Why does PADI recommend planning cold water dives 4m deeper?

[Puppetkon46]

Sorry if this topic has already been addressed, but this PADI recommendation has me curious. Why is it that you are supposed to plan cold water dives as if they were 4m/10ft deeper than your actual dive? I'm more curious about the science behind it than anything else. Have they done experiments to come to this conclusion? Cold water diving might sting a bit at first when not wearing gloves, but I don't really have a problem with it, so other than comfort preference what are the potential dangers associated with not following that rule? And what temperatures actually qualify for this rule? Thanks for any information you can provide as this sort of does concern me for future planned trips to Lake Superior.

 

[darushin]

I am not a cold diver, but My first hunch would be because your body will be cooler and thus less able to absorb a little less gas.

daru

 

[chrpai]

Cold water decreases circulation in the extremities and as a result you don't off gas as efficently. There fore we plan the dive as deeper that way we increase our margins.

 

[DevonDiver]

You start off warm - normal circulation - absorb nitrogen at a typical rate during descent and bottom time.

You get colder - decreased circulation to extremities and surface areas - release nitrogen (from those areas) at a less than typical rate during ascent.

That mis-balance between on-gassing and off-gassing creates a greater disposition to DCS. That issue is compensated for by making the dive planning more conservative.

When dive planning with the RDP, you make a dive more conservative by adding a hypothetical 4m depth increase when planning - the effect of which is shorter dive times at a given depth.

On a dive computer, you could achieve a similar outcome by increasing the conservative factor in the settings.

 

[pasley]

PADI, NAUI, HSA and as far as I know all other Certification agencies all recommend adding depth to your actual dive depth when diving in cold water. Why, I really don't know. But as others have stated we believe it has to do with on and off gassing and the body moving the blood supply around to protect the body. This same logic is why they recommend avoiding hot water (spa, jacuzzi, sauna) after a dive because you are moving the blood supply from the deep tissues where it deposited nitrogen during much of the dive to the surface tissues and because warm promotes bubble formation and offgassing as is evident by the off gassing of a warm sodapop Vs a cold sodapop.

 

[Hawkwood]

...
On a dive computer, you could achieve a similar outcome by increasing the conservative factor in the settings.

Or for those without any settings, leave more NDL time.

 

[TMHeimer]

I too have learned that is has to do with off gassing. I would imagine the 4M/10' deeper rule is very dependent on depth. If I'm doing a dive shallower that 15M/40' I don't figure in the extra 10' no matter what the water temp. Not sure if I am correct in doing this.

 

[shoredivr]

Cold water diving might sting a bit at first when not wearing gloves, but I don't really have a problem with it, so other than comfort preference what are the potential dangers associated with not following that rule? And what temperatures actually qualify for this rule? Thanks for any information you can provide as this sort of does concern me for future planned trips to Lake Superior.

If you are planning on diving any of the Great Lakes, especially Lake Superior, though you might have warmer water above the 30- 60 ft thermocline - say in the mid 60 degrees F, once you get below the thermocline it drops down to 42 to 48, and colder in Lake Superior, at this time of year. Come fall or spring, you lose the surface warm water. You'll want gloves for sure at a minimum. Drysuit is my choice YMMV.

 

[Scuba_Noob]

Cold water diving might sting a bit at first when not wearing gloves, but I don't really have a problem with it, so other than comfort preference what are the potential dangers associated with not following that rule?

Depends on what you think is cold. I figure, since wet gloves aren't expensive, why not be comfortable and wear them? I don't think the planning 4m deeper has anything to do with comfort. As most of the answers say, it's another method to be conservative and reduce the probability of decompression sickness.