Air Sampling Guide BREATHING AIR PURITY. What is important and What to do about it.
EVERYTHING YOU NEED TO KNOW TO SELECT THE BEST VENDOR
Most retail stores that fill air SCUBA and SCBA tanks have no problems with air purity. For them, air sampling/analysis is simply an expensive and seemingly unnecessary routine confirmation of that In fact, however, routine air sampling and analysis is one means to supply a warning that something has already failed – or is about to. There are many other advantages to performing routine purity checks, and most of them impact directly on your store’s ability to survive in a fiercely competitive environment where the client is becoming increasingly sophisticated. Since you can’t do the analysis yourself, you probably have already chosen a lab to provide that service.
There are many labs that can supply sampling equipment, and many more that do air analyses, so why not pick the least expensive one? To the naive, a lab is a lab. But, that’s like saying all doctors and attorneys are the same, so pick a cheap one. Bad advice. Choosing which lab to use is actually an important decision requiring knowledge in an unfamiliar field. We are the experts, but you shouldn’t have to be one in order to choose wisely. When you get the next lab report, you will post it near the compressor, and both you and your clients will rely upon it; but how do you know if that the report is reliable.
SAMPLING STRATEGY – IT ALL STARTS HERE
During normal use, most compressors will shed a small amount of debris: desiccant, water, oil mist and a little carbon monoxide. This debris is ordinarily held back by filters in the system, but under some conditions, the filters can fail to remove the debris, and it ends up in your client’s flask unnoticed until it is breathed.
Discovering contaminants during air purity testing means that you must sample at a flow rate that is similar to the flow rate when you fill flasks at the filling station. One reason for this is that as debris is blown into the piping it usually tends sticks to the walls, and in recessed areas. Sampling at a low flow rates does not disturb these contaminants, whereas the higher flows occurring during tank fills will tend to aspirate it off the walls as an aerosol into the SCUBA or SCBA bottle. Do all labs sample at a high flow rate? No; only 2 labs in the US can sample at a high flow rate. Keep in mind we worry about flow rates, not pressure.
Another reason is that compressors tend to generate carbon monoxide when running hot during the filling of storage tanks. Sampling compressors directly at high flow rates will show the correct CO level that the compressor is producing, whereas sampling at low flow rates will let the compressor run cooler, and the potentially high CO levels will not be discovered.
SAMPLING EQUIPMENT – SOME SPECIFICS.
High pressure systems present a challenge because of the difficulty in controlling the pressure while trying to establish full flow conditions. A variety of approaches are in use, but some are poorly designed and should be avoided. For example, several air kits on the market utilize a pin hole restrictor (~0.018 inch diameter) in the inlet fitting of their kit. This allows them to sample under full pressure, but unfortunately the flow rates are typically about 3 SCFM (about 100 lpm) which is very low flow. Moreover, the tiny hole in their fitting will plug up with even the smallest piece of Teflon tape. Air sampling kits using this technique cannot reveal the correct oil mist plus particulate data on any compressor with a rated output of 5 SCFM or higher.
Some kits solve the pressure problem by attaching a pressure regulator onto the air output port of the compressor. This reduces the pressure and flow rate, however all American regulators contain at least one inlet sintered filter which is meant to protect the regulator. This inlet filter removes the very debris that the lab is trying to find ! Regulators also contain numerous impinging surfaces which also tend to trap out the very debris you are trying to discover. For this reason, air sampling kits that attach a regulator onto the sampling fitting should not be used. Furthermore, the most widely used purity specification in the US is CGA Grades D and E (ANSI S-7). This document specifically disallows the use of a pressure regulator as part of a sampling kit.
Aside from the importance of sampling at full flow is the type of analytical trap used to collect the oil mist plus particulate. Currently all air sampling kits use a membrane type filter. Some kits use large diameter filters (47 mm) while others use very small ones. Analytical filters that are too small (< 37 mm) have such a small filter area, that high velocity air (13 – 18 SCFM) traveling through the filter can push the oil film completely out of the filter. Therefore, we suggest that analytical filters smaller than 37 mm should be avoided.
Gaseous hydrocarbons, carbon monoxide and carbon dioxide are also collected during the sampling process in order to transport them to the laboratory for analysis. These are collected in kits using metal flasks, glass vials or air bags. The air bags (Tedlar or Aluminized Mylar) work reasonably well, but are not very rugged, and in some cases will allow diffusion or loss of hydrocarbons. This is usually not critical unless you need to identify and quantitate toxic hydrocarbons (solvents, paint fumes or oil breakdown products) at levels below 25 ppmv. Glass vials don’t suffer this fault, but glass vials do not contain enough sample to allow repeat analyses. Glass vials use a rubber plug which tend to absorb hydrocarbons (thereby removing them from discovery). Moreover, since glass doesn’t bounce well, glass vials would be dangerous if handled on a wet surface.
ANALYSIS OF THE SAMPLE
Once the laboratory has the sample, the analytical process takes over. There are many ways to analyze a sample: colorimetric indicator tubes; Gas cell infrared spectrometry; Gas Chromatography; Gas Chromatography/Mass Sepctrometry; and analyte specific portable meters.
Unlike environmental analysis which is carefully controlled by EPA, techniques and data quality in compressed breathing air analysis have no controlling or oversight agency. There are certifying organizations such as AIHA, and A2LA which provide general criteria for labs, but analysis of compressor air is so unique that the lab certification does not apply. Put another way, you are just as likely to get useless or misleading results for compressed air analysis from a certified lab as from one that is not certified.
One does not have to be a chemist to obtain and understand the quality of the report, but one must keep in mind the following guidelines
This is going to seem obvious until you read further, but the only way to be certain about the purity of air you charge tanks with is to have a QA program that works properly. In the 23 years that I have been checking air purity, I never met a dive store owner who didn’t have a QA program. But, when we asked a few questions, we found that a QA program meant different things to different people.
Air analysis is the ulitmate way to determine air purity, anything else is useful but not as definitive. In addition to your own preventative maintenance of hardware, you should have your air periodically analyzed. Since we are a laboratory, you might say that we are only feathering our own nest, but here is some hard experience that we have had.
Divers (recreational) generally give blind trust to the quality of breathing air that they purchase from their local dive shop. This trust is based on their assumption that the air supply is tested on a regular basis. Although OSHA has established rules on the frequency and type of air quality management when employees breathe compressor supplied air, the air, recreational use is not federally regulated. Organizations such as PADI, NAUI, SSI and OSHA have established guidelines for checking air that cover frequency as well as purity. The US Navy For example, both use CGA grade D or better for purity criteria. The table at the end of this article lists some common specifications used in the diving community.
There are a wide variety of sampling techniques in use. Most are of questionable validity even though they all appear to offer the same service. Indeed, even the US Navy has a tough time discovering the problems inherent with some of the techniques used in commercial labs. Although a diver need not be a chemist to work through this, the issue is fairly technical, and some seemingly small points are actually crucial to the validity of the data. Afterall, just because the report is printed and presented nicely, doesn’t mean that the data reflects the true quality of the air.
Contact our local service center if you are unable to resolve a contamination issue. Contact us if you need the name of a technical specialist located in your area.