Cold water and computers

[DayBob]

I was reading the skin bends thread in the A&I section and the comment about water temp got my attention. During my training I was told to add 10ft. to a dive when diving cold water. Do computers account for water temperature or is it something I should keep in mind when doing cold water and just be conservative? I looked in my computer manual and saw no reference to cold water diving.

 

[hmarashi]

I have never added or been told to add depth for cold water, but if that somehow becomes a factor, a computer is able to take the environment (depth, time, temp, etc ..) into consideration for its profile.

 

[DayBob]

After some reading, page 220 of my PADI OW Dive Manual states "When using the RDP for planning a dive in cold water...plan the dive for 10 feet deeper than it actually is."

 

[Mr Carcharodon]

There are a few computers that say they do take temperature into consideration. But how does that work if the computer does not know what exposure protection you are wearing? Would your body temperature be different in a 3 mm shorty vs. a dry suit with 400 gram thinsulate? This seems like another case of dive computers being too smart by half.

The core idea here is that on gassing and off gassing to most tissues is driven by blood flow. If you get cold the blood flow to your skin and muscles can drop dramatically. So for a cold dive you will on gas relatively rapidly at the beginning while you are still warm, and off gas more slowly than expected as you get cold. That can increase your bends risk. Of course the best solution is to wear exposure protection that will keep you warm throughout the dive.

 

[TSandM]

That's a good question. I honestly don't know if the algorithms on any of my computers are set up to make any adjustments for cold, but I rather doubt it. I would simply deal with it by not pushing computer limits at all in cold water -- moving up in the water column before I got within 5 minutes of NDL, for example.

 

[Kern]

I have never added or been told to add depth for cold water, but if that somehow becomes a factor, a computer is able to take the environment (depth, time, temp, etc ..) into consideration for its profile.

Most computers use depth & time to decide on N₂ on/off gassing.

For those few that use environmental factors such as ambient temp, how do they know what sort of environmental protection I'm using?

 

[Mr Carcharodon]

Well here's one that uses temperature. From section 4.11 on Decompression Information in my UWTEC manual: "No-stop time is calculated on line and influenced by the current
workload and current water temperature". While those seem like reasonable things to consider it is unclear that a computer can reasonably take them into account.

 

[DayBob]

True, my computer doesn't know what protection I am wearing but the OW manual makes the blanket statement to add 10 ft. I was just wondering if computers used the "add 10ft" rule.

 

[Mr Carcharodon]

I am afraid they are far more random than that. It's pretty clear that not all computers follow DSAT rules, some of them sometimes, sometimes never. Why would you add 10 feet if you are wearing exposure protection that is keeping you warm? But if you are cold adding some conservatism is a reasonable thing to do.

 

[Thalassamania]

No to the "add 10 feet" rule, that's not how computers work. Some (very few) make the deco model more conservative when there's cold water, others permit you to dial in a conservatism factor, if your's doesn't you can get the same thing (more or less) by making sure that you never approach the no-D limit by less than 10 minutes.