systems of the monitors, a number of detailed experiments were carried out. The first experiment was to measure the actual sample intake and exhaust flow-rates and compare these with the flow-rates given by the manufacturer. The rotameter for the REM was marked off in 1/min, while for the Bendix and MEC, a calibration curve supplied by the manufacturer was consulted to convert the rotameter reading to a flow-rate. The true flow-rate was measured by calibrated mass flow meters and in each case several of the readings were verified With a soap bubble flow meter. Figure 1 shows the results. Only the Bendix gave the correct flow-rate (the dashed line indicates exact agreement). The REM had a lower flow-rate than indicated
/.
1.2-
1.0-
O.I-
////////~ //
~
O.6-
i,- O.4-
j ~ l l
(Ixhult)
lll(lltsks)
° 0,2-
, 0.4
~
, 0.6
i
, 0.8
Nnminll FIow-BItt
u
u 13)
i
I. 12
I/will
Fig. 1. Comparison of nominal and true sample flow-rates for the Bendix (B), R E M (R) and MEC (M) monitors.
with a nominal 1 1/min corresponding to 0.66 l/min actual flow. For both the REM and Bendix monitors, the sample intake and exhaust flows were the same. This is to be expected since the exhaust flow includes both the ethylene and sample flows but the 137
former is small ( ~ 2 0 ml/min) with respect to the latter ( ~ 1 l/min). The M E C monitor not only gave an actual flow-rate which was much less than that indicated, but the exhaust and intake flow-rates also did not coincide. The reason for this discrepancy was not determined; a leak downstream of the reactor is a possibility. The reason why two of the rotameters failed to give the correct flow-rate is quite evident when one considers the position of the rotameters in the system. F o r all three monitors, the calibration of the rotameter was done at ambient pressure and temperature. In the Bendix, the rotameter was located upstream of the needle valve and pump so that the pressure was essentially ambient. For the MEC, the rotameter is located in a low pressure region between the needle valve and pump. The R E M rotameter is also downstream o f a flow-restricting device. When the gas pressure in the rotameter is lower than the pressure used at calibration, the true flow-rate will be lower than indicated. Such an effect has been described by Veillon 4 in relation to several laboratory applications o f rotameters. It is apparent that this effect is not recognized by some manufacturers. To determine the flow-rate dependency of the monitors, the response to a fixed ozone concentration was measured for a number of sampling flow rates. The response, R, at a sample flow-rate, f, was compared to the response, Ro, at the flowrate recommended by the manufacturer, fo- The ozone concentration used was 0.8 ppm a n d f o was nominally 1 1/min for all three monitors (see Fig. 1 for the actual flow-rate). The ratios fifo were calculated using the measured sampling flow-rates. Figure 2 shows that each monitor has a different dependence on sampling flow-rate.
|.1-
1.o-
0.0-
0.8
,k
,~,
*'.,
*:o
,~
1'.1
,h
/~
f/t, Fig. 2. Change in response for a change in sample flow-rate for the Bendix (B), REM (R) and MEC (M) monitors. See text for an explanation of the symbols.
The M E C is essentially insensitive to flow-rate changes about fo, while the R E M showed the largest dependence with a slope of 0.83. The Bendix was intermediate with a slope o f 0.31 at fifo = 1, i.e., a 1% change in flow-rate results in a 0.3% change in response. 138
A similar experiment was carried out to determine the dependence of the response on the ethylene flow-rate and the results are shown in Fig. 3. The symbols have the same meaning as for the previous experiment except t h a t f a n d f o are ethylene flow-rates; the ozone concentration was again fixed at 0.8 ppm. The results show that all three monitors behave similarly. The slopes at fifo = 1 are 0.4, 0.3 and 0.2 for the Bendix, MEC and REM monitors, respectively. A 1% change in the ethylene flow therefore results in a response change of less than 0.4% for all three monitors. I 1.1-
1.0-
0.9-
y r/t.
Fig. 3. Change in response for a change in ethylene flow-rate for the Bendix (B), REM (R) and MEC (M) monitors. See text for an explanation of the symbols.
The experiments on flow-rate dependences indicate the importance of keeping flow-rates constant. It is apparent that some monitors are operating in a more optimum region in this respect, particularly for changes in sample flow-rate (Fig. 2). Measurements were also done to determine the changes in sample flow-rate caused by changes in sampling conditions. Specifically, the percent decrease in sample flow-rate O
naB
4O
6o-
20
,'o
2'0
do
4'0
Preseure below Atmospherle
go man Nl~
Fig. 4. Percent decrease in sample flow-rate as a function of intake restriction, measured as a pressure decrease from atmospheric in mm Hg.
139
(from the recommended flow-rate) was measured as a function of inlet restriction. The latter was measured as a decrease in inlet gas pressure, in mm Hg. Figure 4 shows that a restriction causing a 1% drop in inlet pressure results in less than 1% change_ in the sampling flow-rate for the MEC and Bendix monitors b u t a 23% decrease for the REM. The R E M has a small electronic diaphragm p u m p which is very sensitive to any restrictions in the flow. This characteristic was found to preclude the use o f an inlet particulate filter. The other two monitors use mechanical diaphragm pumps with a large pumping capacity; the M E C has in addition a flow regulator in the system. The final experiment on the gas flow system concerned the type o f particulate filters recommended by the instrument manufacturers. The filter supplied by Bendix is made o f Teflon and is held in a Teflon holder. McMillan supplies a plastic filter holder with a membrane filter (probably cellulose acetate). R E M does not r e c o m m e n d the use of a filter with their instrument. Ozone at 0.08 ppm was passed through previously unexposed filters used with the Bendix and M E C monitors and the results are given in Fig. 5. Losses of ozone are significant in the first hour of sampling, particularly for the membrane filter. After about three hours, ozone is no longer destroyed. Ozone Lessos on Filters 0-- Tefl|n 14|mbr|fle
0.9,
(Ol) 0 = O . n prm 0.8 ¸
e
0.7-
0.6-
0.$-
0
I'0
i
2o
i
i
3o Time
4o
t
50
610
rain
Fig. 5. Ozone loss as a function of time for membrane and Teflon filters used with the M E C and Bendix monitors, respectively.
