Environ. Sci. Technol. 1983, 17, 100-103
Laboratory Evaluation of an Airborne Ozone Instrument That Compensates for Altitude/Sensitivity Effects Gerald L. Gregory,* Charles H. Hudglns, and Robert A. Edahl, Jr.
National Aeronautics and Space Administration, Langley Research Center, Hampton, Virginia 23665 One problem encountered in the use of air-quality instrumentation on aircraft is the variation of instrument sensitivity with pressure as the result of altitude changes of the aircraft. Many instruments experience sensitivity changes as much as a factor of 2 a t altitudes of 6 km. Discussed are recent modifications to a chemiluminescent (ethylene) ozone detector that allow the instrument to automatically compensate for pressure/sensitivity effects. The modification provides automated mass flow rate control for both the sample and ethylene gas flows. The flow control systems maintain flow rate to within 15% for a 100-torr instantaneous pressure change, and flow rates are returned to the desired set points within 10 s after the pressure change. During simulated altitude changes (300 m/min from mean sea level to 3-km altitude), flow rates were controlled to within 3% of the set point. Laboratory data are summarized verifying the operation of the instrument for a pressure range of 760 torr (sea level) to 350 torr (=20000 ft) and an ozone concentration range of 20--700 ppb.
Introduction One of the more frequent problems encountered in the use of air-quality instrumentation in aircraft measurement programs is the variation of instrument sensitivity with changing pressure as the result of altitude changes of the aircraft. Many instruments experience sensitivity changes as much as a factor of 2 for an altitude range from mean sea level to 6 km (20 000 ft). Techniques used to correct for these sensitivity changes range from theoretical calculations to the development of altitude-dependent correction factors based on simulation chamber tests of the instrument. In almost all cases, such approaches increase mission data reduction and data-taking requirements and result in an increase in the inaccuracy of the reported data. In many cases, correction factors are not well-behaved functions of altitudes and thus introduce even more error. Often correction factors cannot be developed for a class of instruments but must be independently developed for each instrument. This paper discusses recent modifications to a chemiluminescent ozone detector that allow the instrument to automatically compensate for pressure/sensitivity effects. The simplicity of the modification and its ease of operation suggest that it has potential applications for instruments other than the subject ozone detector. Altitude and Pressure Effect The output response of an air-quality instrument is governed by an equation of the type output
a
cmx
(1)
where c = instrument constant, m = sample mass flow rate, and x = species concentration. A change in any of tpese three parameters results in an instrument output change. The instrument constant is a fixed parameter determined by the manufacturer through its design choices and component selection and as such is not generally a function 100 Envlron. Scl. Technol., Vol. 17, No. 2, 1983
of environmental parameters like pressue. As part of laboratory calibration, the mass flow rate is fixed a t a specified value by maintaining a constant volumetric flow rate (Q) through the instrument. The result of the calibration is a calibration constant or instrument senitivity, which relates instrument output to species concentration. Implicit in the calibration procedures is that both Q and the gas density ( p ) must be those values used in the calibration if m is to be constant as
m =
Qp
(2)
With changing altitude of the aircraft, the pressure and hence the density of the sample change. As a result, m varies from that used in the calibration, and hence, the instrument calibration is no longer valid. Two courses of action are possible to correct for these calibration changes. Through analaytical means or by laboratory testing, a sensitivity/pressure relationship can be determined, which is then used during data reduction to correct the instrument output. Results from simulation chamber testing (I) for this purpose for two ozone instruments are illustrated in Figure 1. In all cases, the uncertainties in obtaining the true relationship between pressure and sensitivity (see standard deviations of Figure 1)result in additional error in the measured parameter as well as require additional data taking and reduction. The second course of action is to modify the instrument or the sample environment to maintain a constant m a t that value used in the calibrations. Modification of the sample environment such as maintaining a constant pressure (2) is an acceptable technique. However, using such techniques requires careful consideration and design to avoid affecting the representativeness of the sample. Modifying the instrument such that riz is a constant that is independent of pressure is, in our opinion, the best solution to the problem for many instruments. This approach minimizes the possibility of inadvertent sample modification and was therefore selected for illustration in this paper.
