SAMPLING AIR INTAKES – Best Practices
SUMMARY: Sampling air at the intake of the compressor is a silly expenditure of time and money. But, like any activity, if it is done, it should be done properly.
AIR INTAKE QUALITY
Several labs offer air intake analyses, therefore, we shall clear the air regarding this (no pun).
When a sample fails a given test (i.e., hydrocarbons), the first question is “where did it come from”. Believing that the answer depends on analysis of the air intake to the compressor is founded in ignorance. With a few questions, any chemist worth his/her diploma can tell within a 90% certainty where it came from without an air intake sample. In the 25 years of sampling and analysis, we have never had a single failure that needed an air intake sample to solve the problem.
We understand the fear that industrial and vehicular pollution can contaminate the air intake with carbon monoxide. However, in Los Angeles – well known for smog and air pollution – records show that the air exceeded 9 ppm CO only 5 times in all of 1999. Obviously, the fear that CO in SCUBA air originated in the ambient air is unfounded. Of course, if you insist in having the intake air tested, we will be happy to spend your money. We have a sampling pump that works perfectly for ambient sampling.
SAMPLING AIR INTAKE – THE RIGHT AND WRONG WAY
For the sake of discussion, let’s look at the sampling process to obtain an air intake sample. Just as sampling at full flow during the entire course of sampling a large volume of air is necessary for the most statistically correct and useful sample, so too would be the case for sampling ambient air.
Several commercial sampling kits use a rubber bulb to suck air into a 20 mL vial. This tiny amount can’t possibly represent the average composition of an 80 cubic ft volume of air taken during the 6 minutes of sampling time for the air system. Common sense would tell you that the wind is blowing, and its composition is going to be different from one second to the next.
Two air analysis labs have carried this idea to even more ridiculous levels. One uses a vial placed near the air intake and tells you to take the cap off while the compressor is running, and then replace it at the end of the compressor test; the other lab uses a jelly jar. This has to be the most unscientific idea I have ever heard of. I guess the lab believes that any “molecules of bad air” are going to magically leap into the jar and stay there! It would be comical except that those labs are serious, and will send you a report (and a bill) on air intake quality using this technique.
By the way they are both “accredited”. I wonder how you could trust any lab to make proper decisions in routine air analysis if they believe that their open jar idea is valid and useful.
The proper technique is to purge the container with the sample. This should be done for the same time period as for the product air sample. Our hand pump will accomplish this and more. It has been used to sample air at 20ft above a compressor and at 50 ft underwater in an open bottom observation bell. This is the same pump we use to sample hyperbaric chambers and the low pressure gas ( 0 – 5 psi) in NASA space vehicles. (See Sampling Chambers.)