MultiDeco v4.19

[inquis]

Yes, density scaled by the temperature ratio.

I guess I'm curious why you would want the actual density at 7C or 13C, when the "threshold" values (e.g., 5.2 g/l or 6.2 g/l) are specified at 0C. Those would need to be scaled (on a per dive basis) for a valid comparison. It seems more intuitive to me to compute the density at 0 deg for an apples to apples comparison to fixed values/limits.

 

[nickbutcher]

I guess I'm curious why you would want the actual density at 7C or 13C, when the "threshold" values (e.g., 5.2 g/l or 6.2 g/l) are specified at 0C.

I'm probably thinking too hard about this, but...

I'd assumed that the ideal density of <5.2g/l was an absolute value. Therefore less than 5.2g/l at whatever temperature it's going through the loop.

Which, come to think about it as I write this, is actually quite a bit warmer than the water temperature, given the heat added by the lungs and via the chemical reaction in the stack.

Maybe I'll ping Simon an email and ask for clarification about that (unless it's actually in some paper/presentation that I've missed).

 

[tursiops]

Sorry, I'm afraid I can't speak to the Mac version. (I was looking at the Android version.)

I see no place in the WIN version to set a temperature for the gas density either,nor does it tell me what temperature is used for the calculation.

 

[inquis]

I'd assumed that the ideal density of <5.2g/l was an absolute value. Therefore less than 5.2g/l at whatever temperature it's going through the loop.

I don't believe that's accurate. Anthony and Mitchell recommend 5.2 g/L (ideal max) and 6.2 g/L (absolute max) in "Respiratory Physiology of Rebreather Diving", 2016. They then state:

Implementation of such a recommendation will require an appreciation of how to calculate gas density for a given respired gas at a given depth. Such calculations begin with knowledge of the density of air and the individual components of gas mixes at 1.0 ATA [...]
Table 1: Gas Density (g·L-1 ) Hydrogen 0.090 Helium 0.179 Nitrogen 1.251 Oxygen 1.428 Air 1.293

Those are 0C constituent densities, but no one dives in 0C water (certainly not fresh water ). Later they use those to compute the density of 16/50 and compare it to the 5.2 g/L (ideal max) and 6.2 g/L (absolute max). I assume the authors made a legit comparison and infer the guideline values must also be referenced to 0C.

If you were diving in 20C and calculated the 20C-based density of your gas at 6.0 g/l, it would be an error to think that's within their guidelines: the 0C-based density of that gas is 6.44 g/l and over the absolute max.

 

[nickbutcher]

I don't believe that's accurate. Anthony and Mitchell recommend 5.2 g/L (ideal max) and 6.2 g/L (absolute max) in "Respiratory Physiology of Rebreather Diving", 2016. They then state:

Those are 0C constituent densities, but no one dives in 0C water (certainly not fresh water ). Later they use those to compute the density of 16/50 and compare it to the 5.2 g/L (ideal max) and 6.2 g/L (absolute max). I assume the authors made a legit comparison and infer the guideline values must also be referenced to 0C.

If you were diving in 20C and calculated the 20C-based density of your gas at 6.0 g/l, it would be an error to think that's within their guidelines: the 0C-based density of that gas is 6.44 g/l and over the absolute max.

I'm not convinced. Surely we're concerned with the physiological consequences of what's happening at the prevailing conditions.

Car engines (those using fossil fuels at least) certainly perform better with cooler air and that's a direct result of the greater air density. They don't perform the same with 20c air as they do with 0c.

Drinking a cup of water at 0c is fine but chugging the same cup at 100c wouldn't end well.

We are somewhat off-topic now and all this good stuff is probably worthy of a separate thread.

 

[inquis]

Surely we're concerned with the physiological consequences of what's happening at the prevailing conditions.

Unquestionably. The issue, however, is how the testing/guidelines were reported. Testing was obviously done in water warmer than 0C. They found the inflection point in density where issues rapidly increased. Did they a) report the actual density experienced at the ambient temperature or b) the density of the gas if it were cooled to 0C.

I maintain it is (b). This is consistent with the walkthrough calculation in the paper. It is also consistent with the common practice of scientists to map things to a standard temperature and pressure. For example, a mole of gas occupies 22.4 L at STP. If you want the volume at other conditions, you have to scale it yourself. Here, the density guidelines of 5.2 or 6.2 g/L are specified at standard temperature (0C). If you want to know the limiting densities at your 10C conditions, you need to scale them yourself. Or just save time and compute the 0C equivalent density of your gas and compare directly (e.g., as your Shearwater computer does).

@Dr Simon Mitchell, if you have a moment, would you please clarify?