Ok, well done both of you..
79%Helium - 21% O2 blending? Why not?
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MSilvia
Contributor
Good info. Thanks Brando.BCS:
Think of it this way. Air is a much better insulator than helium, so it does less to prevent heat from moving from a warm object to a cold object. However, by itself helium holds less heat.
When it's in your drysuit, it does a bad job of preventing heat from moving from your skin to the cold water.
When it's in your lungs, the heat in your body has nowhere to get transferred to besides the helium itself, which doesn't hold a lot of heat.
When it's in your drysuit, it does a bad job of preventing heat from moving from your skin to the cold water.
When it's in your lungs, the heat in your body has nowhere to get transferred to besides the helium itself, which doesn't hold a lot of heat.
Back to the original question.
You certainly CAN use a pure helium/O2 mix for diving but it is rarely the best choice.
Cost of the gas is only one issue but it is important. Even in commercial diving where contractors spend big money on gas reclaim systems to reduce the consumption of He.
Next is the decompression situation. For any given depth and dive duration there is likely to be a best mix that gives you the least deco obligation and a good level of narcosis (low) potential. Adding more He increases the deco without really helping the narcosis situation.
Try it, get modern deco program and run lots of dives with different mixes and see.
Then if you get really deep you get into the HPNS problem. Having enough N2 in the mix greatly reduces the risk of HPNS. Not that many of us will have to worry much about that.
BTW Diving on a rebreather is much warmer more because your body doesn't need to warm every breath from very cold to body temp more than the added heat from the scrubber.
As to He making you colder it is mostly loss of body heat and comes again from commercial diving where the divers are working from a bell and inside the bell is He mix not the same situation as sport divers at all.
Remember, this post is a gross oversimplification and only hits the high points. A good mix course will go into much more detail.
You certainly CAN use a pure helium/O2 mix for diving but it is rarely the best choice.
Cost of the gas is only one issue but it is important. Even in commercial diving where contractors spend big money on gas reclaim systems to reduce the consumption of He.
Next is the decompression situation. For any given depth and dive duration there is likely to be a best mix that gives you the least deco obligation and a good level of narcosis (low) potential. Adding more He increases the deco without really helping the narcosis situation.
Try it, get modern deco program and run lots of dives with different mixes and see.
Then if you get really deep you get into the HPNS problem. Having enough N2 in the mix greatly reduces the risk of HPNS. Not that many of us will have to worry much about that.
BTW Diving on a rebreather is much warmer more because your body doesn't need to warm every breath from very cold to body temp more than the added heat from the scrubber.
As to He making you colder it is mostly loss of body heat and comes again from commercial diving where the divers are working from a bell and inside the bell is He mix not the same situation as sport divers at all.
Remember, this post is a gross oversimplification and only hits the high points. A good mix course will go into much more detail.
Folks,
In reply to the initial post, down to 130 feet, nitrox will give you good results, and is a lot cheaper.
Deeper than that, and you will need the helium. Using it in open circuit diving is very expensive, and getting more so with helium price increases.
Using a helium-oxygen mixture in your diluent cylinder in a re-breather is a lot less expensive. You are filling (on average) a three liter cylinder, and wasting very little of that fill.
You have the added values of breathing a warm, moist gas which improves your over-all level of warmth, and does not leave you with that "Chinese Army marched through my mouth barefoot!" feeling at the end of a long diving day like OC does.
Further, depending upon the decompression algorithm you use, and the procedures you follow, your re-breather will maximize the effectiveness of your decompression.
The downside is the initial cost of the unit, and of the training. If, however, you do a lot of deep diving, you will reach a break-even point rather quickly. I have seen one chart, printed a couple of years ago, which showed that point as being two-hundred hours of dive time. I would suspect that, with the rise of helium prices, that time will shorten down a lot!
Just a few points to think about for later on in your diving career!
Cheers!
Rob Davie :doctor:
In reply to the initial post, down to 130 feet, nitrox will give you good results, and is a lot cheaper.
Deeper than that, and you will need the helium. Using it in open circuit diving is very expensive, and getting more so with helium price increases.
Using a helium-oxygen mixture in your diluent cylinder in a re-breather is a lot less expensive. You are filling (on average) a three liter cylinder, and wasting very little of that fill.
You have the added values of breathing a warm, moist gas which improves your over-all level of warmth, and does not leave you with that "Chinese Army marched through my mouth barefoot!" feeling at the end of a long diving day like OC does.
Further, depending upon the decompression algorithm you use, and the procedures you follow, your re-breather will maximize the effectiveness of your decompression.
The downside is the initial cost of the unit, and of the training. If, however, you do a lot of deep diving, you will reach a break-even point rather quickly. I have seen one chart, printed a couple of years ago, which showed that point as being two-hundred hours of dive time. I would suspect that, with the rise of helium prices, that time will shorten down a lot!
Just a few points to think about for later on in your diving career!
Cheers!
Rob Davie :doctor:
Daryl Morse
Contributor
I'm not a physicist nor am I a trimix diver, but helium is used as a coolant because of its high thermal conductivity. That is a fact.grahamsp:
That's why it is cold to fill your drysuit with... but won't affect anything when inspired.Daryl Morse:
lamont
Contributor
jonnythan:
helium has:
high thermal conductivity == high heat loss through your drysuit
low thermal capacity == low(er) heat loss through breathing
basically helium is like a small pipe, but which allows for high velocity flow. if you hook it up with one end on your skin and the other end to a cold ambient temperature, the heat will flow through with high velocity and you will get cold. on the other hand (this analogy may get a little painful here) when you breathe, you take a fixed length section of pipe and fill it up with every breath -- helium has a smaller diameter than O2 so it carries away less heat.
WarmWaterDiver
Contributor
Helium has a smaller effective molecular diameter than O2, or N2, but that's not really why it has different thermal properties. But, you need the O2 whether the 'inert' portion of the mix is N2, He, or some mixture of the two, so you can't get away from heat issues associated with O2 in the mix.
The thermal properties go back to whether the molecular structure of a gas is monatomic, diatomic, etc. -which is why thermally, O2 and N2 act similarly in this respect - and different from He or say Ar (physical chemistry and/or thermodynamics classes). The effective molecular diameter affects permeability and diffusivity if I remember correctly, but not directly related to thermal properties. Molecular weight, however, does have some relation to thermal properties and helium atoms are lighter than oxygen or nitrogen atoms, much less monatomic He being lighter than diatomic O2 or N2. But, He will permeate faster than N2 or O2.
The thermal properties go back to whether the molecular structure of a gas is monatomic, diatomic, etc. -which is why thermally, O2 and N2 act similarly in this respect - and different from He or say Ar (physical chemistry and/or thermodynamics classes). The effective molecular diameter affects permeability and diffusivity if I remember correctly, but not directly related to thermal properties. Molecular weight, however, does have some relation to thermal properties and helium atoms are lighter than oxygen or nitrogen atoms, much less monatomic He being lighter than diatomic O2 or N2. But, He will permeate faster than N2 or O2.
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