Oil Mist and Particulate

Summary


Oil Mist + Particulate in breathing air is determined as the weight of the debris collected on a pre-weighed filter after a known volume of air has passed through it.  Choosing the right filter for this is important but often overlooked.  Flow and pressure control is critical and can be accomplished several ways; the least accurate of which is through Critical orifices (a tiny pinhole) to control the pressure and flow.  The volume of air tested  can be determined with either a flow meter or a calibrated back-pressure gauge; each has its advantages.  (Note: Sampling Oxygen Compatible air is discussed in its own section.)

 Analytical Filters for Oil Mist + Particulate 

Oil Mist + Particulate levels in SCUBA/SCBA air 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 large volume of rapidly flowing air to: (a)  reproduce SCUBA/SCBA charging  conditions as well as actual use conditions for LP air; (b) to meet 0.10 mg/m3 detection limits; and most (c) to promote movement of debris in the piping to reach the sampling kit's filter intact.

The three filters shown immediately below are from kits used by three different laboratories.  Simply put, some filters are too small or too thin to be effective for most air sampling situations.


 
Analytical Chemists, Inc

47 mm 
Lab. A


37 mm
Lab. B


9 mm



Critical Orifices can Invalidate Some Oil Mist + Particulate results

We have mentioned one of the problems with air kits that use critical orifices to control flow: small holes prevent discovery of large particles.  Here is a case history.   

The filter below is from an HP compressor that had shown no signs of a problem as it was running.  However, our Universal Air Sampling kit revealed that it was actually self-destructing.  


 

Some of the debris from this filter was sprinkled onto the largest of the critical orifice air sampling kits (photo below left). 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
air tanks.



SCUBA Inlets are from 3 different laboratories


     
Lab A: 
 ~0.025 
inch inlet orifice @ 1.5x magnification
Lab B:
 ~ 0.009 
inch inlet orifice at 
2x magnification


Lab C: 
~ 0.014
inch inlet orifice at 
2x magnification
 
Our SCUBA   1/4 inch  inlet orifice at 1.2x magnification



The 3 inlets on the left are enlarged to reveal the tiny (almost invisible) hole that the air must pass through. (These inlets are often hidden from view.)  Our SCUBA inlet (far right) is almost full scale.  When we tried to push the debris from an actual air sample into the inlets of the two well-known critical orifice type air kits  (left and middle) we were unsuccessful.   Clearly, critical orifice sampling devices will keep large particulate debris from reaching the filter. A small piece of Teflon tape will clog the air inlet, thereby preventing the collection of the bulk air sample. This usually results in the lab telling you that you didn't sample properly, and that you will have to order another sampling kit.

If you think that this problem is too unusual to happen to you, check the pile of filters below.  Every filter shown here came from an air system believed to be clean and ready to use before the air test using our sampling kit showed otherwise.





We said that critical orifices can also invalidate oil mist data even though oil mist will readily pass through any hole.  This is because one of the problems with critical orifices is that the flow rate is generally so low, that fugitive oil vapor may simply coalesce onto the walls of the compressor piping instead of being trapped and discovered in the air kit's filter.  During actual charging, however, the high flow rate keeps the oil mist moving, right into the SCUBA or SCBA flask. 

Case history:  a tank of nitrogen was used for high pressure gas tests. The gas was odorless when the tank valve was opened slightly to allow gas to flow moderately fast out of the tank.  However, when the tank valve was opened up enough to let the gas roar out of it, a strong odor of oil filled the room.  When the test was repeated, and the air was allowed to pass through our analytical filter, oil became visible on all surfaces.  A day later, an oily odor was still obvious in the filter!

Some specifications (those involving oxygen additions) set very low allowable limits (less than 0.1 mg/cubic meter) for Oil mist or vapor in the air component.  This requires special sampling protocols as well as analytical techniques to be able to quantitate down to 0.02 mg/cubic meters. This is discussed in the section about Oxygen Compatible Air (OCA).   

The techniques we use are a combination of:  high volume, high velocity air sampling, and sample concentration.  The result is the ability to detect as little as 0.005 mg/m3.