Recreationall TRIMIX

[dfreeman]

There are 4 separation methods that I know of that are commonly used. Three of them (membrane, centrifuge, pressure swing adsorption) require a lot of additional equipment. Distillation is the commonly used method for high purity but must be done at very cold conditions, basically the oxygen and then nitrogen are turned into a liquid at several hundred degrees Rankine (I am too lazy to look up the exact numbers right now). This would present a serious freeflow problem if you tried to run the remaining gas through the regulator.

If you reheated the tank, the stratification would collaspe due to free convection and diffusion forces. I would expect the tank to be well mixed by the time you got it to room tempurature again. The Laws of Thermodynamics would keep it mixed.

The melting point of helium is very close to -460 DEG F, which is absolute 0, or 0 Rankine. Any lower temperature is meaningless in the real world. Again, too lazy to look up.

I worry about all gas analyzers. Oxygen analyzers are famous for drift and calibration problems. All important oxygen processes that I have experience with use three analyzers and a polling control logic to help keep the operation going when one of the analyzers craps out, which it will. If the problem is more severe, an emergency shutdown is initiated.

For the oxygen analyzers that I trust my life with in industry, they are frequently calibrated using air and a calibration gas at 16% O2. This give tha slope and intercept needed to accurately calibrate the instrument. We see sensor heads fail as early as 6 months. They are replaced every 2 years no matter what. Is this the case with your local dive shop? I will let you be the judge of that.

I am not as familiar with helium analyzers. Perhaps someone can relay the technology used in these sensor heads. However, my experience with analyzers is that for $800 you are getting a bear bones piece of equipment. Its is like buying a low end regulator, will it work in warm water? Sure fine. Bottom of Lake Wazee? Oh, boy. I do not have enough background with helium sensors to make a judgement.

As a Nitrox diver, this subject has concerned me quit a lot when considering oxygen analyzers. I do not have an easy answer. I actually trust the blending process over the analyzer. I would prefer to blend my own gases. Give me the nobs.

Doug