Ozone Instrument and Modifications Figure 2 is a schematic of the O3 detector used in the tests. The concentration of O3 is determined by measuring the light produced from the chemiluminescent reaction of ethylene (C,H4) and the O3 in the sample. The light output is detected by a photomultiplier tube and the resulting signal conditioned to a 0-1 V instrument output. The chemiluminescent reaction occurs on a molecular basis (one molecule 03-one molecule C2H4)with a surplus of CzH4 being supplied to the reaction chamber. Instrument detection limit is of the order of 2 ppb with a 90% response of about 3 s. Concentrationsto 1ppm can be detected with the instrument as supplied by the manufacturer. Flow rate (Q) through the instrument (sample and C2H4)is controlled by fixed-orifice systems (restrictors) located for the sample on the exhaust side of the reaction chamber and for C2H4on the supply side. C2H4flow rate changes are made by supply-side gas-pressure changes (regulator) while no provision is made for external adjustments of sample
Not subJect to US. Copyright, Published 1983 by the American Chemical Society
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flow rate. With a sample pressure change, as occurs with a change in aircraft altitude, m and the calibration of the instrument change (Figure la). Sample mass flow rate changes for two reaxons: (1) gas density ( p ) changes; (2) the instrument's pump efficiency and capacity (Q)are also dependent on pressure and density. As shown in Figure la, the calibration constant or instrument sensitivity decreases approximately a factor of 2 from 760 to 400 torr (16000 ft). C2H4 flow rate also changes with sample pressure (reaction chamber pressure), but these changes have little effect on the sensitivity as long as sufficient CpH4 is supplied to satisfy the 1:l O3 to C2H4 reaction. As discussed earlier, the purpose of the modifications to the instrument is to maintain a constant m for the sample independent of the pressure and density of the gas sample. In addition, it was considered advantageous to also maintain a constant ethylene mass flow rate to ensure that C2H4 mass requirements of the reaction are always satisfied. Referring to Figure 2, the modifications were 2-fold. The sample mass flow restrictor and flow meter (exhaust side of reaction chamber) was replaced with a servo-driven bellows valve and mass flow rate (m)sensor. With the mass flow controller output as input to the servo-valve, a specific m (standard cm3/min) was set and maintained regardless of the sample pressure. With de-
creasing pressure and density (increasing altitude), the decrease in p (eq 2) is compensated for by an increase in volumetric flow (Q) as the valve opens. The instrument sample pump has adequate capacity to supply the necessary flow to pressures of about 350 torr. If necessary, this pump can be replaced with a larger capacity pump. A similar modification was made to the C2H4restrictor, which provided flow control for the C2H4. These modifications are illustrated in the schematic of Figure 3. A laboratory test program was developed to evaluate the effectiveness of the modifications and to identify critical instrument parameters like flow rates.
Laboratory Test Program The modified O3 instrument was evaluated in the laboratory for a wide range of pressure (760-350 torr) and O3 concentrations (20-- 700 ppb). The pressure range equates to an altitude range (3) from mean sea level to about 6 km (-20000 ft). Figure 4 is a schematic of the test chamber in which the various O3concentrations and pressure environments were generated. The chamber material is glass, approximately 4 cm (i.d.) by 35 cm in length, with multiple sample ports. All tests were conducted with a constant flowing O3 in air mixture in the chamber, and all tubulation on the O3supply side of the chamber and for the instrument inlets were Teflon. Ozone flow rate into the test chamber was -2-5 times that required by the instruments, thus providing a continuous flow and preventing back diffusion of exhaust gases into the zones being sampled by the instruments. A typical test sequence consisted of generating, in the chamber, a known O3 concentration a t 760 torr and then recording the instrument's outputs for several minutes at these conditions. The pressure in the chamber was then reduced (pressure control valve and pump) 50-100 torr and another set of data recorded. This sequence was continued to about 350 torr (limits of the pump). The entire sequence was repeated for other known O3 concentrations, set a t 760 torr. Ozone was supplied to the chamber from a temperature-regulated ozonator capillary-controlledmixing system having a range of concentration from below 100 to about 1000 ppb. For a fixed ozonator setting, O3 concentrations in the chamber decreased with decreasing pressure. Ozone concentrations in the test chamber equilibrated to a constant value almost immediately (less than 2 s) after a Envlron. Sci. Technol., Vol. 17, No. 2, 1983
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pressure change. No attempt was made to adjust the ozonator output to maintain the test chamber O3 concentration (that value a t 760 torr) constant with changing pressure as the true ozone concentration in the test chamber was measured by a reference technique. The reference technique was a UV absorption 0,instrument, multipoint calibrated in the laboratory a t 760 torr, whose output was corrected for puressure based on simulation chamber testing (1). Concentrations from the modified O3 instrument were obtained by applying only the calibration results (760 torr) to the instrument output. Prior to the instrument evaluations, a series of experiments were conducted to verify the operation of the test chamber, flow control valves, ozonator repeatability and stability, and test procedures. One such study was a repeatability test on ozone concentrations in the test chamber as a function of pressure. Figure 5 shows the results from these tests for three nominal ozone concentrations (chamber concentration at 760 torr). Results from two runs at each concentration are shown. The ozone data are from the reference instrument, and as shown, O3 concentrations decreases with pressure but are highly repeatable. The data of Figure 5, combined with an error analysis of the calculation of true chamber O3 concentration, showed chamber ozone to be accurate to 10-15% or 5 ppb, whichever was largest. The calibration of the reference technique at 760 torr is accurate to about 5%. Discussion of Results Figure 6 summarizes results from the laboratory instrument evaluation. Ozone as measured by the modified instruments is compared to test chamber ozone (reference technique) for the entire pressure range of 760-350 torr. Figure 6b shows only those data below 200 ppb. The line of equal ozone readings are shown ds a solid line. The broken lines (Figure 6b) represent an error level of &lo%. 102
Environ. Sci. Technol., Vol. 17. No. 2. 1983
e.
