Measuring the Electrostatic Charge on a Filter

Particulate matter (PM) emissions from vehicles are measured by passing diluted exhaust through an efficient sampling filter and determining the filter's weight gain. For vehicles meeting the currently regulated levels, the PM mass change may be as little as a few tens of micrograms, and the weighing requirements to accurately detect the change are severe. The difficulty is compounded by the insulating nature of the sampling filters used. Proper neutralizing of electrostatic charge on filters before weighing is critical to achieving accurate, repeatable results. Surface potential is sometimes used to verify that filters have been sufficiently neutralized. Unfortunately, while the surface potential of a conductor is well defined, that of an insulator is not and cannot be measured uniquely. This paper provides background electrostatic calculations needed to interpret the measurements. It shows that the electric field above a charged filter depends on the proximity of the filter to grounded surfaces and that a location can be found where the distance dependence is minimized. It presents manual weighing results from filters with varying degrees of neutralization and shows that improperly neutralized filters exhibit both a systematic error toward higher weights, as well as greater variability.