I did some investigating last year as I was trying to figure out if the difference between running 21/35 and air was in my head or real.
One of the first thing I noticed is that you'll often see people discuss how efficient a gas is at retaining heat based on its thermal co-efficient... You normally end up with something like:
You'll then normally get told "so use Argon because you won't cool down as fast".
What I didn't understand was what happened when you mix two gasses together. After a bit of research on rubicon and other places, I found a couple of answers...
Firstly, mixing gases together has an effect on thermal coefficients. While the relationship between gas makeup and co-efficient is not linear, in the case of Argon and Air, you'll end up with something roughly in the middle if you mix them, so in that case, you would already have lost half the advantage given by Argon.
The second issue to consider is what does that gas co-efficient actually mean? This is where I haven't been able to find any references that actually explain what's going on, so I now have to resort to pure guessing. (I have no idea if the following is correct but no one I've asked has been able to tell me whether the following has any basis in reality or not. So if anyone knows, please share)
The temperature that you'll feel confortable in will depend on how much clothing you're wearing and your activity (ie, think about how warm you feel running vs just lying down).
There's a bunch of tables on the Internet that give you some ideas, but I nothing that made me think "Aka! that's it!". So I made an excel spreadsheet showing a target temperature - the ambient water temperature and multiplied that different by different coefficients. I then calibrated that judging how cold I felt between when I had a mix with some helium in it in my suit, vs when I was running air in my suit, and checking which temperature ranges gave me the same co-efficients (ie, 22C in the suit with Trimix, feels about the same as 20C with Air), and the coldest I've ever been in my drysuit was the day I went to 16C on Trimix, which felt colder than 13C on Air. This is obviously mostly guess work as it will depend on how much you're moving around etc, but the numbers don't seem totally incorrect, and it gives me a target temperature of 26C, which seems plausible as well. Trying to fill this spreadsheet out with other people gives me other temperatures, but within the 25C-32C range which seems to make sense to me. Everytime you put on a thicker undergarnment, better hood and similar, you will be bringing down your target temperature.
What this means is that the further away the water temperature is from the target temp (in my case 26C), the bigger the difference between different mixtures, by the time you get to 10C, there's a marked difference. 10C in Air works out the same as 4C in Argon. I've never dived below 12C so I can't comment, but it at least helps explain why there's so many different opinions. It also explains why when people start wearing thicker undergarnments, they start saying that they don't notice a difference.
I fear that getting a better opinion may involve rectal thermometers, and I have so far found no one willing to be either a test subject or a test administrator, so I'll leave it at that.
Hopefully this post will be helpful in at least getting someone started in finding the right answer.
Pasc
One of the first thing I noticed is that you'll often see people discuss how efficient a gas is at retaining heat based on its thermal co-efficient... You normally end up with something like:
- Helium: 0.142
- Air: 0.024
- Argon: 0.016
You'll then normally get told "so use Argon because you won't cool down as fast".
What I didn't understand was what happened when you mix two gasses together. After a bit of research on rubicon and other places, I found a couple of answers...
Firstly, mixing gases together has an effect on thermal coefficients. While the relationship between gas makeup and co-efficient is not linear, in the case of Argon and Air, you'll end up with something roughly in the middle if you mix them, so in that case, you would already have lost half the advantage given by Argon.
The second issue to consider is what does that gas co-efficient actually mean? This is where I haven't been able to find any references that actually explain what's going on, so I now have to resort to pure guessing. (I have no idea if the following is correct but no one I've asked has been able to tell me whether the following has any basis in reality or not. So if anyone knows, please share)
The temperature that you'll feel confortable in will depend on how much clothing you're wearing and your activity (ie, think about how warm you feel running vs just lying down).
There's a bunch of tables on the Internet that give you some ideas, but I nothing that made me think "Aka! that's it!". So I made an excel spreadsheet showing a target temperature - the ambient water temperature and multiplied that different by different coefficients. I then calibrated that judging how cold I felt between when I had a mix with some helium in it in my suit, vs when I was running air in my suit, and checking which temperature ranges gave me the same co-efficients (ie, 22C in the suit with Trimix, feels about the same as 20C with Air), and the coldest I've ever been in my drysuit was the day I went to 16C on Trimix, which felt colder than 13C on Air. This is obviously mostly guess work as it will depend on how much you're moving around etc, but the numbers don't seem totally incorrect, and it gives me a target temperature of 26C, which seems plausible as well. Trying to fill this spreadsheet out with other people gives me other temperatures, but within the 25C-32C range which seems to make sense to me. Everytime you put on a thicker undergarnment, better hood and similar, you will be bringing down your target temperature.
What this means is that the further away the water temperature is from the target temp (in my case 26C), the bigger the difference between different mixtures, by the time you get to 10C, there's a marked difference. 10C in Air works out the same as 4C in Argon. I've never dived below 12C so I can't comment, but it at least helps explain why there's so many different opinions. It also explains why when people start wearing thicker undergarnments, they start saying that they don't notice a difference.
I fear that getting a better opinion may involve rectal thermometers, and I have so far found no one willing to be either a test subject or a test administrator, so I'll leave it at that.
Hopefully this post will be helpful in at least getting someone started in finding the right answer.
Pasc
