OXYGEN COMPATIBLE AIR (OCA): SPECIFICATION

Background

Analysis of OCA is also complicated by the misunderstanding as to what specification is appropriate.   The most common specifications for OCA come from AAUS, ANDI, IANTD and the US Navy.  Although all agree on lower levels of condensable hydrocarbons (0.1 mg/m³ ), a problem exists regarding particulate.

Expecting an air sample taken from piping in a Dive Shop to be free of  2 microns particles is difficult to achieve.  Think about it.  Aside from the absence of experimental data to support the 2 micron theory, the specification is even far more stringent than for a Class 100 clean room.  It is also more stringent than that used by the world’s best authority on oxygen systems: NASA.

We have designed a specification which utilizes: existing specifications, NASA criteria for oxygen handling equipment, and a knowledge of what is reasonable with current sampling and analysis techniques.  We give it here as a draft version, and we have included our rational to allow peer review criticism.

Our Rational for the Above Draft Version

Carbon Dioxide (1000 ppmv): Some OCA specifications above have reverted to the previous version (500 ppm) of CGA Grade E, but we have not discovered any supporting arguments for this. We, therefore, decided to retain the CGA Grade E criterion for carbon dioxide. Our experience has shown that carbon dioxide levels in compressor air will increase as the chemical filters become saturated, and we use this as an indicator of filter condition but we don’t get too excited until the level climbs above 800 ppm.
Carbon Monoxide (2 ppmv):
This is unnecessarily stringent, but we defer to the majority opinion to avoid appearing omniscient.
Condensible Hydrocarbons (0.1 mg/cubic meter ):
This seems reasonable since condensible hydrocarbons can build up over time.
Total Volatile Hydrocarbons (15 ppmv including CH4): 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.
* Particulate (a particle count) per NASA:
101 – 250 µm (93); 251 – 300 µm (3); > 300 µm (0). Although other specifications use a 1 or 2 micron limit, we have seen no evidence to support that idea. We make note of particles smaller than 100 µm, but we actually count and limit larger particles as listed above.
* Fibers per NASA are < 25 µm diameter particles having a >3:1 aspect ratio:
0 – 500 µm (20); 501 – 1000 µm (3); >1000 µm (0).

Values for particulate and fibers were taken from NASA’s Oxygen Cleaning Specifications which involve analysis of the rinsate from 1 sq ft of surface area. This is basically an absolute number or total count rather than a concentration per volume of gas. We, therefore, have also considered particle limits as absolute or a total number. By sampling at a velocity much higher than during oxygen blending, we hope to approach the effectiveness of rinsate sampling. It has been our observation that particulate from a compressor is more likely to form clusters of small particles which could act as a large particle during impingement. In this regard, our approach seems more likely to discover clusters of debris than would be true of a rinsate sample. In other words, (depending on the plumbing design) it may well be closer to reality than a simple rinsate sample.
Laboratory Analysis of the OCA Sample
Laboratory analysis of the OCA sample is far more tricky than for SCUBA air. For example, when checking normal compressed air for Oil mist + Particulate, one simply weighs the filter before and after sampling. This allows some room for error without diver endangerment (providing that the sample was properly taken). For OCA, however, this simple test – even with more sensitive balances – is not very reliable; measuring non-volatile hydrocarbons below 0.1 mg/cubic meter includes new factors more significant in OCA than in SCUBA air. Although 0.1 mg/cubic meter seems like a respectably low quantity, even trace levels of sticky debris can build up to a point where they can become an ignition source. This possibility makes sensitive, specific, and reliable tests even more important. Moreover, aside from hydrocarbons, determining acceptable levels of solid particulate further complicates things because ignition factors include mass and morphology.
With all these factors in mind, we examined several techniques for OCA analysis. We use microscopy for particle analysis (100 microns or greater). Inasmuch as the less expensive gravimetric method is less reliable for measuring condensible hydrocarbons on the filter, we decided to measure condensible hydrocarbons according to NASA protocol.
Oxygen Compatible Air: Sampling Kit
With this appreciation of basic principles, we have made modifications in our sampling kit preparation procedures to allow our standard Sport Diving air sampling kit to be use for OCA as well as SCUBA air sampling. (See Sport Diving .)