Performance of single point monitors for measuring aerial ammonia and hydrogen sulfide

Performance of 43-44 Single Point Monitors (SPMs) was evaluated for measuring aerial ammonia (NH ₃, 0-30 ppm) and hydrogen sulfide (H ₂S, 0-90 ppb) under laboratory and field conditions. Calibration gas or sample air with various levels of moisture content or dew-point temperature (t _dp) and gas concentrations were introduced simultaneously to the SPMs and a reference gas analyzer - chemiluminescence analyzer for NH ₃ and pulsed-fluorescence analyzer for H ₂S. Linear relationships were observed between readings of the SPMs and those of the respective reference analyzer, which were influenced by moisture content or t _dp of the calibration or sample air. Specifically, average H ₂S readings by the SPMs were, respectively, 66%, 80%, 87% and 97% of those by the reference analyzer for calibration gas at t _dp of -22 (dry), 9, 13 and 16°C. In comparison, average NH ₃ readings by the SPMs were 42%, 86%, 102% and 178% of those by the reference analyzer for calibration gas at t _dp of -22, 8.5-10, 12.5-14, and 16-17°C respectively. Coefficient of variation of "as-is" readings among the tested SPMs was up to 15% for H ₂S and up to 25% for NH ₃. Regression equations were developed to compensate for the moisture effect on SPM readings of both gases. The correctional regression equations were able to achieve over 90% of the reference H ₂S readings; however, such equations were not as effective in predicting or correcting NH ₃ readings by the SPMs.