Errors in sampling ambient concentrations with instruments employing setpoint temperature compensated mass flow transducers

Abstract

Although the USEPA will permit the use of other flow rates, EPA has concentrated on the High-Volume type of sampler, with a nominal flow rate of 1.13 m 3/min (40 cfm). All available devices with that flow rate use a controller of the Kurz Model 310 variety. Laboratory studies confirm that this is a constant mass controller, and, when operating correctly, it will control the flow rate to a constant volume but only at reference conditions (specified by EPA as 25°C, 101.3 kPa)—not operating conditions. However, the proposed amendments to 40 CFR Part 50, Appendix J, proceed on the assumption that flow controllers will yield constant flow at ambient, not reference conditions. It has been demonstrated herein that errors in actual volumetric flow rate of 25–30% can result if the flow controller is operated at temperatures differing from the setpoint temperature, due to both diurnal and seasonal temperature variations. Thus, the value for total volume of air sampled used in the denominator for calculating ambient concentration levels will bear little resemblance to the actual volume sampled. Finally, size-specific inlets will not experience their design linear flow rate, and sampler cutpoints could be affected significantly.