Cold water and computers

[Bob DBF]

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.

Considering how clear a lot of manuals are, it might be trying to say: No-stop time is calculated on line (not with all variables). No-stop time can also be influenced by the current workload and current water temperature, take that into consideration when planning.

When someone writes, they know what they want to say and what they write makes perfect sense to them, but oft times is confusing to the reader. You can find examples of that all day long on ScubaBoard.



Bob
----------------------
I may be old, but I’m not dead yet.

 

[Mr Carcharodon]

Here’s a couple quotes from the Galileo Luna manual (with emphasis added). Care to re-write these to what you think they mean? I would be hard pressed to say they are not using temperature as a factor in RBT calculations.

“The RBT calculation is based on your current breathing rate, accounts for any existing and upcoming decompression obligation and for any temperature gradient in the water.”

“Dive planning is done using the following as basis:
- selected oxygen concentration
- selected water type
- selected microbubble level
- water temperature of the most recent
dive
- altitude range (if any)
- status of saturation at the time the
planner is started
- a normal workload of the diver and
observance of the prescribed ascent rates.”

 

[RonR]

Uwatec does say they take temperature into account- it would have to be by adding some factor that increases the underlying conservatism in cold water, trying to account in the model for lowered circulation to extremities and skin. That would amount to a more sophisticated, digital version of the "add 10 feet" rule.

I have always been skeptical about the value of water temperature as an input to the algorithm- beyond jazzing up marketing copy, anyway. The temperature of the diver is what matters, not the water temperature. And the dive computer has no idea what the diver's temperature is. A diver with good insulation can be very warm, even overheat, in cold water, and a diver with too little insulation can become hypothermic in quite warm water. Individuals also vary quite a bit in their response to cold. You could argue that it is useful as a rule of thumb, if some assumptions are made about how divers are generally protected from cold. But I think it adds to the general impression that decompression algorithms are highly precise, and that just is not the case.

Ron

 

[Insta-Gator]

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.

Agree. I would also take a longer SI than required to try to get my core temp up before the second dive.

 

[RTee]

If diving a Suunto (one of my computers is a Vyper), this is how the DC would compensate for cold water but only based on your input...ie by adjusting the Personal Adjustment sub-menu from P0 to P1 or P2 (if more than one condition applies to the dive in question). In other words, if you do not change the Personal Preference, the DC will not adjust the conservatism level solely on the water temperature it is recording.

The personal factors which tend to increase the possibility of DCI include, but are not
limited to:
• cold exposure —water temperature less than 20 °C/68 °F
• below average physical fitness level
• fatigue
• dehydration
• previous history of DCI
• stress
• obesity
• patent foramen ovale (PFO)
• exercise on or after dive
This feature is used to adjust the computer to be more conservative, according to
personal preference, by entering the suitable Personal Adjustment setting with the
help of
Table 5.5, Personal Adjustment settings. In ideal conditions, retain the default
setting, P0. If conditions are more difficult, or any of the factors which tend to increase
the possibility of DCI exist, select P1, or even the most conservative P2. The dive
computer then adjusts its mathematical model according to the entered Personal Adjustment
setting, giving shorter no-decompression times.

 

[hmarashi]

Learn something new everyday. I primarily dive cold water here in SoCal. The temp tonight at 60' was 50-52 deg. I dive wet with a 8mil suit.
I know if you're cold you will go through your air faster, but never thought about adding depth to the profile to compensate for temp.

 

[NCadiver]

Good question DayBob,
Computer and dive table work differently from each other table use a square profile where as a computer credits your nitrogen load as you ascend.
Let say you use tables for a 100ft for 15min dive you have 5min left on your dive profile. On the computer you ascend to 50ft now you have at least 20min of dive time left because the computer is crediting you for shallower depth.
You can always turn the conservative factor on in the dive computer if you are worried about the cold water but I don’t think the book or tables tells you what is cold water.

I guess next year feature the computer will come with thermometer that you swallow and transmit your core temp to the computer. So do you recycle or buy new thermometers.:shocked2:

 

[Rred]

Seeing that this is still somewhat unanswered...Even in the 1980's "no one" in the Northeast US believed that the USN tables were specifically "warm water tables". I had been taught (by a very good ex-frogman who started in WW2) that the USN had ":cold water tables" and that if we needed a wetsuit, we needed to be using the cold water tables--not the "normal" ones.

I had a chance in the 80's to ask a USN Chief Diving Medical Officer about this. I said, am I crazy? Or is the entire industry up here nuts? And he said "If you need to wear a wetsuit, you SHOULD NOT BE USING THE TABLES. They are designed for warm water and you should be using the cold water tables." Which are basically the same tables, but adding one safety group, i.e. adding an extra ten feet to your depth, or going back one extra time column, or even doing both if you want to be more conservative, because the USN tables are not "safe". They are designed to allow for a reasonable number of combat losses in perfectly healthy vigorous young divers in peak condition.

What the USN consider acceptable losses, might be unacceptable to many civilian sport divers.

So with a modern computer, that has no integration of thermal factors? That's what the (typically three) levels of additional "personal" factor are for. If you are wearing a wetsuit, you need to add one bump. If you are somewhat older, add two bumps. How an industry that is so concerned with liability and litigation continues to ignore this issue and basically go against the tables, eludes me.

Anyone who has a problem with you doing that, should be encouraged to just dive without limits, tables, or computers.

FWIW, the current PADI tables *now* also finally note that in cold water, an extra group shift (time or depth) should be added for safety.