Rapid Descent Rate
- Thread starter RobPNW
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@tursiops ' reply might have gotten lost in the follow-up discussion about weights, but it bears repeating. The biggest concern with a fast descent rate would be some sort of barotrauma.
@Edward3c also brought up an interesting point about descent rates and printed dive tables - if they assume a certain maximum descent rate and a diver exceeds that, there's going to be a theoretical risk of ongassing more quickly than the tables predict. AFAIK that's a theoretical risk.
Best regards,
DDM
What I am about to describe does not apply to NDL dives--just deeper technical dives.
When you plan your dives using dive planning software or other similar means, that plan assumes a specific rate of descent. The time of descent is included in your planned bottom time. A typical planned rate of descent is 50 FPM. If you descend to 300 feet at 50 FPM, you will arrive there at 6 minutes, so for the first 6 minutes of your dive, you will have been at an average depth of about 150 feet. If you instead descend at 100 FPM, you will get there at 3 minutes, so for the first 6 minutes of your dive, you will be at an average depth of 225 feet (3 minutes at 150 and 3 minutes at 300).
That creates a significant difference in decompression obligation for the dive. If you are using a computer, the computer will know that, but your gas needs will be different from what you had planned. If you are not using a computer, you may not realize the difference and go with the decompression schedule you had planned, not realizing it is no longer appropriate.
When you plan your dives using dive planning software or other similar means, that plan assumes a specific rate of descent. The time of descent is included in your planned bottom time. A typical planned rate of descent is 50 FPM. If you descend to 300 feet at 50 FPM, you will arrive there at 6 minutes, so for the first 6 minutes of your dive, you will have been at an average depth of about 150 feet. If you instead descend at 100 FPM, you will get there at 3 minutes, so for the first 6 minutes of your dive, you will be at an average depth of 225 feet (3 minutes at 150 and 3 minutes at 300).
That creates a significant difference in decompression obligation for the dive. If you are using a computer, the computer will know that, but your gas needs will be different from what you had planned. If you are not using a computer, you may not realize the difference and go with the decompression schedule you had planned, not realizing it is no longer appropriate.
pauldw
Contributor
If I'm weighted well, I barely start down upon emptying my BC, and the descent is slow, period. It can't be fast (unless I invert and swim down). That's fine, because I get barotrauma regularly so I don't like to descend fast anyway. Once I was with a buddy in a class, and when she dumped her BC she plummeted. She was out of sight into the gloom in seconds. I can invert and try to swim hard after someone who is overweighted, but it's far from ideal (and it also makes mask clearing difficult on the way down). You fast descenders: don't leave us behind!
Astrobratt
Registered
I recently trained on the Hydro Atlantic on Pompano Beach, where we used a negative entry. We were diving with double steel tanks plus a deco aluminum 80. This entry requires negative buoyancy and immediate descent to the wreck. My computer recorded a descent rate of 108 feet per minute, I feel like the biggest danger here is not being able to equalize my ears on descent. But besides that, we could complete this rapid descent without any issues.
I'm curious about the assumption of a certain descent rate and impact on bottom time calculations. It doesn't make sense to me. What if you don't descend to your deepest depth immediately? It seems like the computer would dynamically adjust it's calculations based on your actions in real time as opposed to making assumptions.
Yes, it's not an issue with a functional computer on your wrist.
Those who use tables or pre-planned profiles (even as backup) need to consider any deviation from the various assumptions. Some differences make the plan more conservative, others (like a faster descent rate or a slower ascent when in deco) make it worse.
Yes, for computer users, the computer will adjust, but it can still be an issue. When planning a technical dive, you preplan how much gas you will need, and you carry an appropriate amount (with reserve). In the example I used, you would use more gas than planned on the deepest part of the dive, and you will use more on deco because of your longer deco times. You should have more than enough in your planned reserve, but if you have an emergency that requires that reserve, you may not have enough.
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