If a higher ozone concentration is passed through the filters, e.g., 1 ppm, the losses after one hour of sampling are reduced to 3 and 0% for the membrane and Teflon filters, respectively. The results of Fig. 5 indicate that an incorrect calibration will result when a new filter is used and the instrument is calibrated with an external ozone source. In such a case, the best procedure is to remove the filter. For o z o n e measurement, it must be realized that some losses will occur but that this is only important in the first few hours of sampling.
140
Response times Three instrumental response times were measured: lag time, rise time to 95% response and fall time to 95% response. Lag time is the time interval from the introduction of a sample at the instrument inlet to the first response at the output. Care was taken in the measurement that no interruption in flow occurred when ozone was introduced. Table 1 shows the lag times found and compares these with the manufacturers' specifications and those given in ref. 1. Our values are similar to those TABLE
1
MONITOR
LAG
TIMES
(sec)
Monitor
Measured
Specified
Ref. 1
REM
7
-
10
MEC Bendix
3 2
3 3
10 10
specified by the manufacturers. The somewhat longer lag time for the REM results from the comparatively large tubing in the monitor (3/8 in. O.D.) and the slow flowrate (0.67 1/min). The lag times given in ref. 1 appear unusually long but this point is not discussed by the authors. Rise and fall times were measured using an ozone concentration of 0.8 ppm. The strip chart recorder used for the measurements had a pen speed of 0.5 sec full scale, which is fast compared to the response times measured. For all three monitors, rise and fall times were found to be equal so that only rise times are given in Table 2. TABLE
2
MONITOR
RISE
TIMES
(to 95%) (sec)
Monitor
Time constant
REM MEC
--
Bendix
* To 90%
Measured
Specified
Ref. 1
1
5 14
-5*
40 60
10 1
55 4
-7*
-54
10 40
30 108
---
---
response.
Two of the instruments had time constant selectors on the output and the response times were measured for each setting. The response time of the Bendix fell within specifications but the response of the MEC did not; no response time was given for the REM. As expected, response times increased for larger time constant settings. The values given in ref. 1 are shown for comparison. 141
REM
OUTPUT
I
(0.2 ppm 0 3 =-10 V)
m m m m m m m m m m m m m m m m m m m mmm~ m m n ~llln m I N N mml~m m mmmm mmmmmu, IrlrllflriPr,
m m m m m m m m m
m m m m m m m m , mmmmm m m m
m m
inmn Ilmmn m m mmlml m m mmm Ilmmm m ~ mllmm inmm toNi Jmml mmmmmmmmlmlm m m m m m m m w m imml m ~ lM
LrlIIIP
wPl,qlal,
r,ln
Imp
m m
m m
m m
m m
m m
A
m m
i N m n i N N mmm m m l m m n m m m m m m m ~ m n m m l m mmrm mmm m m mFrmmmmmm m ~ m , m m m m n m
p'
IP'P
71mlll~llqlfPpm
m m m
m m mr~p
m
m
i,m,n B dc
lOOmy
_ _
i * .......
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)
i
T ........
~ .......
I ........
~T
-"/
......
I I,..o Fig. 6. REM output on the recorder (A) and CRO (B); 0.2 ppm Os = 10 V.
142
REM OUT PUT II (0.2 ppm 03-10 V)
A dc
4 mv
_1
I_
-I
I~o msec
B de
4 mv
l -I Ilmsec Fig. 7. R E M output o n the C R O ; 0.2 p p m Oa = 10V.
143
Output signals Any thorough evaluation must include a comprehensive look at the o u t p u t signal. The noise that is invariably superimposed on the desired signal results f r o m several sources and may constitute a significant interference depending on the t y p e o f device used for recording the output 5. Electronic noise can result directly f r o m the inherent noise of certain components such as photomultipliers and operational amplifiers, or it may result from spurious noise caused by ineffective shielding, p o o r choice of components or poor power regulation. A third source of noise can be t r a c e d to malfunctioning components, such as oscillating amplifiers or noisy electrometers. Two different approaches were used to examine the output signals o f the monitors. The first was to record the output on a standard strip chart r e c o r d e r (Yokagawa, Model 3047) to display low frequency noise. In some cases, certain characteristics of the noise were linked to certain components or functions o f the monitor. The nature of the low frequency noise (or short term drift) allows an estimate of the minimum measurable concentration to be made. A second approach was to measure noise of a higher frequency nature t h a t may not constitute interference to a recorder but may be interference to data acquisition and transmission systems. High frequency content ( > 2.0 Hz) of the output was measured with a Tektronix RM 564 cathode ray storage oscilloscope. It was c o n n e c t e d directly to the unaltered output of the monitors, loading the output with 1 0 6 ~ and 47 pF. Using this device the output of the three ozone monitors was examined in both the milli- and micro-second time domains. The measurements were made with minimum damping on the output signals, i.e., one second time constant f o r the Bendix and M E C and direct output on the REM. The nominal measurement ranges were 0--0.2 ppm for the Bendix and R E M and 0-0.1 ppm for the MEC. Figures 6 and 7 show the output of the REM monitor. Figure 6 shows the standard strip chart tracing and its equivalent on the CRO screen. The oscilloscope shows the same magnitude as the recorder tracing but outlines the wave shape o f the interference. This shape contains wave fronts having a time constant o f less than 0.2 sec. In addition, there is evidence of a 60 Hz envelope. Figure 7 shows details of the power line interference with a peak-to-peak voltage level of 4 mV. Accompanying this is a phase-oriented spike. Using extended time as shown in the bottom photograph (B), a more detailed view of the phase-oriented spike is seen. It appears to come from some level switching control within the system. The next three figures show the output of the Bendix monitor. Figure 8 shows the standard strip chart tracing and its equivalent on the CRO screen. Both have the same magnitude; however, the CRO shows more spikes. Figure 9 examines the wave shape in detail both in the differential and singleended sensing mode. In these tracings we see evidence of transients greatly attenuated by the instrument's electronic integration. AC coupling of this output is shown in Fig. 10. The top photograph shows level changes taken in the storage mode. I n the bottom photograph we see a grouping of oscillations that appear in the o u t p u t approximately every 10 min. They were of significant size and very similar to wave shapes associated with switching transients 5. 144
BENDIX
OUTPUT
(0.2 ppm 0 3 -1 V)
A
oL lOOmy mm i
l
l
i
l
l
I ~mi.
