Back to the OXY HACKER page

Excerpts from THE OXYGEN HACKER'S COMPANION

 

From the Introduction:

"Oxygen is a very useful and potent gas with many applications, especially for the diver and the aviator. Unfortunately for the individual who wants to learn more about O2 handling, high pressure gases, especially oxygen, exist in a gray area deep inside a bewildering maze of Compressed Gas Association (CGA) guidelines, Dept. of Transportation (DOT) regulations, Food and Drug Administration (FDA) laws, state laws, and industry codes. To which a diver using O2 can add NOAA and Navy guidelines, and the dictates of a half dozen "tech diving" certifying organizations. Aviators can add the Federal Aviation Administration (FAA).

The sheer pressure involved - 200 atmospheres or more - are so much higher than most of us are used to dealing with, or can obtain in our basement or garages, and this to adds an aura of black art to the subject. Dive shop and gas suppliers are both well aware of this mystique, and often as not seem to enjoy their role as initiates of a discipline which is shrouded in mystery, and are often reluctant to share information.

There is also always certain overcaution and "political correctness" at work with large institutions that leads them to be overly cautious in how they deal with such subjects. DAN is probably not about to advise you about using welding O2 in emergency treatment. PADI may reluctantly and half-heartedly allow itself to be dragged into the nitrox certification game, but isn't about to tell where to buy your own oxygen mixing whip - let alone how to save a few bucks by making one. And your local industrial/medical gas dealer probably won't offer you much help with how to clean and fill your own O2 tanks.

As a result, straight forward information about using and handling O2 is hard to come by, and much of what you hear will be third hand and of dubious validity."

 

On Oxygen Cleaning:

"One way or another, it seems clear that there are a lot of people doing partial pressure mixing of nitrox who are not using professionally 02 clean tanks or hydrocarbon free air and appear to be getting away with it. Does that mean you should?
Don't expect me to tell you it's OK. It's one of those things that you may get away with ten or a hundred times, but if on the thousandth time it blows up in your face youčre going to feel pretty stupid in those last micro-seconds.
Rather, take it to mean one doesn't have to be overly intimidated. Oxygen cleaning, it appears, is not rocket science, as we've been led to believe in the past, but rather kitchen science - a matter of detergent, hot water and common sense. The important thing is to use cleaning agents that will remove any hydrocarbons, but not contribute any themselves. Flammable solvents, obviously are out.

The Navy dive manual. the CGA, and other technical publications have leaned towards heavy duty solvents and detergents such as anhydrous technical grade trisodium phosphate, trichlorethylene and liquid freon, followed by rinsing with deionized water. These are hard to find, environmentally suspect in some cases, and, in the case of trichlorethylene, leave a bad smell in the tank that's almost impossible to get rid of.

Fortunately, there are some much milder and more easily obtained detergents that are popular with tech divers and seem to do the job just fine. Two of the most frequently mentioned are Simple Green, which can be used diluted, and Formula 409 which is usually used straight. The tank is filled with a hot solution of the detergent of choice, shaken, scrubbed, or tumbled, then thoroughly rinsed and dried.

Cleaning tanks can be done better, and a whole lot easier, if one improvise a washing and drying stand that will hold the tank upside down and a few inches off the ground, so it can drain freely. Then the tank can be throughly rinsed by sticking a hose up inside and blasting it out with flowing hot water. "

 

On Nitrox Mixing Calculations:

"The place where a lot of home mixers get into trouble in partial pressure mixing is figuring out how much O2 to prime the tank with to get a desired percentage of O2. The really dumb ones say "let's see, I want 32% nitrox, and it's a 3000 psi tank, so I'll add - let's see, 32% of 3000 is 960 - 960 psi of O2!"
The slightly smarter ones figure, "I want to make 32% nitrox, but since therečs already 21% O2 in air, that means I should add - uhmmm, 21 from 32 is .... 11% more O2! 11% of 3000 is 330 psi, so I'l l add 330 psi of O2. "
The first method is obviously totally bogus since it does not take into account the O2 already in the air. That diver would end up with something like 46% nitrox, and, very likely, CNS poisoning.