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Data below 200 OYb
Flgure E. Instrument evaluation resuns, 760-350 torr: (a)all (b) data below 200 ppb.
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In each case, the measured O3 values are within 15% of the true chamber ozone, and all but two are within 10%. Instrument flow rates for those data of Figure 6 are (1) C,H, = 110 standard cm3/min and (2) sample = 310 standard cm3/min. (Flow rate units of standard cm3/min can be equated to m, i.e., if flow rate is constant a t 310 standard cm3/min, then m is also constant.) Initial tests using the manufacturer's recommended flow rates (C2H4 = 55 standard cm3/min and sample = 310 cm3/min) showed that at pressures below 500 torr, only 60-70% of the test chamber ozone was being detected. Analysis showed that in the process of maintaining m constant (by increasing sample flow velocity), the residence time of the sample in the instrument reaction chamber decreased to the point that 0, left the reaction chamber
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without reacting with CzH4. Increasing the C2H4from 55 to 110 standard cm3/min allowed complete reaction to occur and alleviated this problem. Figure 7 is a plot of ozone ratio (0, measured by the modified instrument to test chamber 0,)vs. pressure. Shown a t each test pressure is the average ratio calculated from 3 or 4 data points a t different concentrations. The shaded area is a l a standard deviation of the averages, and for reference, the earlier 10% uncertainity bands are shown as broken lines. The initial conclusion from Figure 7 is that at pressures below 450 torr, the instrument is not as effective for correcting for pressure effects as at the higher pressures. Closer examination of the data indicated that this did not appear to be the case but rather was an artifact of the data and procedures used. The data a t the lower pressures were also the data a t the lower concentrations and, as such, on a relative scale less accurate. For example, at 380 torr the data points at 34 and 54 ppb had an ozone ratio of 0.82 and 0.88, respectivley. This increase in error at the low concentrations is most likely due to calibration (sensitivity and zero) errors of the instruments, which traditionally increase error percentages a t the lower concentrations. For the above ratios of 0.82 and 0.88, the modified instrument O3 value and test chamber ozone differed by only 6 ppb. Studies to determine any effect of the modifications on the response time of the instrument showed no measurable effect. The 90% response to a stepped O3concentration (at constant pressure) was of the order of 3 s, the same as that of the unmodified instrument. Data a t the different pressures suggested a slight improvement in response time at the lower pressures but could not be quantified (at most 0.5-9 improvement a t -400 torr). The servo valves and flow-rate controller performed well in maintaining a constant flow rate during a pressure change. For example, a 100-torr instantaneous pressure change resulted in only a 15% flow-rate change, which was automatically adjusted to the control value within 10 s. Simulation of an aircraft altitude profile (spiral a t -300 m/min from sea level to 3-km altitude) showed no flow-rate changes above 3%. At
constant pressure, flow-rate control is of the order of 2 % of the set point.
Concluding Remarks A chemiluminescent ozone detector was modified to automatically correct for pressure altitude (density) effects on instrument sensitivity. The modification consisted of providing positive and automated mass flow rate control (mass flow controller and servo valve) for both the sample and ethylene gas flows. The flow control systems maintained flow rate to within 15% for a 100-torrinstantaneous pressure change, and flow rates were returned to the desired set points within 10 s after the pressure change. During simulated altitude changes (300 m/min from sea level to 3-km altitude), flow rates were controlled to within 3% of the set point. The modified instrument was evaluated in the laboratory for a pressure range of 760-350 torr and for O3 concentrations from 20 to 700 ppb. In all cases, the instrument automatically corrected for pressure changes, resulting in an O3 reading generally within 10% of the true ozone concentration. Current research plans for the instrument include flight packaging of the hardware, laboratory testing in pressures corresponding to about 10-km altitude, and flight testing. Potential applications of the instrument include programs like NSF's GAME-TAG and NASA's Global Tropospheric Experiment. Registry No. 03, 10028-15-6. Literature Cited (1) White, J. H.; Strong R.; Tommerdahl, J. B. "Altitude Characteristics of Selected Air Quality Analyzers"; NASA CR 159165, 1979. (2) Reck, G. M.; Briehl, D.; Perkins, P. J. "Flight Test of Pressurization System Used to Measure Minor Atmospheric Constituents from an Aircraft"; NASA T N D-7576,1974. (3) U.S. Standard Atmosphere, 1976; NOAA publication S/T 76, 1562, 1976. Received for review June 15,1982. Accepted October 28,1982.
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