B 10 mv
dc
Fig. 8. Bendix o u t p u t o n the recorder (A) and C R O (B); 0.2 p p m 0 3 = 1 V.
145
BENDIX OUTPUT II
(0.2 ppm 0 3 - 1 V)
Differential
k 4mv
do
-I
I:lO msec
Single Ended
B 4 mv dc
rl
l~10 msec
Fig. 9. Bendix output on the CRO, differential (A) and single-ended (B); 0.2 p p m Oa = 1 V.
146
BENDIX
OUTPUT III
(0.2 ppm 0 3 : 1 V)
A 2 mv
ac l p f
_1
I-
-I
to.2 msec
B 2 mv
ac
100 pr
ms e o Fig. 10. Bendix output o n the CRO with AC coupling; 0.2 p p m Oa = 1 V.
147
The next three figures show the output of the MEC monitor. Figure 11 shows the standard recorder tracing along with wave analysis on the CRO screen. The regular waveform results from the action of the thermoelectric cooler, i.e., the o u t p u t increases when the cooler is off and decreases when the cooler is on. The CRO trace (B) shows high frequency noise of two magnitudes associated with the low frequency variations. Figure 12 analyzes this further by showing the difference in magnitude of the high frequency noise with the cooler off and on. Noise of still higher frequency was present and this is shown in Fig. 13. The peak-to-peak voltage of this interference is five times that of the interference on the recorder tracing. The exact waveform of these spikes is shown in Fig. 13B. All measurements were made with the ethylene flow turned off, so t h a t no signal was being detected. Noise appearing in the output can therefore be attributed to the electronic systems of the analyzers. It is apparent from the results that the instruments are quite dissimilar. Each processes the signal from the detector in a somewhat different way with the result that the noise differs both in magnitude and character. The recorder traces in Figs. 6, 8 and 11 allow an estimate of the minimum detectable ozone concentration to be made. The minimum detectability can be defined as twice the noise level, where noise is the maximum peak-to-peak excursion o n the recorder over a 5-min period. Table 3 gives the measured minimum detectabilities and compares these to the manufacturers' specifications and those from ref. 1. The TABLE 3 MINIMUM DETECTABILITY (ppm ozone) Monitor
Measured
Specified
Ref. 1
REM MEC Bendix
0.020 0.005 0.010
0.0001 0.001 0.001
0.015 0.005 0.005
agreement between the values measured by us and those of ref. 1 is quite good considering that a different definition for noise was probably used in ref. 16. The values given by the manufacturers are much lower in each case. It would appear that these values are overly optimistic and may be based on a detection system with greater damping. A longer time constant will decrease the minimum detectable concentration at the expense, however, of time resolution. Comparison o f measurement
The monitors were run side by side for a period of 32 days to compare the ozone readings. Ambient air was drawn from the outside through a 6 m Teflon line to a glass manifold and then to the analyzers. Twelve times during the experiment (about every three days) zero-drift, span-drift and changes in flow-rates weft n o t e d and adjustments made if necessary. The outputs of the monitors were connected to 148
MEG OUTPUT I (0.1 ppm 03 ~ 100 mv)
A 10 mv I-'~
j O-
i I--1 min
B 2 mv
dc
Fig. 11. MEC o u t p u t o n the recorder (A) and CRO (B); 0.1 p p m Oa = 100 mV.
149
MEG OUTPUT II (0.1 ppm 03 : 100 mv)
Cooler on
A 1 mv
ac 1/~f
ilO
msec
Cooler o f f
B 1 mv
a© l p f
i i 0...o Fig. 12. M E C o u t p u t on the C R O with cooler on (A) a n d off (B); 0.1 p p m 0 3 = 100 m V .
150
MEG OUTPUT III (0.1 ppm 03 = 100 mv)
A ac 100 pf
10 mv
LI
I_
-I
Io.2 .,se~
1
B ac 100 pf
10 mv
11).02 msec Fig. 13. M E C o u t p u t o n the C R O : 0.1 p p m Oa = 100 mV.
151
strip chart recorders for a permanent record. A span check was performed using a concentration of 0.8 p p m ozone from a previously calibrated ozone source. The ozone concentration was referred to a standard NO in N 2 cylinder through the gas phase titration of O a with NO 7. The raw data from the recorders were reduced to 711 average hourly values using visual integration. Correlation coefficients were calculated for each p a i r o f analyzers using all 711 values. The very good correlation obtained is shown in T a b l e 4. TABLE 4 CORRELATION MATRIX
MEC Bendix REM
MEC
Bendix
REM
1.000 0.976 0.976
0.976 1.000 0.992
0.976 0.992 1.000
The hourly values were also averaged for the 32 days on an hourly basis to p r o d u c e an average diurnal profile for each monitor. Figure 14 shows the results and again shows close agreement. It should be noted that the ozone levels measured were quite low and that different results might be obtained from the measurement of different atmospheres. Clark and coworkers t found a correlation of 0.95 for the Bendix and M E C but a lower value of 0.80 for the Bendix-REM and M E C - R E M combinations. The average ozone levels were somewhat higher in these experiments t, i.e., a b o u t 0.035 p p m compared to 0.019 p p m for our experiment.
0"O3O 1 O.02S
O.OISan
0.010-
-
. . . .
S
. . . .
