SUMMARY: Oil Mist + Particulate in air is determined as the weight of the debris collected on a pre-weighed filter. Very thin and small diameter membrane filters do a poor job. Critical orifices to control the pressure and flow can prevent discovery of debris. The volume of air tested can be determined with either a flow meter or a calibrated back-pressure gauge.

Analytical Filters for Oil Mist + Particulate
Oil Mist + Particulate levels are determined from the increase in weight of a pre-weighed filter after passage of a known volume of air through it. Proper sampling requires: (a) a large volume of rapidly flowing air to reproduce charging conditions and to meet 0.10 mg/m3 detection limits; and (b) the air path must allow all debris to reach the filter intact.

The three filters shown immediately below are from kits used by several competitors. Simply put, some air kit filters are too small to be effective.

Critical Orifices can Invalidate Some Oil Mist + Particulate results. A Case History
In Sampling Kits a Common Sense View, we mentioned one of the problems with air kits that use critical orifices to control flow: small holes prevent discovery of large particles.

The filter below is from the sampling of an HP compressor which had shown no signs of a problem as it was running. However, tests with our Universal Air Sampling kit revealed that it was actually self-destructing. When we tried to push the debris into the inlets of two well-known critical orifice type air kits (also shown below) we were unsuccessful. Clearly, critical orifice sampling devices will keep large particulate debris from reaching the filter.

The photograph (below) at the left is an enlarged view (2x) of the SCUBA adapter in a popular sampling kit. The debris from the filter below is too large to enter the 0.025 inch critical orifice inlet. The photograph in the middle is an enlarged view (2x) of the inlet of another popular sampling kit. We removed it from the SCUBA fitting to reveal the tiny critical orifice which is barely visible. That orifice is only 0.009 inches in diameter. The photograph at the right is our built-in SCUBA/SCBA adapter in our Dedicated HP Air Sampling kit. Nothing is held back by it.

Actual size of our filter is (1.8 in);

Some of the debris from this filter was sprinkled onto the largest of the critical orifice air sampling kits (left side). Most of the debris is too large to pass through the inlet, and smaller particles are likely to pile up

This nasty surprise is not a routine situation, but the collection of filters below (from several years of sampling with our sampling kits) suggest that problems like these are not entirely rare. Indeed, the last thing you want is for your clients to experience a surprise in their SCUBA tanks.

SCUBA Inlet Adapters from 3 popular air sampling kits