Testers have found that the performance of the filter can be effected by the water that is present in wet scrubber exhaust systems. This is due to the water either becoming trapped or absorbed by the filter medium which can cause it to swell. Additionally it has been found that laboratory DPM testing after a wet scrubber produces inconsistent and variable results making comparable analysis of filter performance inaccurate.
MSHA tests new or altered filters (or media) by comparing the candidate filter to a standard paper filter on which their DPM rule was based (2.5g/per hour) This is done in order to directly compare the performance of the new filter to a standard media. This method avoids issues such as assigning an efficiency value to each media, which is undependable, as any efficiency percent (and final DPM output) assigned is very dependent on the test engine emissions and after treatment conditions, and one setup used for testing may not yield the same result as another in terms of percentage reduction.
Testers use the same engine, exhaust treatment, and filter canister, etc. for all tests in order to reduce variability to a minimum. When tested the performance of a media is compared to the standard filter in back to back tests under the same sample conditions. If they show equivalent results, then the new media is deemed in compliance and the filter is added to the list and can be used to meet the requirements of allowable DPM emissions.