10
"
-
1 I~S . . . .
Rim |lmdix
....... -
JO
Nours
Fig. 14. Average diurnal profiles (32 days) measured for ambient air. T h e sample and ethylene flow-rates remained surprisingly constant for the duration o f the measurements. For all three monitors flow-rate changes for any period (about three days) were not significantly greater than the errors introduced in setting 152
and measuring the flow-rates, i.e., about 1-2%. Zero-drift was found to be less than 0.001 ppm and within the accuracy of setting the zero. The M E C monitor showed a somewhat higher zero fluctuation but this could be attributed to difficulties in setting the zero because of the wavy nature of the ouput (see Fig. 11). The twelve span checks carried out over the 32 day test period showed an average span drift of about 1% for all three monitors. This is well within the accuracy of preparing a constant ozone mixture, The largest span drift observed, 3% for a 5 day period (Bendix), was still well within the manufacturer's specification. Maintenance None of the monitors had any mechanical or electrical failures during the course of the experiments. The REM and Bendix, which have been operating a total of 2 and 1½ years, respectively, have not had any breakdown. The M E C experienced one electrical failure; the resistor in the input circuit to the cooler power supply burned out and was replaced. The monitors can also be compared for the ease in which maintenance may be performed. The best in this respect is the Bendix ,where four thumbscrews remove the top and sides to expose all physical and electrical components. The R E M is somewhat less convenient. It slides out of its case but all plumbing connections at the rear of the instrument must first be disconnected. The MEC was judged the most difficult from a maintenance point of view. The top is easily removed but this does not allow access to many of the components. In most cases the side and bottom panels must also be removed and some components disconnected.
CONCLUSIONS The observations and comparisons made during the study allow the following conclusions to be drawn: (1) The design of the gas flow systems is different in the three monitors, which results in significant differences in certain flow parameters. Two of the monitors had sample flow-rates much lower than indicated and one had an unusually large dependence on sampling restrictions. (2) The dependence of the response to changes in sample and reagent flowrate varied considerably. It is apparent that some monitors operate in a more optimum region in this respect. (3) The use o f particulate filters may lead to serious errors particularly in calibration. This is more likely for filters not made of Teflon. (4) The three monitors showed different lag and response times, which in some cases did not meet the manufacturer's specifications. (5) Significant electronic noise was found in the output from each monitor. In every case minimum detectable concentrations were higher than those given by the manufacturer. Analysis of the output with an oscilloscope showed the presence of considerable high frequency noise in some cases. (6) Correlation coefficients of greater than 0.97 were obtained for the monitors 153
when a m b i e n t air was sampled c o n t i n u o u s l y for one m o n t h . Zero- a n d span-drifts were within specifications. There were n o mechanical or electrical failures d u r i n g the test. REFERENCES 1 T.A. Clark, R. E. Baumgardner, R. K. Stevens and K. J. Krost, in Instrumentation for Monitoring Air Quality, ASTM Special Technical Publication 555, American Society for Testing and Materials, Philadelphia, Pa., 1974, pp. 101-111. 2 C. F. Cole, Amer. Ind. Hyg, Ass. J., 34 (1973) 159. 3 D. M. Steffenson and D. H. Stedman, Anal. Chem., 46 (1974) 1704. 4 C. Veillon and J. Y. Park, Anal. Chem., 42 (1970) 684. 5 G. Dowd and L. Dubois, Sci. Total Environ., 3 (1974) 179. 6 United States Federal Register, Vol. 38, No. 197, p. 28443, October 1973. 7 J. A. Hodgeson, R. E. Baumgardner, B. E. Martin and K. A. Rehme, AnaL Chem., 43 (1971) 1123.
154
DETAILED EVALUATION OF T H R E E C H E M I L U M I N E S C E N T O Z O N E MONITORS
W. J. FINDLAY, G. DOWD and N. QUICKERT Instrument Evaluation Section, Chemistry Division, Technology Development Branch, Air Pollution Control Directorate, Ottawa (Canada)
(Received January 30th, 1975)
ABSTRACT A detailed evaluation of three gas phase chemiluminescent ozone analyzers was undertaken. Experiments were carried out to study the characteristics o f the gas flow systems and significant differences were found in certain areas. The instrument response was measured as a function of sample and ethylene flow-rates and the dependences were found to be different for each monitor. Errors introduced by using particulate filters on the input stream were also investigated. Response times were measured and compared with those given by the manufacturers. An analysis of the output signals was performed using a standard strip chart recorder and a storage oscilloscope. The latter provided an indication of the noise content at various frequencies; in some cases significant high frequency noise was found. The study also included a comparison of operating characteristics over an extended period. The instruments sampled ambient air for one month and flow-rate changes and zero- and span-drifts were noted. Measurement data from each monitor were compared and correlation coefficients calculated. The correlation between each pair was better than 0.97.
