Incorrect interpretation of moving-filter continuous particulate air monitor responses

摘要

The graphs supplied by the vendors of moving-filter continuous particulate air monitors (CPAMs) in their sales literature show linear curves on a log-log scale, with net count rate on one axis and concentration on the other. The implication is that the monitor user is to read the concentration from the graph, given an observed net count rate, at any time. For the nominal filter speeds commonly used for these monitors, using the graph in this way is incorrect. The graphs do not state the limitations of the calculation: (1) the nuclide measured must be long-lived; (2) the concentration of that nuclide in the sampled air must remain constant; and (3) the reading of the net count rate must be obtained after a specific time, called the "transit time." This time is typically on the order of several hours. Reading the net count rate at any time earlier than this will result in an incorrect concentration estimate. Given that a major purpose of a CPAM is to alert plant personnel to a change in airborne radioactivity concentrations, by definition when this happens the concentration is not constant. Thus, using the supplied curves will result in an incorrect estimate of that concentration. The solution is to use instead a fixed-filter CPAM and a previously-published quantitative method. With this approach, there is no need to attempt to estimate a concentration, much less to assume that it is constant over long periods of time or that it can only change in a stair-step manner. With this alternative to a moving-filter CPAM, a signal proportional to the time-integrated worker intake can be generated continuously for any time-varying air concentration, including the sums-of-exponentials shapes expected during transient events in compartmental systems.