Rock Bottom Pressure and Turn-Around Pressure

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FPDocMatt

Contributor
Messages
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Location
Middletown, Maryland, USA
# of dives
25 - 49
I just read this excellent article on gas management by Grateful Diver.

It looks like turn-around pressure is for shore dives and rock bottom pressure is for boat dives. Is this correct?

I understand how to calculate turn-around pressure.

But rock bottom pressure is a bit much for me to handle. Does it have to be that complicated? Is there a simpler way to figure this out? What if I don't know my air consumption rate yet?

It looks like the time it takes to ascend can be considerable. I mean heck, if you have to start ascending when your tank is only half empty...
 
The type of dive doesn't matter. The idea is to make sure that you have enough gas to ascend while supplying air to you AND your dive buddy, under stress, while completing mandatory safety stops. That takes a bit more air than the "back on the boat with 500PSI" that people are commonly taught in OW class.

If you do not yet know your SAC, then you cannot do the math. Your only option, until you can figure your own, is to use a figure like .75, which is a not at all uncommon SAC rate for new divers. Personally, I'd go through my log books, which should probably tell you how much gas you had when you got out of the water, and calculate your SAC rate. Then add a buffer. Remember that if you're diving AL80 with 3000PSI, you're actually starting with (most likely) 77.4CF, not 80.
 
The concept of rock bottom / min gas is pretty simple (i.e. "reserve enough gas to get you and a buddy up from depth doing a controlled ascent and stops"). The details are perhaps a bit confusing initially, but you can certainly glean some more generalized rules after doing a few calculations.

For my teams, we've basically settled on:

100' - 40cf
80' - 30cf
60' - 20cf
Shallower - 500psi (we dive at least 100cf cylinders)

Our math assumes 1 min at max depth to handle the OOG, an ascent to half depth at 30'/min, and 1 minute stops from there to the surface, all using a very conservative 1cf/min/ata/diver consumption rate.

I find it best to do all calculations in cf, and then just convert to cylinder pressure (here's where tank factors come in handy; perhaps GD's article covered those as well).

Good luck!
 
Okay. Well, the first thing I need to do is figure out my air consumption rate. I'll put that on my list of things to do in Grand Cayman. Thanks, guys!
 
It's good to know whether your gas consumption rate is higher than the assumptions made in the numbers Rainer has given you, because for some very new divers, an UNSTRESSED SAC rate can be 1.0. But otherwise, knowing your precise SAC rate is unnecessary for the "rock bottom" numbers. It's really used for planning.

And "rock bottom" applies to ALL dives, not just boat dives. If you swim out from shore to 80 feet of depth and your buddy's reg freeflows and you need to share gas with him, you STILL need enough gas to get him AND you to the surface, don't you?

You are troubled by the fact that rock bottom for deep dives is a large proportion of the gas you take with you. That IS troubling, and it should be. What it tells you is that, when you do deep dives on small tanks, you either get a very short dive, or you play Russian Roulette with your gas. But do remember that rock bottom is a dynamic number during the dive -- if you spend a little time at 100 feet, but then you move up to 80, your rock bottom has gone down from 40 to 30 (what that means is that you have YOUR part of the original rock bottom now to use). If you then move up to 60, it goes down again. So, at each depth, you have a number where you have to begin to ascend -- but if you get to the next depth without having to share gas with anyone (as you usually would) you have some more gas to play with at the new depth.

So I guess what I'm saying is that, if you plan a 100 foot dive on an Al80 (which I wouldn't, but it's up to you), you don't have to get back to shore or back on the boat with 1600 psi. You don't even have to do a direct ascent from 100 feet if you hit 1600 psi. You need to move up to where you have more gas than the rock bottom for that depth, and then keep doing that, so that you always keep at least the reserve you need for that depth in your tank.

"Turn pressure" is relevant to all dives. If you are doing a drift dive off a boat, you have all available gas (everything but your rock bottom) to use. But if you are doing a dive off an ANCHORED boat, you at the very least want to divide your usable gas in half, so you swim away from the boat with half of it, and come back to the boat with the other half. If you really, truly HAVE to make it back to the boat (open ocean wreck dive, well offshore, anchored boat) then you need to think about being even more conservative, because you might have to get you and your buddy back to the anchor line, as well as ascend along it.

The numbers you're staring at are the reason why a lot of people go to bigger tanks.
 
Matt, when you figure your consumption in GC don't be surprised if it is different when figured again in colder, less comfortable waters. Diving in 200ft viz and 82F water is not the same as 20ft viz and 55F water, and your consumption rate will probably reflect that.
 
Matt, when you figure your consumption in GC don't be surprised if it is different when figured again in colder, less comfortable waters. Diving in 200ft viz and 82F water is not the same as 20ft viz and 55F water, and your consumption rate will probably reflect that.

In 57-degree water in Spain last month I was breathing quite a bit faster than usual. I don't know if it was the cold, or just the fatigue of lugging all that weight down to the shore, fighting the waves to enter, and swimming out to the dive site.

Oh, and it's also the extra exertion to overcome the inertia of all that weight. You might be weightless in the water, but the mass still has to be overcome when you accelerate from rest.

But anyway, I concluded that my air consumption is higher in cold water.
 
But anyway, I concluded that my air consumption is higher in cold water.

Perfectly normal, and it will all go down as you dive more.
 
Question....where does the RMV factor in? I was told that you should basically scrap your SAC rate and use your RMV. Is this true? It does give you a much more limiting dive time but seems to make sense.
 
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