INTRODUCTION Both Canada and the United States have chosen the chemiluminescent reaction of ozone with ethylene as the reference method for ozone analysis, with the result that a number of instruments based on this reaction have become commercially available in the past few years. This poses a problem for anyone wishing to purchase ozone monitoring equipment since a choice must be made from among a number of quite similar instruments. Some information has been published recently comparing three different chemiluminescent ozone monitors 1, including a study of response characteristics, calibration stability and measurement correlations. This work I concluded that the three monitors agreed closely in their operating characteristics. 135
It is also stated that differences in engineering and design cause minor differences in response but this point is not elaborated. This paper describes an evaluation which takes a somewhat different a p p r o a c h . Rather than restricting the evaluation to overall performance characteristics, a m o r e detailed study was carried out. This included a study of the response as a f u n c t i o n of a number of instrument variables, such as reagent and sample flow-rates. A l s o included is an analysis of the electronics, with emphasis on the noise content o f the output at various frequencies. Overall design criteria have been investigated and are related to performance characteristics. EVALUATION
The instruments chosen for the study were a R E M ozone monitor, M o d e l 612 from R E M Incorporated, Santa Monica, California, a Bendix ozone m o n i t o r , Series 8000 from Bendix Corporation, Ronceverte, West Virginia and a M E C o z o n e meter, Model 1100 from McMillan Electronics, Houston, Texas. A detailed description of these instruments can be obtained from the manufacturers or their i n s t r u m e n t manuals. A brief general description is given below. Certain details which are i m p o r tant to performance characteristics will be described in the appropriate sections o f the evaluation. The R E M monitor was obtained in February, 1972 and was one of t h e first chemiluminescent ozone monitors available commercially. The reaction cell is m a d e o f glass and neither the reactor nor photomultiplier are temperature-controlled. The sample flow is driven by an electronic diaphragm pump with adjustable p u m p i n g speed. The sample and ethylene flow-rates are indicated by front panel rotameters. The measurement ranges are 0-0.2 and 0-2 ppm and the output is shown on a digital display. An analogue output (0-10 V) is available at the rear of the instrument. The Bendix monitor was obtained in August 1972 and has a built-in z e r o gas and calibration source. Thermoelectric coolers are placed at the reaction c e l l - p h o t o multiplier interface; the reaction cell is made of aluminum. The sample flow is indicated by a rotameter and driven by a mechanical diaphragm pump. A needle valve controls the flow. The ethylene flow-rate can be controlled at the instrument by a pressure regulator but the actual flow is not indicated. Seven measurement r a n g e s are available, from 0--0.01 to 0-1 ppm. The output is indicated by a front panel m e t e r and two analogue outputs are available: 0-10 mV at the front and 0-1 V at the r e a r o f the instrument. The MEC monitor was acquired in March 1973. It has a glass reactor, which together with the photomultiplier and electrometer are contained in an insulated unit which is thermoelectrically cooled. Both ethylene and sample flow-rates are indicated by rotameters on the front panel. The sample flow is adjustable by a needle valve and is driven by a mechanical diaphragm pump. Four measurement ranges from 0-0.05 to 0-1 ppm are available. A front panel meter indicates the o u t p u t ; analogue outputs (0-0.1 V and 0-1 V) are available at the rear of the instrument. Most of the detailed evaluation was carried out over a period o f several 136
months in 1973. The REM had been operating continuously for 1½ years before the tests and the Bendix about one year. The MEC had been in operation for only several months.
Gas flow systems Ozone-ethylene chemiluminescent monitors require a fixed flow-rate of sample and ethylene to be mixed at constant pressure and temperature. Only under these conditions is the chemiluminescent intensity proportional to the ozone concentration. Further information on this point can be obtained from Cole 2, who has given an analysis of the dependence of the response on flow-rate, temperature and pressure, and Steffenson3, who has expanded the output dependence to the kinetic parameters of the reaction. In order to evaluate the different properties of the flow systems of the monitors, a number of detailed experiments were carried out. The first experiment was to measure the actual sample intake and exhaust flow-rates and compare these with the flow-rates given by the manufacturer. The rotameter for the REM was marked off in 1/min, while for the Bendix and MEC, a calibration curve supplied by the manufacturer was consulted to convert the rotameter reading to a flow-rate. The true flow-rate was measured by calibrated mass flow meters and in each case several of the readings were verified With a soap bubble flow meter. Figure 1 shows the results. Only the Bendix gave the correct flow-rate (the dashed line indicates exact agreement). The REM had a lower flow-rate than indicated
/.
1.2-
1.0-
O.I-
////////~ //
~
O.6-
i,- O.4-
j ~ l l
(Ixhult)
lll(lltsks)
° 0,2-
, 0.4
~
, 0.6
i
, 0.8
Nnminll FIow-BItt
u
u 13)
i
I. 12
I/will
Fig. 1. Comparison of nominal and true sample flow-rates for the Bendix (B), R E M (R) and MEC (M) monitors.
with a nominal 1 1/min corresponding to 0.66 l/min actual flow. For both the REM and Bendix monitors, the sample intake and exhaust flows were the same. This is to be expected since the exhaust flow includes both the ethylene and sample flows but the 137
former is small ( ~ 2 0 ml/min) with respect to the latter ( ~ 1 l/min). The M E C monitor not only gave an actual flow-rate which was much less than that indicated, but the exhaust and intake flow-rates also did not coincide. The reason for this discrepancy was not determined; a leak downstream of the reactor is a possibility. The reason why two of the rotameters failed to give the correct flow-rate is quite evident when one considers the position of the rotameters in the system. F o r all three monitors, the calibration of the rotameter was done at ambient pressure and temperature. In the Bendix, the rotameter was located upstream of the needle valve and pump so that the pressure was essentially ambient. For the MEC, the rotameter is located in a low pressure region between the needle valve and pump. The R E M rotameter is also downstream o f a flow-restricting device. When the gas pressure in the rotameter is lower than the pressure used at calibration, the true flow-rate will be lower than indicated. Such an effect has been described by Veillon 4 in relation to several laboratory applications o f rotameters. It is apparent that this effect is not recognized by some manufacturers. To determine the flow-rate dependency of the monitors, the response to a fixed ozone concentration was measured for a number of sampling flow rates. The response, R, at a sample flow-rate, f, was compared to the response, Ro, at the flowrate recommended by the manufacturer, fo- The ozone concentration used was 0.8 ppm a n d f o was nominally 1 1/min for all three monitors (see Fig. 1 for the actual flow-rate). The ratios fifo were calculated using the measured sampling flow-rates. Figure 2 shows that each monitor has a different dependence on sampling flow-rate.
|.1-
1.o-
0.0-
0.8
,k
,~,
*'.,
*:o
,~
1'.1
,h
/~
f/t, Fig. 2. Change in response for a change in sample flow-rate for the Bendix (B), REM (R) and MEC (M) monitors. See text for an explanation of the symbols.
The M E C is essentially insensitive to flow-rate changes about fo, while the R E M showed the largest dependence with a slope of 0.83. The Bendix was intermediate with a slope o f 0.31 at fifo = 1, i.e., a 1% change in flow-rate results in a 0.3% change in response. 138
A similar experiment was carried out to determine the dependence of the response on the ethylene flow-rate and the results are shown in Fig. 3. The symbols have the same meaning as for the previous experiment except t h a t f a n d f o are ethylene flow-rates; the ozone concentration was again fixed at 0.8 ppm. The results show that all three monitors behave similarly. The slopes at fifo = 1 are 0.4, 0.3 and 0.2 for the Bendix, MEC and REM monitors, respectively. A 1% change in the ethylene flow therefore results in a response change of less than 0.4% for all three monitors. I 1.1-
1.0-
0.9-
y r/t.
