ARS | Publication request: Model-based flow rate control for an orfice-type low-volume air sampler

Submitted to: ASABE Annual International Meeting
Publication Type: Abstract Only
Publication Acceptance Date: July 31, 2012
Publication Date: July 31, 2012
Citation: Koller, A.A., Buser, M.D., Whitelock, D.P. 2012. Model-based flow rate control for an orfice-type low-volume air sampler. ASABE Annual International Meeting. PRESENTATION ONLY; Number 121336720.

Technical Abstract: The standard method of measuring air suspended particulate matter concentration per volume of air consists of continuously drawing a defined volume of air across a filter over an extended period of time, then measuring the mass of the filtered particles and dividing it by the total volume sampled over that time period. Consequentially, the accuracy of the computed average concentration depends both on the ability to accurately measure the mass of the collected particles as well as the accuracy with which the sampled volume can be controlled. The process of measuring the collected mass of particles is well defined by the Environmental Protection Agency's reference method. The standard gravimetric method requires that the filter is exposed to a defined temperature and moisture environment before and after sampling to establish known conditions before the mass of the filter, or the filter with the collected particles in the case of post-sampling measurements, is determined. The difference in mass between pre- and post-sampling represents the collected mass of particles. Commonly available laboratory scales are able to determine these small masses to a very high degree of accuracy. The control of the air volume drawn across the filter, however, is less well defined. EPA requires that air samplers implement a means to measure directly or indirectly and control the flow rate. Many air sampling systems use restriction-type indirect flow rate measurement concepts such as a venturi tube, an orifice, or a flow nozzle. The relationship between the pressure drop across the restriction and the flow rate is commonly calibrated in a laboratory setting and the actual flow rate in the field for a given pressure drop may differ significantly from the laboratory observations due to changing environmental conditions. This introduces measurement error in two ways: 1) the total sampler volume may differ significantly from the presumed volume, hence leading to an inaccurate computation of the particulate matter concentration and 2) the actual flow rate – and hence, the gas velocity through the sampler head – may influence the cut-off characteristics of the impacter, leading to an unintended skewing of the size distribution of the collected particles. This paper proposes a model-based closed-loop flow rate control concept that makes use of environmental sensory data to compensate for the change in local sampling conditions. The data collected in the laboratory during the calibration procedure is reduced to obtain a flow rate dependence on Reynolds number and geometric parameters. The implementation of this model in the air sampler allows the continuous precise flow rate control during the entire sampling period.