Equivalent Air Depth question

[Nick_Radov]

Heat Loss
Respiratory heat loss is much higher on HeO2, which sat divers manage with open circuit hot water heated wetsuits and gas heaters.

Actually, respiratory heat loss is slightly lower on helium because it has a lower heat capacity than nitrogen. (Just don't use helium to inflate your drysuit because it's a much better heat conductor.)

Fighting Exposure – Is Helium a “cold” gas? - International Training - SDI | TDI | ERDI | PFI

So, now you should understand why it does not deserve to be called a “cold gas” when inhaled from a scuba regulator as compared to air. This is good news.

www.tdisdi.com

 

[Akimbo]

Actually, respiratory heat loss is slightly lower on helium because it has a lower heat capacity than nitrogen.

The thermal conductivity of Helium is 5.9 times higher than Nitrogen or Oxygen:

Thermal Conductivity Relative to Air
Times as conductive as air​	W/(m·K)	Name
1.0​	0.024​	Air (gas)
0.7​	0.016​	Argon (gas)
0.6​	0.0146​	Carbon dioxide (gas)
5.9​	0.142​	Helium (gas)
7.0​	0.168​	Hydrogen (gas)
90.8​	2.18​	Ice (0°C, 32°F)
1.0​	0.024​	Nitrogen
1.0​	0.024​	Oxygen
24.2​	0.58​	Water

Shirt sleeve comfortable in a chamber at 850'/260m on HeO2 is in the 92°F/33°C range. Divers will shiver uncontrollably under these conditions at 85°F/30°C. The comfort zone increases with depth, largely because the Oxygen percentage drops. The comfort zone also gets very narrow, like ±0.5°F/0.25°C.

Divers will develop pneumonia within an hour below 600'/180m breathing HeO2 that is not heated to at least 60°F/21°C. Considering that the area of an adult human lung is about the size of half a tennis court, there is a lot of energy transfer that results in core temperature loss.

As a diver that has lost heating in a sat chamber and hot water when locked out, I can assure you that the respiratory heat loss is dramatically higher or HeO2 than air of trimix.

 

[GF99/99]

The main reason is that helium is very expensive, nitrogen is free.
So a fill with 40% N2, 40% He, 20% O2 costs half than 80% He, 20% O2.
And such a trimix is effective avoiding narcosis at, say, 60m max depth.
Regarding the resulting deco obligation I do not possess enough technical knowledge for evaluating if the trimix is better or worst than Heliox.

If cost is the only driver for trimix vs heliotrox, wouldn’t it make sense to use the latter for CCR where the dil cost per dive is 10-15 times lower than OC?

I am not trying to make a point but rather genuinely trying to learn

Full disclaimer: Non commercial diver here but from a recreational diver stand point

For me cost is not really the main driver. For me dives sub 150m I actually want Nitrogen in the mix and even sometime dive higher END's/gas densities just to keep as much as I can in the mix. Main reason is HPNS. Most dives sub 130m I plan at 30m/min decent rate and easily hit 40-43m/min at the peak. This is like drag racing take off speed compared to commercial divers decent rate so HPNS is a major concern that is manage by having Nitrogen in the mix. Second is to reduce the deco penalty as a recreational diver we try and dive the "best mix" that still gives a comfortable END and gas density while still minimizing the amount of deco. There is no point in diving a 6/94 and raking up loads of deco when you could be diving a 6/75 (yes a 6/94 would give you an amazing END and super nice gas density but there is a trade off). Lastly is to minimize ICD, once again I want Nitrogen in the mix to "flatten" out the nitrogen spike if you are doing gas switches/bailed out, especially when you in the shallows. This is commonly why you see sub 130m dives require your 50% to have 10-15% He in it or you need to do you gas switch at 15m-18m and not 21m.

ICD is probably the most over looked item in dive planning. There are many many many different trains of thought on ICD that you could start an entire thread about and people can argue over, but personally I believe in it and am very very cautious and concerned about it and plan my dives accordingly. You really need to run through all different B/O scenarios at all different depths and loss of gas scenarios and when and where you can use different gasses. I find that lots of times gasses you actually can not plug in at there o2 calculated MOD in reality its actually a lot shallower due to an ICD warning.

All these thing are items that commercial divers are not as concerned about as recreational divers (once again non-commercial diver speaking here so apologies if I am wrong) Yes I'm sure they are taken into account in commercial diving but the environment is so much more controlled and there decent and ascent rates are sooooooooo much slower. Plus once they are fully saturated there desat time I am assuming is a fixed standard procedure so it really does not matter how long to them this is just apart of the work process.