Fig. 3. Change in response for a change in ethylene flow-rate for the Bendix (B), REM (R) and MEC (M) monitors. See text for an explanation of the symbols.
The experiments on flow-rate dependences indicate the importance of keeping flow-rates constant. It is apparent that some monitors are operating in a more optimum region in this respect, particularly for changes in sample flow-rate (Fig. 2). Measurements were also done to determine the changes in sample flow-rate caused by changes in sampling conditions. Specifically, the percent decrease in sample flow-rate O
naB
4O
6o-
20
,'o
2'0
do
4'0
Preseure below Atmospherle
go man Nl~
Fig. 4. Percent decrease in sample flow-rate as a function of intake restriction, measured as a pressure decrease from atmospheric in mm Hg.
139
(from the recommended flow-rate) was measured as a function of inlet restriction. The latter was measured as a decrease in inlet gas pressure, in mm Hg. Figure 4 shows that a restriction causing a 1% drop in inlet pressure results in less than 1% change_ in the sampling flow-rate for the MEC and Bendix monitors b u t a 23% decrease for the REM. The R E M has a small electronic diaphragm p u m p which is very sensitive to any restrictions in the flow. This characteristic was found to preclude the use o f an inlet particulate filter. The other two monitors use mechanical diaphragm pumps with a large pumping capacity; the M E C has in addition a flow regulator in the system. The final experiment on the gas flow system concerned the type o f particulate filters recommended by the instrument manufacturers. The filter supplied by Bendix is made o f Teflon and is held in a Teflon holder. McMillan supplies a plastic filter holder with a membrane filter (probably cellulose acetate). R E M does not r e c o m m e n d the use of a filter with their instrument. Ozone at 0.08 ppm was passed through previously unexposed filters used with the Bendix and M E C monitors and the results are given in Fig. 5. Losses of ozone are significant in the first hour of sampling, particularly for the membrane filter. After about three hours, ozone is no longer destroyed. Ozone Lessos on Filters 0-- Tefl|n 14|mbr|fle
0.9,
(Ol) 0 = O . n prm 0.8 ¸
e
0.7-
0.6-
0.$-
0
I'0
i
2o
i
i
3o Time
4o
t
50
610
rain
Fig. 5. Ozone loss as a function of time for membrane and Teflon filters used with the M E C and Bendix monitors, respectively.
If a higher ozone concentration is passed through the filters, e.g., 1 ppm, the losses after one hour of sampling are reduced to 3 and 0% for the membrane and Teflon filters, respectively. The results of Fig. 5 indicate that an incorrect calibration will result when a new filter is used and the instrument is calibrated with an external ozone source. In such a case, the best procedure is to remove the filter. For o z o n e measurement, it must be realized that some losses will occur but that this is only important in the first few hours of sampling.
140
Response times Three instrumental response times were measured: lag time, rise time to 95% response and fall time to 95% response. Lag time is the time interval from the introduction of a sample at the instrument inlet to the first response at the output. Care was taken in the measurement that no interruption in flow occurred when ozone was introduced. Table 1 shows the lag times found and compares these with the manufacturers' specifications and those given in ref. 1. Our values are similar to those TABLE
1
MONITOR
LAG
TIMES
(sec)
Monitor
Measured
Specified
Ref. 1
REM
7
-
10
MEC Bendix
3 2
3 3
10 10
specified by the manufacturers. The somewhat longer lag time for the REM results from the comparatively large tubing in the monitor (3/8 in. O.D.) and the slow flowrate (0.67 1/min). The lag times given in ref. 1 appear unusually long but this point is not discussed by the authors. Rise and fall times were measured using an ozone concentration of 0.8 ppm. The strip chart recorder used for the measurements had a pen speed of 0.5 sec full scale, which is fast compared to the response times measured. For all three monitors, rise and fall times were found to be equal so that only rise times are given in Table 2. TABLE
2
MONITOR
RISE
TIMES
(to 95%) (sec)
Monitor
Time constant
REM MEC
--
Bendix
* To 90%
Measured
Specified
Ref. 1
1
5 14
-5*
40 60
10 1
55 4
-7*
-54
10 40
30 108
---
---
response.
Two of the instruments had time constant selectors on the output and the response times were measured for each setting. The response time of the Bendix fell within specifications but the response of the MEC did not; no response time was given for the REM. As expected, response times increased for larger time constant settings. The values given in ref. 1 are shown for comparison. 141
REM
OUTPUT
I
(0.2 ppm 0 3 =-10 V)
m m m m m m m m m m m m m m m m m m m mmm~ m m n ~llln m I N N mml~m m mmmm mmmmmu, IrlrllflriPr,
m m m m m m m m m
m m m m m m m m , mmmmm m m m
m m
inmn Ilmmn m m mmlml m m mmm Ilmmm m ~ mllmm inmm toNi Jmml mmmmmmmmlmlm m m m m m m m w m imml m ~ lM
LrlIIIP
wPl,qlal,
r,ln
Imp
m m
m m
m m
m m
m m
A
m m
i N m n i N N mmm m m l m m n m m m m m m m ~ m n m m l m mmrm mmm m m mFrmmmmmm m ~ m , m m m m n m
p'
IP'P
71mlll~llqlfPpm
m m m
m m mr~p
m
m
i,m,n B dc
lOOmy
_ _
i * .......
, -t- .......
)
i
T ........
~ .......
I ........
~T
-"/
......
I I,..o Fig. 6. REM output on the recorder (A) and CRO (B); 0.2 ppm Os = 10 V.
