Assessment of In-line Filter Type and Condition on Measurement of Ammonia Concentration

Gas analyzers are commonly protected from impurities in air sampling via use of in-line filters to ensure operational performance and longevity of the instruments. This is especially true for extended period of air monitoring under conditions where airborne dust exists. Prices for commercially available filters vary substantially. A question that has often come up but has not received much investigation is how the filter media type (e.g., paper vs. Teflon) and condition (clean vs. dirty) impact measurement of the gaseous concentration. The study reported here was conducted toward addressing this issue. Specifically, the study assesses the magnitude of ammonia (NH3) adsorption to different types of in-line filters and conditions often used or encountered in air sampling for animal feeding operation air emission studies. The type of filters evaluated in this study included Teflon (most expensive), paper (less expensive), and stand-alone fuel filters, being either clean (new) or dust-laden. Three nominal NH3 levels (20, 45, 90 ppm, generated with poultry manure) coupled with two nominal airflow rates (4 vs. 8 l/min or 8 vs. 16 l/min) through the filters were used in the evaluation. The type of dust used in this study included broiler house dust and starch. Simultaneous measurements of NH3 concentrations before and after the tested filter were made with two photoacoustic gas spectrometers. The results revealed that initial NH3 adsorption was highest for the fuel filter but negligible for the Teflon filters. However, after 30-min exposure the relative NH3 adsorption by the filters were mostly below 1%. During fresh-air purging of the fuel filters laden with broiler house dust, ammonia was initially released but quickly diminished after 15 minutes. Flow rate was inversely related to NH3 adsorption by the filter, particularly dust-laden filters. The result suggest that when used properly, the in-line filters tested in this study (fuel, paper and Teflon) all offer viable options for air emission measurement applications.