SAC Rate and the effects of bottom temperature?

[TheTrickster]

Here I am working out my SAC rates for a few recent dives and then thought about the effects on temperature on them.

I have my SAC calculations nailed (I hope), but I'm wondering if I could/can some how see if the bottem temp has effected them, so next time I go diving in cold water I can see roughly what I need.

For example:

Surface temp: 27'c
Bottom temp: 15'c
(sp) Starting pressure: 210bar (taken on the surface before decending)
(ep) Ending pressure: 90bar (taken on the surface after the dive)
(ts)Tank Size: 15ltr
(d)Average Depth: 21m
(t)Dive Time: 42mins

(((sp-ep)*(ts))*t)/(d/10)+1

I get a SAC rate of 14.16. I was thinking the colder it is, the higher the SAC rate would be, however using Charle's law I get a lower SAC rate (due to the slower molecue movement)

Any ideas?

 

[The Kraken]

Interesting work.
About the only thing you can do, without a massive and complex amount of data manipulation, is to generate a matrix of numerous dives, group the dives into subgroups which have as close to the exact same dive profile as possible, with exception of temperature, and then try to find a relationship of the dive profile with the temperature being the only variable.

Lots of luck . . .

the K

 

[Charlie99]

If you have the right thermal insulation for the temperature, then the effect on SAC shouldn't be significant.

If the water temperature affected your breathing gas in some way, then it would change the SAC. The air in the tank has the same mass, independent of tank temperature, so that doesn't have any direct effect.

If we didn't warm the air to body temp, 0C/273K/32F air is about 9% more dense than 27C/300K/81F air so even if the gas we were breathing was at ambient (and it isn't) the change would only be 9%. The body does both heat and humidify the gas we breathe, so the temperature of actual gas breathed is relatively independent of ambient temperature. If one desired, you could work out the energy involved in warming 5 pounds of air up to body temperature over the length of a dive and figure out how much oxygen that is going to use, but I doubt that it is going to be dramatic.

I'll bet that normal dive-to-dive variations in SAC overwhelm any temperature related effects.

 

[Luc Dupas]

My SAC definitely varies over the year as a function of water temperature !
Mid summer, when the sea water is at a mild and comfortable 21C (70F) my SAC drops down to the 14 liter/min range... and in autumn when the water temperature reaches the 10C (50F) level the SAC already goes up to the 17-18 liter/min range... and towards december when the water temperature goes down to just a few degrees (<40F) my SAC raises to about 22 liter/min (all with a 7mm one piece wetsuit!).
When the water then really gets cold I step over to my 7mm neoprene wetsuit and my SAC drops down to about 17 liter/min... even at 1C (34F) sea water temperature...
In spring time when the water temperature comes close to the 10C (50F) mark I switch over again to my wetsuit and the first dives the SAC jumps up again to the 20-22 liter/min level and gradually when the water temperature raises again the SAC drops down to the 14 liter/min range.