142
REM OUT PUT II (0.2 ppm 03-10 V)
A dc
4 mv
_1
I_
-I
I~o msec
B de
4 mv
l -I Ilmsec Fig. 7. R E M output o n the C R O ; 0.2 p p m Oa = 10V.
143
Output signals Any thorough evaluation must include a comprehensive look at the o u t p u t signal. The noise that is invariably superimposed on the desired signal results f r o m several sources and may constitute a significant interference depending on the t y p e o f device used for recording the output 5. Electronic noise can result directly f r o m the inherent noise of certain components such as photomultipliers and operational amplifiers, or it may result from spurious noise caused by ineffective shielding, p o o r choice of components or poor power regulation. A third source of noise can be t r a c e d to malfunctioning components, such as oscillating amplifiers or noisy electrometers. Two different approaches were used to examine the output signals o f the monitors. The first was to record the output on a standard strip chart r e c o r d e r (Yokagawa, Model 3047) to display low frequency noise. In some cases, certain characteristics of the noise were linked to certain components or functions o f the monitor. The nature of the low frequency noise (or short term drift) allows an estimate of the minimum measurable concentration to be made. A second approach was to measure noise of a higher frequency nature t h a t may not constitute interference to a recorder but may be interference to data acquisition and transmission systems. High frequency content ( > 2.0 Hz) of the output was measured with a Tektronix RM 564 cathode ray storage oscilloscope. It was c o n n e c t e d directly to the unaltered output of the monitors, loading the output with 1 0 6 ~ and 47 pF. Using this device the output of the three ozone monitors was examined in both the milli- and micro-second time domains. The measurements were made with minimum damping on the output signals, i.e., one second time constant f o r the Bendix and M E C and direct output on the REM. The nominal measurement ranges were 0--0.2 ppm for the Bendix and R E M and 0-0.1 ppm for the MEC. Figures 6 and 7 show the output of the REM monitor. Figure 6 shows the standard strip chart tracing and its equivalent on the CRO screen. The oscilloscope shows the same magnitude as the recorder tracing but outlines the wave shape o f the interference. This shape contains wave fronts having a time constant o f less than 0.2 sec. In addition, there is evidence of a 60 Hz envelope. Figure 7 shows details of the power line interference with a peak-to-peak voltage level of 4 mV. Accompanying this is a phase-oriented spike. Using extended time as shown in the bottom photograph (B), a more detailed view of the phase-oriented spike is seen. It appears to come from some level switching control within the system. The next three figures show the output of the Bendix monitor. Figure 8 shows the standard strip chart tracing and its equivalent on the CRO screen. Both have the same magnitude; however, the CRO shows more spikes. Figure 9 examines the wave shape in detail both in the differential and singleended sensing mode. In these tracings we see evidence of transients greatly attenuated by the instrument's electronic integration. AC coupling of this output is shown in Fig. 10. The top photograph shows level changes taken in the storage mode. I n the bottom photograph we see a grouping of oscillations that appear in the o u t p u t approximately every 10 min. They were of significant size and very similar to wave shapes associated with switching transients 5. 144
BENDIX
OUTPUT
(0.2 ppm 0 3 -1 V)
A
oL lOOmy mm i
l
l
i
l
l
I ~mi.
B 10 mv
dc
Fig. 8. Bendix o u t p u t o n the recorder (A) and C R O (B); 0.2 p p m 0 3 = 1 V.
145
BENDIX OUTPUT II
(0.2 ppm 0 3 - 1 V)
Differential
k 4mv
do
-I
I:lO msec
Single Ended
B 4 mv dc
rl
l~10 msec
Fig. 9. Bendix output on the CRO, differential (A) and single-ended (B); 0.2 p p m Oa = 1 V.
146
BENDIX
OUTPUT III
(0.2 ppm 0 3 : 1 V)
A 2 mv
ac l p f
_1
I-
-I
to.2 msec
B 2 mv
ac
100 pr
ms e o Fig. 10. Bendix output o n the CRO with AC coupling; 0.2 p p m Oa = 1 V.
147
The next three figures show the output of the MEC monitor. Figure 11 shows the standard recorder tracing along with wave analysis on the CRO screen. The regular waveform results from the action of the thermoelectric cooler, i.e., the o u t p u t increases when the cooler is off and decreases when the cooler is on. The CRO trace (B) shows high frequency noise of two magnitudes associated with the low frequency variations. Figure 12 analyzes this further by showing the difference in magnitude of the high frequency noise with the cooler off and on. Noise of still higher frequency was present and this is shown in Fig. 13. The peak-to-peak voltage of this interference is five times that of the interference on the recorder tracing. The exact waveform of these spikes is shown in Fig. 13B. All measurements were made with the ethylene flow turned off, so t h a t no signal was being detected. Noise appearing in the output can therefore be attributed to the electronic systems of the analyzers. It is apparent from the results that the instruments are quite dissimilar. Each processes the signal from the detector in a somewhat different way with the result that the noise differs both in magnitude and character. The recorder traces in Figs. 6, 8 and 11 allow an estimate of the minimum detectable ozone concentration to be made. The minimum detectability can be defined as twice the noise level, where noise is the maximum peak-to-peak excursion o n the recorder over a 5-min period. Table 3 gives the measured minimum detectabilities and compares these to the manufacturers' specifications and those from ref. 1. The TABLE 3 MINIMUM DETECTABILITY (ppm ozone) Monitor
Measured
Specified
Ref. 1
REM MEC Bendix
0.020 0.005 0.010
0.0001 0.001 0.001
0.015 0.005 0.005
agreement between the values measured by us and those of ref. 1 is quite good considering that a different definition for noise was probably used in ref. 16. The values given by the manufacturers are much lower in each case. It would appear that these values are overly optimistic and may be based on a detection system with greater damping. A longer time constant will decrease the minimum detectable concentration at the expense, however, of time resolution. Comparison o f measurement
The monitors were run side by side for a period of 32 days to compare the ozone readings. Ambient air was drawn from the outside through a 6 m Teflon line to a glass manifold and then to the analyzers. Twelve times during the experiment (about every three days) zero-drift, span-drift and changes in flow-rates weft n o t e d and adjustments made if necessary. The outputs of the monitors were connected to 148
MEG OUTPUT I (0.1 ppm 03 ~ 100 mv)
A 10 mv I-'~
j O-
i I--1 min
B 2 mv
dc
Fig. 11. MEC o u t p u t o n the recorder (A) and CRO (B); 0.1 p p m Oa = 100 mV.
