Abstract
If a room containing a contaminant emission source is well-mixed, and if a series of concentration measurements are made with a real-time sampling device, it is possible to deduce the contaminant generation rate function G(t), mg/sec. However, if the space is not well-mixed, and if the real-time sampler inlet is located within a concentration gradient near the emission source, the deduced G(t) function can differ markedly in shape and magnitude from the true G(t) function. This article describes the theoretical difficulty in deducing G(t) from concentration measurements by examining hemispherical diffusion and two generation rate functions—a constant generation rate and an exponentially decreasing generation rate. A scenario is posed in which the room volume is 25 m3, the supply/exhaust air rate is. 042 m3/sec (6 nominal air changes per hour [ACH]), the eddy diffusivity coefficient is. 0033 m2/sec, and measurements are made at 1.0 m from the emission source. For a constant generation rate G, the deduced G(t) function increases from zero to a maximum that is 3.8-fold G, then gradually decreases to a steady-state value that is 2-fold G. For the exponentially decreasing emission rate function with an initial rate G0, the deduced G(t) function increases from zero to a maximum that is 3.5-fold G0, and then gradually declines to zero. It is shown that both the eddy diffusivity coefficient and the measurement distance from the source affect the shape and magnitude of the deduced G(t) function. Therefore, to validate a G(t) function deduced from serial contaminant concentrations in the workplace, one must either establish that the workplace is well-mixed, or demonstrate that a scaled version of the emission process in a well-mixed test chamber produces a similar G(t) function.