PADI dive table question

[boulderjohn]

Muddiver is correct on this. While diving when you are cold is no fun and has many problems, the reason in reference to the tables is to the increased risk of DCS.

If you look at all the potential factors that increase the risk of DCS, the most common factor is circulation. An important part of the process of releasing nitrogen is the efficiency with which the body circulates blood. When you are dehydrated, for example, your blood does not flow as well.

When you are cold, your body limits circulation to your extremities in order to preserve core warmth.

There can be no clear definition of cold because it depends so much on thermal protection. If I am diving 55 degrees with my dry suit and my heavy underwear, I will be toasty warm for quite some time. If I am in a 3 mm wet suit, I will be shivering in minutes.

You have to use some judgment with this.

 

[Crush]

Many of these topics are covered in http://www.scubaboard.com/forums/ba...04882-differences-warm-cold-water-diving.html. Basically, adding 4 m to your dive profile is a way of limiting your time underwater in cold conditions so as to minimize your risk of DCS. Adding depth is not necessary if you and your extremities have not cooled, but be careful - you can easily cool off to a point where a depth penalty is appropriate but you might not notice that you have cooled.

To reiterate what others have written, at the beginning of a dive you are more likely to be warmer than at the end of a dive. Thus, "ongassing" should occur at a near-normal rate as you begin to dive. Near the end of your dive you will be colder, and blood-flow in the extremities will be poorer. The effect of cooler fluids (in cold limbs) holding more dissolved N2 is negligible. However, the decreased perfusion (i.e., delivery of arterial blood to [and from] a capillary bed) will result in your body off-gassing more slowly than at normal body temperatures, increasing the likelihood of the bends. The obvious things to do would be to stay warm, call the dive when you being to feel like you are getting too cool, dive a conservative profile, and take your time when surfacing. Strenuous activity just prior to or during a dive also increases the likelihood of DCS owing to the formation of microbubbles.

As for how this relates to your dive plan: "Cold can translate to higher-than-usual air consumption and affect your motor coordination and mental capacity. It can also cause an increased propensity toward decompression illness and oxygen toxicity.... Thermal protection by a wetsuit, drysuit or other survival-type suit will dramatically lessen the immediate effects, but no matter where you dive, heat loss will still occur over time, and eventually you can get chilled. This can lead to hypothermia, a dangerous condition for any diver." See "Cold-Water Protection: California Diving During the Early 1950s" at Alert Diver Online for more information.

Also, check out these two articles:
Divers Alert Network : Alert Diver Articles
Divers Alert Network : Alert Diver Articles

The author of article 292 (Neal Pollock) has also published an article "Cold stress complicates decompression stress" in dirQuest, vol. 4(1), pp. 14-16, 2003. He generously provided access to this article at: http://rubicon-foundation.org/Download/Pollock/Cold_Stress_Complicates_Deco_dirQuest2003_4_1 14-16.pdf

Stay warm!

 

[TMHeimer]

Lots of variables, as stated. 60 F for me in my 7 mil is Caribbean warm. 50s as well. On a warm day for the SI, mid to high 40s is fine. Besides personal tolerance, I would think depth would be a biggee. If you're only diving to 30 ft as opposed to 100 you are ongassing much less nitrogen, so planning that dive as if it were 40 ft probably is way less important than planning the 100 ft one to 110 (assuming you're bottom time at 30 is not 2 hours).

 

[Peter Guy]

If you're only diving to 30 ft as opposed to 100 you are ongassing much less nitrogen

Just to be a bit picky, Not really.

On gassing is a combination of Depth (partial pressures) AND Time.

A short deep dive may well have much less on-gassing than a long, shallow one -- not to mention that the compartments which may control may (will) be different.

 

[TMHeimer]

That's why I said I'm assuming the 30' bottom time is not 2 hours (PADI NDL for 35' being over 3.5 hrs.). A dive to 100 feet to the limit of 20 minutes basically fills the 5 min. compartment. An hour's dive to 30 feet probably doesn't come close to filling the 120 min. compartment (I may be off on this a bit?). So with a reasonable bottom time at 30 feet in cold circumstances I would think it's less important to plan the extra 10 feet on the tables than it would be going to 100 feet, which normally means, say, a 15-17 minute dive on air (closer to the NDL).