149
MEG OUTPUT II (0.1 ppm 03 : 100 mv)
Cooler on
A 1 mv
ac 1/~f
ilO
msec
Cooler o f f
B 1 mv
a© l p f
i i 0...o Fig. 12. M E C o u t p u t on the C R O with cooler on (A) a n d off (B); 0.1 p p m 0 3 = 100 m V .
150
MEG OUTPUT III (0.1 ppm 03 = 100 mv)
A ac 100 pf
10 mv
LI
I_
-I
Io.2 .,se~
1
B ac 100 pf
10 mv
11).02 msec Fig. 13. M E C o u t p u t o n the C R O : 0.1 p p m Oa = 100 mV.
151
strip chart recorders for a permanent record. A span check was performed using a concentration of 0.8 p p m ozone from a previously calibrated ozone source. The ozone concentration was referred to a standard NO in N 2 cylinder through the gas phase titration of O a with NO 7. The raw data from the recorders were reduced to 711 average hourly values using visual integration. Correlation coefficients were calculated for each p a i r o f analyzers using all 711 values. The very good correlation obtained is shown in T a b l e 4. TABLE 4 CORRELATION MATRIX
MEC Bendix REM
MEC
Bendix
REM
1.000 0.976 0.976
0.976 1.000 0.992
0.976 0.992 1.000
The hourly values were also averaged for the 32 days on an hourly basis to p r o d u c e an average diurnal profile for each monitor. Figure 14 shows the results and again shows close agreement. It should be noted that the ozone levels measured were quite low and that different results might be obtained from the measurement of different atmospheres. Clark and coworkers t found a correlation of 0.95 for the Bendix and M E C but a lower value of 0.80 for the Bendix-REM and M E C - R E M combinations. The average ozone levels were somewhat higher in these experiments t, i.e., a b o u t 0.035 p p m compared to 0.019 p p m for our experiment.
0"O3O 1 O.02S
O.OISan
0.010-
-
. . . .
S
. . . .
10
"
-
1 I~S . . . .
Rim |lmdix
....... -
JO
Nours
Fig. 14. Average diurnal profiles (32 days) measured for ambient air. T h e sample and ethylene flow-rates remained surprisingly constant for the duration o f the measurements. For all three monitors flow-rate changes for any period (about three days) were not significantly greater than the errors introduced in setting 152
and measuring the flow-rates, i.e., about 1-2%. Zero-drift was found to be less than 0.001 ppm and within the accuracy of setting the zero. The M E C monitor showed a somewhat higher zero fluctuation but this could be attributed to difficulties in setting the zero because of the wavy nature of the ouput (see Fig. 11). The twelve span checks carried out over the 32 day test period showed an average span drift of about 1% for all three monitors. This is well within the accuracy of preparing a constant ozone mixture, The largest span drift observed, 3% for a 5 day period (Bendix), was still well within the manufacturer's specification. Maintenance None of the monitors had any mechanical or electrical failures during the course of the experiments. The REM and Bendix, which have been operating a total of 2 and 1½ years, respectively, have not had any breakdown. The M E C experienced one electrical failure; the resistor in the input circuit to the cooler power supply burned out and was replaced. The monitors can also be compared for the ease in which maintenance may be performed. The best in this respect is the Bendix ,where four thumbscrews remove the top and sides to expose all physical and electrical components. The R E M is somewhat less convenient. It slides out of its case but all plumbing connections at the rear of the instrument must first be disconnected. The MEC was judged the most difficult from a maintenance point of view. The top is easily removed but this does not allow access to many of the components. In most cases the side and bottom panels must also be removed and some components disconnected.
CONCLUSIONS The observations and comparisons made during the study allow the following conclusions to be drawn: (1) The design of the gas flow systems is different in the three monitors, which results in significant differences in certain flow parameters. Two of the monitors had sample flow-rates much lower than indicated and one had an unusually large dependence on sampling restrictions. (2) The dependence of the response to changes in sample and reagent flowrate varied considerably. It is apparent that some monitors operate in a more optimum region in this respect. (3) The use o f particulate filters may lead to serious errors particularly in calibration. This is more likely for filters not made of Teflon. (4) The three monitors showed different lag and response times, which in some cases did not meet the manufacturer's specifications. (5) Significant electronic noise was found in the output from each monitor. In every case minimum detectable concentrations were higher than those given by the manufacturer. Analysis of the output with an oscilloscope showed the presence of considerable high frequency noise in some cases. (6) Correlation coefficients of greater than 0.97 were obtained for the monitors 153
when a m b i e n t air was sampled c o n t i n u o u s l y for one m o n t h . Zero- a n d span-drifts were within specifications. There were n o mechanical or electrical failures d u r i n g the test. REFERENCES 1 T.A. Clark, R. E. Baumgardner, R. K. Stevens and K. J. Krost, in Instrumentation for Monitoring Air Quality, ASTM Special Technical Publication 555, American Society for Testing and Materials, Philadelphia, Pa., 1974, pp. 101-111. 2 C. F. Cole, Amer. Ind. Hyg, Ass. J., 34 (1973) 159. 3 D. M. Steffenson and D. H. Stedman, Anal. Chem., 46 (1974) 1704. 4 C. Veillon and J. Y. Park, Anal. Chem., 42 (1970) 684. 5 G. Dowd and L. Dubois, Sci. Total Environ., 3 (1974) 179. 6 United States Federal Register, Vol. 38, No. 197, p. 28443, October 1973. 7 J. A. Hodgeson, R. E. Baumgardner, B. E. Martin and K. A. Rehme, AnaL Chem., 43 (1971) 1123.
154