EPA-R2-72-121 ARKON Sci.n«mc Labs DEVELOPMENT OF AN INFRARED FLUORESCENT GAS ANALYZER By E.A. McClatchie ARFR-14 August 1972 Distribution of this report is provided in the interest of in- formation exchange. Responsibility for the contents resides with the author or organization that prepared it. The work upon which this publication was based was performed pur- suant to Contract No. CPA-70-152 with the National Air Pollution Control Administration by: ARKON SCIENTIFIC LABS O3O OwIgM Way. Berkeley. Calif. 94710 (415) 849-1377 ------- DEVELOPMENT OF AN INFRARED FLUORESCENT GAS ANALYZER By E.A. McClatchie ARFR-14 August 1972 Distribution of this report is provided in the interest of in- formation exchange. Responsibility for the contents resides with the author or organization that prepared it. The work upon which this publication was based was performed pur- suant to Contract No. CPA-70-152 with the National Air Pollution Control Administration by: ARKON SCIENTIFIC LABS 930 Dwight Way Berkeley, California 94710 ------- TABLE OF CONTENTS Page 1. INTRODUCTION ± 2. THEORY OF OPERATION 1 3. RESULTS TAKEN WITH THE PROTOTYPE ANALYZER 4 4. CONCLUSIONS 7 ------- 1. INTRODUCTION This report covers the work performed by Arkon Scientific Labs under Contract CPA-70-152 to the National Air Pollution Con- trol Administration (NAPCA). The aim of this contract was to develop a prototype model low level carbon monoxide analyzer using the Arkon Scientific Labs fluorescent cell and negative chopping techniques to achieve a de- vice superior to state of the art NDIR analyzers in stability and cross-sensitivity to other gaseous species. The prototype analyzer, a completely selfcontained unit powered from 115 V 60 HZ line, was delivered to NAPCA in January 1971. It showed an ultimate detectivity to CO of approximately 1 ppm, and no measurable cross-sensitivity to any other gaseous species at the level normally found in the ambient atmosphere. 2. THEORY OF OPERATION The analyzer uses the principle of infrared fluorescence, where a vibrationally excited molecular species decays electro- magnetically and emits a line spectrum of infrared radiation which is an exact match of the infrared absorption spectrum of that mole- cule. Two isotopic forms of carbon monoxide are used in the fluorescent mode. One, the normally abundant species CO16, was used to provide the measuring beam of the instrument since CO16 fluorescent radiation is strongly absorbed by normal carbon monox- ide. The second beam, CO18 fluorescent radiation, was used to provide a reference beam since the variety of radiation is only weakly absorbed by the low abundance of this isotope of carbon monoxide in nature. Referring to Figure 1, the operating principles of the in- strument are as follows: ------- Infrared radiation from the black body source is used to vibrationally excite the carbon monoxide molecules in a sealed fluorescent cell. The fluorescent cell contains a mixture of two isotopes of carbon monoxide at pressures of approximately 8.0 torr each and is filled to 1 atmosphere total pressure with Argon as a buffer infrared neutral gas. The gases in the cell are of extreme- ly high purity so that the vibrationally-excited molecules de- excite by re-emission of electromagnetic radiation; i.e., fluores- cent radiation. Thus, the cell continuously emits both CO16 and Diode -w- o *a •o f Black Boc Source lyc K Phototrai CO18 Argon * k isi stor — co" — co" ->to processing electronics 1 metre long sample cell f / « / Interference F: r"l_! v\ i^ to processing electronics fs PbSe detector Chopper Fluorescent Cell Figure 1. Arkon 0-20 ppm Carbon Monoxide Analyzer ------- and CO18 fluorescent radiation. The emitted radiation is chopped by negative chopper cells containing CO16 and CO18 alternately. The negative chopper cells contain impure carbon monoxide so that they absorb the appropriate variety of fluorescent radiation. The complete chopper consists, as shown in Figure 1, of two blades rotating on a common shaft. When rotated, the fluorescent cell is alternately exposed to exciting radiation from the black body source and then allowed to shine its