OXYGEN COMPATIBLE AIR (OCA)
SUMMARY: Safe addition of oxygen to diver’s air involves several well- studied considerations, but eventually the question of air purity has to be determined by analysis of the air. The primary concerns are condensible hydrocarbons (oil), and particulate. There is no industry consensus for a specification, and this has given birth to a good bit of confusion. Oxygen Compatible Air needs a single well documented specification.
We are analytical chemists, but we are not experts in techniques or equipment for preparing oxygen blends. Our concern is sampling and analysis of breathing gas systems. All of our information below comes from published sources, and personal experience about what is analytically possible and reasonable.
BACKGROUND – THE DILEMMA
There are three specifications used in testing ordinary SCUBA air: US Navy Diver Manual, CGA Grade E, and Federal Specification BB-A-1034b. However, there is no specification that enjoys industry wide consensus for sampling and analysis of OCA (Oxygen Compatible Air). There are at least 4 different published OCA specifications in the United States and they originate from: US Navy, ANDI, IANTD, and AAUS. (The CGA does not yet offer an OCA specification.)
Some labs analyze air per “Modified CGA Grade E” requirements which is actually defined by each lab. A few labs have tried to use CGA Grade J, despite the fact that ordinary air compressors cannot possibly meet that specification. Some labs have touted “Modified CGA Grade J” as the proper specification, but a quick review shows no similarity to the original CGA Grade J specification. That would be like saying a Volkswagen is a modified Porsche.
Curiously, we have not yet found any storeowner who actually realized that these different specifications exist and/or understood their differences. Most stores simply pick an accredited lab based on their fee schedule, and then rely on the lab to establish effective sampling and analysis procedures.
So, if your laboratory provides you with a report that says your air is Oxygen Compatible and cites one of the above specifications, does that mean that your air might not be compatible if the air had been analyzed by a different lab that happened to use a different OCA specification? But, if the reverse were true, who would you believe?
WHY THE FUSS?
We brought up the entire issue of specifications for 3 reasons. Firstly, we believe that paranoia has replaced science in the rush to establish an Oxygen Compatible Air specification to meet the demand for Nitrox. Secondly, we would like to avoid the education process when store owners ask why we do not use the same specification they have been relying on with another laboratory. They ask; “What is the difference, and is your specification better?” The answers are very technical, not easily demonstrated, and often add more confusion. You can even see their eyes glaze over when we try to explain oxygen ignition. Thirdly, aside from dew point choices, shouldn’t there be just one OCA specification that every lab uses?
The table below lists the current specifications that our clients request, and one that we prefer.
OUR RATIONAL FOR THE DRAFT VERSION ABOVE
Carbon Dioxide (500 ppmv) : CO2 will remain at a low concentration as long as the chemical filters are effective, an elevated CO2 level provides advanced warning of a nearly spent filter.
Carbon Monoxide (2 ppmv): This is reasonable, and we defer to the majority opinion.
Condensible Hydrocarbons (0.1 mg/m3 ) : This is reasonable, and we defer to the majority opinion.
Total Volatile Hydrocarbons (15 ppmv): In our experience, levels of Volatile Hydrocarbons above 15 ppmv are unusual, and in this business, unusual is generally a warning to investigate. Accepting 25 ppmv as a limit is ignoring the unusual.
Oil Mist + Particulate (1 mg/m3): This is essentially the same as NASA’s NVR (Non-volatile Residue). The only time this value is useful is if the OCA sample had a large amount of particulate smaller than 100 microns, or a discoloration despite negligible amount of Condensible Hydrocarbons. It is sort of a catch-all criterium.
Particulate (a particle count): We make note of particles smaller than 100 µm, but we actually count and limit particles as follows: 101 – 250 µm (93); 251 – 300 µm (3); > 300 µm (0).
Fibers (5:1 aspect ratio) < 25 µm diameter: 0 – 500 µm (20); 501 – 1000 µm (3); >1000 µm (0)
The values for particulate and fibers came from NASA oxygen cleaning specifications. The specification is based on 1 sq ft, but we ignore the surface area criteria, in favor of assuming that the total amount of debris – rather than surface area considerations – is more important. This assumption errs on the side of safety.
Unlike the other specifications, the criteria in the composite specification we propose are complete. For example, the AAUS, ANDI and IANTD specifications do not say how many 1 or 2 micron particles are allowed. In fact, the IANTD specification (taken from its website) does not list any particle-associated criteria. The heart of the modified OCA specification is taken from the NASA Safety Manual because we believe that NASA’s Oxygen Cleaning specification is the most appropriate guide. Unlike the other specifications NASA’s oxygen safety manual is realistic, is based on experimental data and real-world experiences.
OTHER CONSIDERATIONS of 2 MICRON OCA CRITERIA
When we began researching OCA specifications and encountered the 1 and 2 micron particle specifications, we wondered about how realistic this test could be in the laboratory. It is apparent to microscopy labs, that attempting to do an honest evaluation of debris at 1 or 2 microns is difficult, tedious and very time consuming. The time factor alone would become a large factor in the cost of the analysis and consume most of the profit. This always leads to shortcuts. In other words, the chance to miss a single 1 or 2 micron particle would be very high, thereby casting serious doubt on the validity of the test.
But, aside from that, consider this: At least two air sampling kits pass the air through a soft plastic hose before it reaches the filter. At least one lab includes a wire to “clean out the needles”. Another lab has you remove the filter from the sampling device, and place it in a container to mail back to the lab. I presume one would use a tweezers, but handling a naked filter so casually puts the risk of micro-contamination very high. So, if an OCA sample “fails” because of particulate, did it come from your system, the room you were standing in when you assembled the kit, the kit itself, or just during handling of the filter in the laboratory? There is no way to tell; it could even be all of the above. The 2 micron particle specification basically requires levels of airborne debris in the sampling kit and during all manipulations to be cleaner than in a class 100 clean room.
And, finally, if the lab fails a sample because it found a single,10 micron particle, what do you think the store owner would say? What if it happened twice in a single year? It is likely that the storeowner would look for a different lab. Inasmuch as the lab knows this, how many times do you think the lab will fail a client’s sample? My guess is not more than once despite any lab results to the contrary.
The conclusion we reached is that the 1 and 2 micron particulate specifications are not realistic, and should be replaced.
Our Draft Specification for OCA does not suffer from any of these problems while relying on the experience and expertise embodied in the NASA Oxygen Cleanliness specifications.
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.