The flaw with the second method is that it doesn't take into account that the straight O2 one puts in the tank displaces air that will otherwise be in the tank, along with the O2 that would be in that air, so that the final mix would have less O2 than calculated. In the second case, the diver put 330psi, or 11% O2 to the tank That leaves only 89% of the tank for air, so diver will end up with the equivalent of 89% of 21% of O2 from the air, which works out to 18.69%, along with the 11% of straight O2. Add them together, and it comes out to only a 30% mix.

 

Or to look at it in terms of cubic feet, let's call it a 100 cf. tank (just to make the percentages easier) and you put in 11% O2 into it. That's 11 c.f., which would leave 89 c.f. of air which at 21% O2 equals 18.69 c.f of O2, for a total of 29.69 c.f. of 02. So in order to figure out how much O2 to add for any desired nitrox percentage you must take into account the O2 in the air. Fortunately, there's an easy formula that does so:

(FO2 mix - FO2 air) x final psig = psi O2
FN2 air

Where:
FO2 mix is the fraction of O2 you want in the finished mix.
FO2 air is the fraction of O2 in the air you are using (usually 21%)
FN2 air is the fraction of nitrogen in the air (usually 79%)
Final psig is the pressure of the tank once it is completely filled.

 

On Trimixing:

"There are really two separate, if overlapping issues when it comes to the accuracy of a mix; the FO2, and the N2 to He ratio. The problem with the latter is that there is no easy way to measure it the way there is for O2, so that it's harder for the tri mixer to track his or her mixes and come up with a personal fudge factor for correcting for compressibility.

Fortunately, the FO2 is the only percentage which is really critical. A 5% error in the FO2 - and that would be a gross error by any standard - could put a diver in serious danger of oxtox or seriously compromise a deco schedule. A 5% error in the amount of He, on the other hand, - as long as the FO2 was OK - would only mean a diver would be exposed to a level of narcosis slightly greater or less than planned since He and N2 behave fairly similarly in deco. Dr. Bill Hamilton, the deco specialist, has been quoted as saying that even a 10% error in the N2 to He ratio would not be significant. Actually, there's no excuse for even a 5% error. If the gases are added according to the calculations, and the FO2 measures within the target range, then the He to N2 ratio should, by inference, be acceptable too.

However, that isn't going to happen if the mixer doesn't take into account compressibility. If a trimix is prepared by adding O2 to an empty tank, then He, and topping up with air based on ideal gas calculations, one can expect the final mix to contain approximately 10% more oxygen than the intended (note that this is 10% of the desired FO2, not 10% of the total; if one is aiming for a 12% mix, the error would be 10% of 12%, or about 1%, not 12% plus 10% = 22%).

The 1971 U.S. Navy Diving-Gas Manual (not to be confused with the U.S. Diving Manual Part 2) has procedures for dealing scientifically and precisely with compressibility when PP mixing, but they are complicated enough that almost no one bothers to use them. Indeed, the more recent U.S. Navy Diving Manual Part 2 (Mixed Gas) acknowledges this, saying, "An awareness of the differences in the compressibility of various gases is usually sufficient to avoid the problems which are often encountered when mixing gases. When using the ideal-gas procedures which follow, a knowledgeable diver should add less O2 than called for, analyze the resulting mix and compensate as necessary".

In short, fudge."

 

On Refilling Aviation O2 Sets:

"Also, medical O2 refills for small tanks tend to be fairly expensive, at $10+ a hit. Fills at airports can cost anything from $5 to $50. If your airport is at the higher end of the scale, or simply does not have O2, this can provide a powerful incentive to fill your own.

