Performance of Single Point Monitor in Measuring Ammonia and Hydrogen Sulfide Gases

Performance of Single Point Monitors (SPMs) was evaluated for measuring aerial ammonia (NH3, 0 to 30 ppm) and hydrogen sulfide (H2S, 0 to 90 ppb) under laboratory and field conditions. Calibration gas or NH3/H2S−ladden air at various dew−point temperatures (tdp) were introduced simultaneously to the SPMs under evaluation and a chemiluminescence NH3 analyzer or a pulsed−fluorescence H2S analyzer. Coefficient of variation for “as−is” readings among the SPMs, a measurement of unit interchangeability, was up to 15% for H2S and up to 25% for NH3. Linear relationships existed between readings of the SPMs and those of the respective gas analyzer, with the slope increasing with moisture content of the calibration or sample air. Specifically, H2S readings by the SPMs averaged, respectively, 66%, 80%, 87%, and 97% of those by the analyzer for calibration gas at tdp of −22 C (dry), 9 C, 13 C, and 16 C. In comparison, NH3 readings by the SPMs averaged 42%, 86%, 102%, and 178% of those by the analyzer for calibration gas at tdp of −22 C, 8.5 C to 10 C, 12.5 C to 14 C, and 16 C to 17 C, respectively. Correctional equations were developed to compensate for the moisture interference effect on SPM readings of both gases. The corrected SPM readings for H2S measurement with an overall correctional equation generally achieved 90% to 107% agreement with the respective analyzer readings. However, such corrections for NH3 measurements proved not as effective (59% to 90% agreement). To improve the quality of H2S data obtained with SPMs moisture content of the sample air should be concurrently measured and moisture compensation can be made using an overall correctional equation.