Accurate measurements of toxic pollutants are essential to a proper assessment of air, fluid and gas quality. In particular, ambient air particulate matter less than or equal to 10 microns in size (PM10) has been recognized as matter which can be inhaled deeply into the respiratory system and which may cause adverse health effects. The greater the concentration of particulate matter in the ambient air, the greater the risk of health problems caused by the ambient air. Particulate matter measuring devices measure the mass concentration of particulate matter within ambient air, gases or fluids to determine the quality. The measuring devices use different sensing techniques to provide continuous monitoring of particulate mass concentration. Examples of sensing techniques are beta radiation attenuation and optical sensing methods. These measuring devices can provide a warning to a user when detecting a relatively low air quality based upon a relatively large particulate mass concentration within the air.
A particulate measuring device of the beta attenuation type is used to determine a mass concentration of a desired range of sizes of particulate matter through beta attenuation. FIG. 1 depicts a particulate mass measuring device of the beta attenuation type 10 known in the art. The monitor features a nozzle 12 having an ambient airflow chamber 14 with an axial ambient airflow pathway 16 therein. The airflow chamber has a neck 18 with an ambient air inlet 20 and curved surfaces 22 extending from the neck. A beta radiation source housing 24 disposed within the ambient airflow chamber 14 houses a beta radiation source 26 used in the determination of particulate mass concentration. During operation, the ambient air 28 being sampled (or other sample) enters the chamber through the inlet and flows to a filter tape 30 beneath the beta radiation source. Typically, before the ambient air enters the ambient airflow chamber, particles greater than a select size, (e.g. particles greater than 10 microns) are separated from the rest of the sample using impaction separation. Beneath the filter tape 30 is a beta radiation detector 32. The filter tape 30 collects the particulate matter present within the ambient air over time and the remainder of the sample flows out the outlet 34. As the amount of particulate matter collected by the filter tape 30 increases, the particulate matter attenuates the beta radiation emitted from the beta radiation source 26 as detected by the detector (i.e., the beta radiation detector senses less radiation from the beta radiation source). Thus, the attenuation of the beta radiation detected by the beta detector is related to the mass of the particulate matter collected by the filter tape 30 in a specific location. A beta radiation signal produced by the beta radiation attenuation device is used to calculate a mass concentration of particulate matter within a sample over a specified period of time.
With reference to FIG. 2, a problem with the prior art measuring devices is that when ambient air 28 enters the ambient airflow chamber 14, particulate matter 36 from ambient air (in particular, larger diameter or massive particulate matter having a size of 10 microns or less (PM10)) may impact and be retained on the surfaces of and/or within the ambient airflow chamber, such as curved or bent surfaces 22. Particulate matter having a size of 2.5 microns or less PM2.5 may become stuck also, however, the problem of particulate loss is more pronounced in PM10 particulate matter samples. This is because larger particles having greater inertial force are not as influenced by the air sample trajectory as the smaller particles and eventually hit the curved or bent surfaces of or within the nozzle 12 and become stuck. These stuck particles 38 will then not be accounted for when mass measurements are made often resulting in an inaccurate measurement. Further, particles stuck on surfaces of the ambient airflow chamber may accumulate on the ambient airflow chamber surfaces until random, naturally occurring phenomena cause them to dislodge and generate anomalous mass measurements in the ambient air sample.
What is desired is a particulate mass measuring device that provides more accurate mass measurements of particulate matter within a sample.
What is desired is a method for improving the accuracy of particular mass measurements in a particulate mass measuring device.