fluorescent radiation through a nega- tive chopper cell and sample tube to the detector. Thus the infrared detector receives alternate pulses of CO16 and CO18 fluorescent radiation which have passed through the gas sample to be measured. Referring to Figure 2, if the CO16 and CO18 B, CO 18 A, CO 16 a. B, CO 18 A, CO 16 B, CO 18 A, CO 16 B, CO 18 A, CO 16 2. b. Fig. 2. a' Detector signals with no CO in sample tube b.. Detector signals with CO in sample tube ------- signals are defined as the "A" and "B" signals, then since the natural isotopic abundance of CO18 is only 0.2% of CO16, only the "A" signal will be attenuated by the presence of CO gas in the analyzer sample tube. A light emitting diode and phototransistor on the chopper wheel (see Figure 1) provide logic signals to the electronics for identification of the A and B pulses. The A and B pulses are then routed into separate demodulators where they are synchronously demodulated and smoothed. In addition, an automatic gain control circuit maintains the "B" signals at fixed amplitude regardless of variations of source power or detector response. The processing electronics then generates an output proportional to the quantity B - A B and this expression should only be sensitive to differential absorp- tion of the A and B signals which to first order can only occur due to the presence of carbon monoxide in the sample tube. 3. RESULTS TAKEN WITH THE PROTOTYPE ANALYZER Part of a chart recorder trace of the analyzer output is shown below in Figure 3. In this case the analyzer was set up for 50 ppm CO full scale and the chart recorder trace shows the varia- tion of CO level over a two mile run through the downtown section of Berkeley, California, near our plant. The analyzer is obviously capable of accurately measuring CO concentration in the 0-50 ppm range. ------- 10 20 30 40 mins Figure 3. Carbon monoxide distribution in down- town Berkeley, California, streets ------- Under constant temperature laboratory conditions, a series of carbon monoxide readings taken every 10 seconds over a period of 5 minutes showed a variation from a highest recorded value of 50.5 to a lowest value of 46.0 with nominal 49 ppm CO span gas flowing through the instrument. A root mean square analysis of the 30 recorded readings showed that the 'R.M.S. noise of the analyzer was 0.7 ppm CO. Possible cross-sensitivities of the prototype analyzer to other gases were investigated specifically for the gases C02, CH4 and H2O. For 100% CO2 the analyzer showed a downscale reading of - 25 to 30 ppm CO, or a rejection ratio of - 30,000:1. For 100% CH4 and saturated H2O vapor at room temperature (approximately 1%) the instrument showed no deflection within its noise limit. The performance of the prototype analyzer can therefore be summarized as follows: Minimum detectable value of CO: 0.7 ppm Rejection ratio to other gases - CO2: 30,000:1 H20: > 10,000:1 CH4: > 106:1 Only limited temperature coefficient data was obtained with the prototype analyzer. The measured zero drift was of the order of 5 - 10 ppm CO per °C change in ambient temperature. It was identified that the major contributor to this drift was the change in temperature of the infrared detector. The temperature change in the detector element caused a differential change in 16 18 its response to CO and CO fluorescent radiation and hence a severe zero drift in the analyzer. ------- 4. CONCLUSIONS A prototype carbon monoxide analyzer using an infrared fluorescent source has been built and undergone a preliminary test- ing program. It is clear that this type of analyzer has the capa- bility to advance the state of the art in NDIR carbon monoxide mea- surement devices in the areas of stability and freedom from inter- ferences by other gases. More effort will have to be devoted to an investigation of the effects of ambient temperature and pressure variation on the span and zero stability of the fluorescent CO analyzer before its true performance advantages can be assessed. A potential factor of 10 improvement in CO noise level would be attainable by a thorough upgrading of the optical components of the analyzer. ------- |