 

[Nick_Radov]

The thermal conductivity of Helium is 5.9 times higher than Nitrogen or Oxygen

You're missing the point. Thermal conductivity is largely irrelevant to respiratory heat loss. Any breathing gas will be heated up to body core temperature in the lungs regardless of conductivity. It's heat capacity that matters.

The reason divers in saturation chambers lose heat faster when surrounded by helium is due to thermal conduction from the skin. That's separate from respiration. And it's not a factor for sport divers because we're not in chambers. If we're surrounded by gas it's going to be air or argon in a drysuit. No one uses helium for suit inflation.

 

[Wibble]

Does anyone use argon for suit inflation?

Edit: Have never seen a drysuit diver using anything but air in their drysuit inflation cylinder.

 

[rx7diver]

Does anyone use argon for suit inflation?

Well, not me anymore. I sold my three suit inflation cylinders a few months ago. (Closed - Three 3,000 psig, 6 cu ft Al Argon Cylinders, No valve)

rx7diver

 

[Akimbo]

You're missing the point.

I get the point you are trying to make but heat capacity is trivial compared to thermal conductivity in diver respiration, except maybe in the warmest tropical water. We are not just talking about sat divers in chambers but also immersed. That is why gas heaters are required below about 600'.

Thermal conductivity is largely irrelevant to respiratory heat loss. Any breathing gas will be heated up to body core temperature in the lungs regardless of conductivity.

Exhalation gas temperature on air is rarely at body temperature unless ambient temperature is high to start with. It is very close on HeO2 even starting with cold gas.

We know that core temperature drops significantly faster on immersed divers breathing HeO2 in hot water suits than air divers. It was not uncommon in the early days of deep bell-bounce and saturation HeO2 dives for divers to keep asking for hotter water to the point that skin burns developed, but they were still quite cold. That doesn't occur in shallower water with richer O2 mixes or on air.

Granted, the heat capacity of Helium goes up slightly at higher compression levels but so does Nitrogen's. Heat capacity is only part of the thermal dynamics calculation but not a big part in this application because the gas volumes are very high and the thermal equalization time is low.

It is a little better on diver-mounted CCRs because there is respiratory thermal conservation in the loop plus a slight increase from the exothermic reaction of the absorbent. The problem in cold water is the thermal transfer across the loop's components is pretty high.

The US Navy ran a series of covert saturation dives on CCRs in arctic waters to tap Soviet communications cables. They had to heat the loop with hot water along with the suits.

Does anyone use argon for suit inflation?

Argon would be more effective in a drysuit that air if you could prevent water vapor (perspiration) from contaminating the thermal conductivity of Argon. Check the chart above. Argon is about 30% less conductive than air but water is 2500% higher.

 

[Wibble]

Argon would be more effective in a drysuit that air if you could prevent water vapor (perspiration) from contaminating the thermal conductivity of Argon. Check the chart above. Argon is about 30% less conductive than air but water is 2500% higher.

Absolutely. However argon is much more expensive than air, is uncommon in dive shops and the benefits are relatively marginal.

 

[Akimbo]

Absolutely. However argon is much more expensive than air, is uncommon in dive shops and the benefits are relatively marginal.

Wibble and I concur in case some readers are unsure. To be clear, the thermal conductivity of Argon in a drysuit with a sweating diver inside is not significantly more effective than vapor laden air in the same suit. Argon use in a drysuit is a classic case where the data sounded good but the application was not thought through. Save the money and hassle of Argon and just use air or Nitrox, whichever you are packing anyway.

It makes more sense to use air to inflate your drysuit than Trimix more because of gas cost than thermal efficiency, though air is a little better even with high humidity.

 

[Dr Simon Mitchell]

You're missing the point. Thermal conductivity is largely irrelevant to respiratory heat loss. Any breathing gas will be heated up to body core temperature in the lungs regardless of conductivity. It's heat capacity that matters.

In theory you are correct, but in practice there are data that suggest it is more complicated than the theory alone. For example, see here (pdf is downloadable). Air environment, respiratory heat loss was greater breathing helium vs nitrogen.

There are many many many different trains of thought on ICD that you could start an entire thread about and people can argue over, but personally I believe in it and am very very cautious and concerned about it and plan my dives accordingly.

There have indeed been many threads and many arguments, but most of it is based on incorrect or half correct perceptions. I am not necessarily criticizing your statement or advocacy for considering ICD in dive planning, but such strong advocacy deserves some balanced commentary because as I point out in the final paragraph of this post, excessive focus on ICD could introduce more risk than it resolves. You might be interested in the NEDU paper I have attached here in which they trialed switches from pure heliox to air (which would cause a rule of fifths calculator to melt down).

Does anyone use argon for suit inflation?

I think some do, but its benefit is questioned. None of the relevant studies have tested it in the very long cold dives undertaken by some technical divers, but there are some discouraging data, see here and here.

Simon M