Mailing List Archive Mailing List Archive
---- Microsoft Mail "VMS Mail" message ---- From: HeimannJ on Tue, Feb 16, 1993 10:16 AM Subject: RE: Trimix + Re: Hose wars To: techdiver Response to Nick Simicich: >About PP mixing of He: You should know that the ideal gas equations >don't apply to helium at 3000 PSI and room temperature. They aren't >close enough to be safe. As a former math&physics student I am well aware of this behavior (I think I can even remember the Van der Waals equations from my statistical mechanics course ten years ago). I raised the question with a couple experts in the field (Bill Deans and Randy Bohrer). Both stated, and Mount's book "Mixed Gas" agrees, that the deviation from ideal gas law in typical trimixes are not enough to worry about. Apparently, if you mix and analyzed the gas properly, the discrepancy introduced by non-ideal gas behavior is not enough to worry about, and is within the tolerance of the decompression model. "Mixed Gas" claims that, for typical mixes, the decompression model is most sensitive to FO2, which is what you analyze with your Mini-Ox or Teledyne or whatever. Note that the analysis is done from gas at 1 atmosphere pressure, not at tank pressure, so the analyzed gas behaves much more like an ideal gas at this pressure. In a sense you don't care what the high pressure gas mix in tank is, as long as the low pressure gas you are breathing (and for any reasonable depth, the pressure of gas you are breathing is much lower than tank pressure) is within the tolerance of what your deco model requires. Now I should note that I am a little leery of trimix technology for a number of reasons: 1) There is little flexibilty on the dive. When diving air, I plan by tables and my known surface consumption rate and pick a turn-around time based on air. On the dive, I typically watch my air gauge and the total deco time indicated on my computer (adding some time for safety). I may choose to modify my dive plan on the bottom if I am using more or less air than planned. I can also ascend as slowly as I choose (possibly modifying the deco schedule in the process). Finally, I often use O2 or EAN50 for deco on deep dives, but plan the dive so that I can do the hang on air if necessary. With gas, one must adhere to the preplanned bottom time, ascent rate, and gas switches. 2) There is less backup When doing "technical" dives on air, one typically uses redundant tanks/regs, so there is a complete backup if one component of your life support system should fail. When going to mix, you still retain redundancy of your bottom mix, but lose it for your deco mix. You MUST decompress on the nitrox and O2 carried in your stage bottles or you will be bent. If you lose a bottle, or have a regulator failure, you must now contemplate the possibility of life as a paraplegic. One could solve this by using redundant stage bottles, or Y-valves on each stage (putting the number of regs required at seven, counting your argon suit inflator), but nobody I know does. You could also have support divers tending your deco - Billy Deans does this, and it's a damn good idea for any deco dive - but few people in the Northeast seem to be doing this. 3) There have been sufficiently few mix dives that most if not all deco models are still being validated - on YOU. This does not mean that I won't do this kind of diving at some time in the future - but I am concerned that trimix is thought of as a safe alternative to air. It is no doubt safer than air for really deep (well below 200') dives, if only because of the O2 toxicity risk for air, but I suspect that most people shouldn't even consider gas until they are routinely pushing this limit on air.
Navigate by Author:
[Previous]
[Next]
[Author Search Index]
Navigate by Subject:
[Previous]
[Next]
[Subject Search Index]
[Send Reply] [Send Message with New Topic]
[Search Selection] [Mailing List Home] [Home]