A whip for refilling aviation O2 tanks can be much simpler than one for mixing dive gases, since one will usually be trying to get as much into the tank as possible, and not trying to meter out an exact amount, making a gauge or precision valve unnecessary. A suitable high pressure O2 hose, like the Western PF2-4-36, with a CGA 540 on one end and the appropriate fitting to mate with the aviation set (often another CGA 540) is all it takes.

A basic refilling station for av O2 will cost a bit over $100 - about $50 for a one year tank lease, $40 or so for the whip, plus the cost of the gas. For more efficient gas use you'll need more tanks, at $50/yr. each - see the discussion of cascades later on. If you have an aluminum tank and are refilling it yourself, you should know that these tanks must always be filled at very slow flow rates, since rapid filling can overheat a tank, and aluminum tanks are very susceptible to damage from overheating.

Incidently, since glider pilots regularly use O2, and tend to use smaller portable sets, you are much more likely to get a reasonably priced, hassle-free refill at an airport serving gliders than one serving bizjets. Another alternative is a dive shop that caters to tech divers and is set up to do O2 fills on the spot.

Since you can use an portable aviation O2 set for emergency medical treatment, it's only fair that it should work the other way around. The catch is, for the most efficient aviation use, you only want a trickle flow of supplemental O2, not the flood that is used in treating diving emergencies, and many medical regulators dončt allow such precise adjustment. However, you can easily modify them by using one of the kits, consisting of a needle valve, flow meter and conserving cannula, made to upgrade older aviation sets, and available from aviation O2 suppliers."

 

On Getting Privately-Owned Oxygen Tanks Filled:

"If you own your own tank (or have an orphan tank of dubious pedigree) this can create a dilemma when you have it filled. On one hand, if they swap it for a full one, youčre relieved of the responsibility of maintaining it. But if you swap it your bill of sale (and you do have one, don't you?) will no longer matches the tank in your possession, and some time in the future you may have a hard time proving that the tank is really yours.

Anytime, then, you swap a privately owned tank you should have a clear understanding with the supplier about ownership. I know one fellow who took his privately owned tank to be filled and swapped it without difficulty; a year later he got a bill for tank lease. When he asked what was going on, he was told in their view he'd never owned the tank, they'd given him a year's free lease for "turning in" an orphan tank. What could he do? The tank they'd given him when he swapped his was clearly marked "Property of" the gas dealers, and he had no paper on it to show otherwise.

The company I'm using now, on the other hand, when they exchanged my welding tank was willing to give me a letter stating that I owned a tank outright. I suppose if someone in this situation wanted to be absolutely safe they'd get the old and the new tank's serial numbers on the invoice each time they got a tank filled, so they'd have a paper trail back to the original bill of sale.

Occasionally one will run into tanks for sale with questionable pasts. Sometimes they are leased tanks which were never returned to the rightful owner, but often you'll run into tanks that were sold outright back when that was more common, or tanks left in the field when a compressed gas supplier went out of business.

Since these tanks can often be had for less than the price of a year's lease, they would seem to be, on first sight, a real bargain. But whether these "orphan tanks" are worth anything to you depends on how tolerant a gas supplier you can find. Many gas dealers operate on the theory that if you can't absolutely prove a tank is yours, then it must be theirs, never mind that they have absolutely no claim to it. Linde tends to be the hardest to deal with; other vary. If you do buy such a tank, be sure to get a bill of sale for it, preferably on a commercial letterhead.

If you can get the original bill of sale made over to you, so much the better. Legitimate bills of sale for privately owned gas cylinders are worth their weight in gold, since without them the tank may very well be useless.

Cale when confronted by an orphan tank without its bill of sale (after confirming, of course, that it is not a stolen one) simply grinds the name off the collar if it is a well known one, and makes up an "original" bill of sale on his computer ("Acme Welding Supply") and laser prints it - see the appendix for a blank sample of his work."

Back to the OXY HACKER page

Back to Airspeed Press main home page