Decompression planning and cave diving

Nirvana · Apr 25, 2013

This is a question about a type of diving I am not (but hopefully will be sometime next year) qualified to make, but about I have been thinking nevertheless.

It is my understanding that dives made in caves do not usually correspond to a square profile, or even a multilevel profile with readily identifiable levels. Therefore, how does one go about planning required bottom and decompression gases? It doesn't seem to be a case of simply following the thirds rule.

I appreciate any light shed on the subject, and would like to make clear the information I seek is not intended to be used to make any dives I do not have formal training to do, but merely to satisfy my curiosity.

cephasiii · Apr 25, 2013

Yep, you will get flamed. Sadly like all people who ask questions out of curiosity.

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Nirvana · Apr 25, 2013

cephasiii:
Yep, you will get flamed. Sadly like all people who ask questions out of curiosity.

There is always hope an intelligent answer will come by. I've seen a fair number of smart posts here on scubaboard.

Besides, I believe that if anyone thinks this topic should not be discussed he or she can simply choose to ignore it.

PfcAJ · Apr 26, 2013

Average depth. Max depth of cave is shallower than or equal to your bottom gas MOD. Deco gases left at cave entrance and are dependent on cave avg. depth and bottom time.

Lorenzoid · Apr 26, 2013

Nirvana:
. . .
Besides, I believe that if anyone thinks this topic should not be discussed he or she can simply choose to ignore it.

Unfortunately, people don't ignore it and, as you saw in your other thread where you asked a purely theoretical question, quickly muddy the original question to the point where those of us who would actually be interested in a theoretical discussion lose interest. I wish the people who feel compelled to fight each other over tangential issues would not fill the thread with rubbish that the rest of us then have to sift through.

Nirvana · Apr 26, 2013

PfcAJ:
Average depth. Max depth of cave is shallower than or equal to your bottom gas MOD. Deco gases left at cave entrance and are dependent on cave avg. depth and bottom time.

Thanks for the reply. But how do you know what the time-averaged depth will be before the start of the dive?

PfcAJ · Apr 26, 2013

Ask around or look at maps for depths, and that plus how much gas you're carrying and SAC rate will let you know how long your bottom time will be.

rjack321 · Apr 26, 2013

Nirvana:
Thanks for the reply. But how do you know what the time-averaged depth will be before the start of the dive?

Its heavily dependent on the profile of the cave and how much gas you have with you. Depth + SAC rate + volume you have will put an upper limit on dive time. That dive time will vary with average depth and actually the "worst" depth for deco ends up being a fair bit shallower than the MOD of whatever gas you've chosen. That is, you accumulate the most deco a little shallower than you expect from just the MOD and max depth of the cave because your consumption is lower and you stay longer. Drop enough deco gas to cover this maximum deco time or cut your penetration time short.

ucfdiver · Apr 29, 2013

Nirvana:
This is a question about a type of diving I am not (but hopefully will be sometime next year) qualified to make, but about I have been thinking nevertheless.

It is my understanding that dives made in caves do not usually correspond to a square profile, or even a multilevel profile with readily identifiable levels. Therefore, how does one go about planning required bottom and decompression gases? It doesn't seem to be a case of simply following the thirds rule.

I appreciate any light shed on the subject, and would like to make clear the information I seek is not intended to be used to make any dives I do not have formal training to do, but merely to satisfy my curiosity.

Well, you start with what you know.

Most swim cave dives are 40-50fpm. So if you want to go somewhere that's 1000ft in, look at the average depth on the map and then bring enough gas for 1000/40fpm = 25min penetration gas (so 3x that number to adhere to 1/3rds).

If you know the average depth is around 80ft, and you're going 3000ft in at 40fpm, you know that you'll end up doing 150min at 80ft. With an o2 bottle, that's going to require 9min of backgas deco at apprx 30ft, and 24min o2 deco. Since this back gas isn't part of penetration gas, you subtract it before you calculate your 1/3rds. In the case of an o2 bottle failure, some people consider the 1/3 as counting towards the gas you would use to compensate for a failed o2 bottle, others don't-- by the time you're doing that long of a dive you should be able to decide for yourself.

When you cannot find a map, usually a local shop can tell you, or if it's unknown, other local caves can be used to have a somewhat accurate estimate. Most caves in Tallahassee eventually get to 300ft, with lots at 150-190ft, but there are a select few which never go below 60ft. In Marianna nearly all caves are <100ft depth. In Mexico, most are 10-20ft depth, but there are a select few which hit 70-- again you can usually find this out by asking the shop first.

Nirvana · Apr 29, 2013

ucfdiver:
Well, you start with what you know.

Most swim cave dives are 40-50fpm. So if you want to go somewhere that's 1000ft in, look at the average depth on the map and then bring enough gas for 1000/40fpm = 25min penetration gas (so 3x that number to adhere to 1/3rds).

If you know the average depth is around 80ft, and you're going 3000ft in at 40fpm, you know that you'll end up doing 150min at 80ft. With an o2 bottle, that's going to require 9min of backgas deco at apprx 30ft, and 24min o2 deco. Since this back gas isn't part of penetration gas, you subtract it before you calculate your 1/3rds. In the case of an o2 bottle failure, some people consider the 1/3 as counting towards the gas you would use to compensate for a failed o2 bottle, others don't-- by the time you're doing that long of a dive you should be able to decide for yourself.

When you cannot find a map, usually a local shop can tell you, or if it's unknown, other local caves can be used to have a somewhat accurate estimate. Most caves in Tallahassee eventually get to 300ft, with lots at 150-190ft, but there are a select few which never go below 60ft. In Marianna nearly all caves are <100ft depth. In Mexico, most are 10-20ft depth, but there are a select few which hit 70-- again you can usually find this out by asking the shop first.

That was very helpful!:cool3:

Thank you.

Decompression planning and cave diving

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