Rock Bottom?

I put a page describing rock bottom up at http://jonnythan.com/rockbottom.html .. mostly for the use of my dive buddies and local club.

Does the info here seem reasonable and accurate? Does anyone think having this page up might be a bad idea?

I am guessing that you are assuming a SAC of 1.0 for both divers for this...the only thing I see as a problem is if you don't have a known SAC rate for both divers....

I also assume you are using similar tanks where 28 cu. ft. in one tank is the same psi as 28 cu. ft. in another tank (remember the difference between steels and ALs.

A static chart like that will not work, too many dynamic variables. You breathe differently depending on exertion levels, anxiety levels, water temp, etc.

If you followed that "rock bottom" formula and were about to surface, had a large sea urchin spine go through your hand, I guarantee you will start breathing very fast from the pain. At that point you would already be "in the red" as far as air reserves are left. (speaking of that, its always a good idea to surface with 500psi left in case of unforseen problems)

What about shore exits that you need the reg in your mouth to accomplish?
What if you needed to share air with somebody?
What about bcd air?
Too many variables...

Not a very good idea if you ask me.

I am guessing that you are assuming a SAC of 1.0 for both divers for this...the only thing I see as a problem is if you don't have a known SAC rate for both divers....

I also assume you are using similar tanks where 28 cu. ft. in one tank is the same psi as 28 cu. ft. in another tank (remember the difference between steels and ALs.

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This is a *sample* calculation, not a definite "rock bottom for 90 feet is 1100 psi." It's a sample indicative of my club and the dives we do - lakes and quarries on Al 80's. A more precise calculation taking into account buddies (maybe raising the SAC) and tanks (at the moment, we dive pretty exclusively Al 80's) should be part of the pre-dive plan.

What about bcd air?

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No BC air will be required for an ascent.

If you followed that "rock bottom" formula and were about to surface, had a large sea urchin spine go through your hand, I guarantee you will start breathing very fast from the pain. At that point you would already be "in the red" as far as air reserves are left.

Click to expand...

It's pretty unlikely that a sudden loss of air will occur simultaneously with a significant injury. You won't be "in the red" unless you or your buddy had an OOA at the absolute end of your dive.

(speaking of that, its always a good idea to surface with 500psi left in case of unforseen problems)

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This takes the place of that 500 psi reserve, doesn't it?

What if you needed to share air with somebody?

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This rock bottom is intended FOR air sharing scenarios. Read the first sentence: "Rock Bottom" is the minimum amount of air necessary for you and your buddy to make a safe (30 fpm + safety stop) ascent from the deepest point in the dive.

SARmedic once bubbled...
(speaking of that, its always a good idea to surface with 500psi left in case of unforseen problems)

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That's exactly the point going on here. Where does that 500 psi come from? It's a guesstimate of 'unforseen problems'. The rock bottom concept shown in jonnythan's example actually has a 550 surfacing psi.... One of the things his math shows is that aiming to surface with 500 psi wouldn't have been enough to get his buddy to the surface.

Jonnythan....

Don't ignore the SAC rate... put it in your math anyway... otherwise people don't know where that goes.

When calculating rock bottom, you wouldn't use your normal resting SACs, you would use a working SAC so that you have some leeway if you _do_ end up stressed when ascending.

But one of the important points that SARmedic inadvertantly brings up is that if you ever calculate out a rock bottom over 1000 psi, you know that 'surfacing with 500' wouldn't have been enough to get your buddy to the surface.

Put in a calculation to convert the rockbottom in cf, to psi, based on volume of tank at working pressure.

To illustrate,

Diver Bob is using HP100 @ 3500psi

28 / 100 * 3500 = 980psi rockbottom for Bob

Dive Joe is using LP104 @2640psi

28 / 104 * 2640 = 710psi rockbottom for Joe

So the math is:

rockbottom(cf) divided by tank volume(cf) multiplied by working pressure(psi) = rockbottom(psi)

First off- jonnythan -
I thought it was just an example, I totally agree with the fact pre-dive planning would accomplish the questions I had.

Second

SARmedic once bubbled...
A static chart like that will not work, too many dynamic variables. You breathe differently depending on exertion levels, anxiety levels, water temp, etc.

Click to expand...

This 500psi vs. rock-bottom thing is only something I have started to look into, but see a lot of common sense behind rock-bottom as opposed to "surface with 500psi"

By using a Rock-bottom calculation, you use a conservative SAC rate (based on previous dives and your charting) which should account for your heavy breathing during "exciting" situations.

as far as air reserves are left. (speaking of that, its always a good idea to surface with 500psi left in case of unforseen problems)

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Rock bottom replaces this thinking by telling you where to turn a dive if something hasn't happened and can occur...thus accounting for a reserve if something should happen. IT is the point where you turn a dive and one of the two in the buddy pair can surface safely on one diver's air supply.

What about shore exits that you need the reg in your mouth to accomplish?

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add it in to the calculation if you wish

What if you needed to share air with somebody?

Click to expand...

already accounted for

What about bcd air?

Click to expand...

why are you worried about this? when you get to the surface, and want air in your BC, orally inflate the thing. If your rig is too heavy to swim up...add more air to your rock bottom calc. to compensate

Now my question to you....you are at 80 feet diving with your buddy...when do you start your ascent to be sure you have enough gas to compensate for a catastrophic loss of your buddy's gas and you both have to make it back to the surface one one diver's supply?

Big-t-2538 once bubbled...

why are you worried about this? when you get to the surface, and want air in your BC, orally inflate the thing. If your rig is too heavy to swim up...add more air to your rock bottom calc. to compensate

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I am not worried about it, but it's much easier to use tank air instead of orally inflating it. In choppy waters (northern ca) it is a PITA to do that once you are at the surface. That was my only point with the bcd.

Spectre once bubbled...


That's exactly the point going on here. Where does that 500 psi come from? It's a guesstimate of 'unforseen problems'. The rock bottom concept shown in jonnythan's example actually has a 550 surfacing psi.... One of the things his math shows is that aiming to surface with 500 psi wouldn't have been enough to get his buddy to the surface.

Jonnythan....

Don't ignore the SAC rate... put it in your math anyway... otherwise people don't know where that goes.

Click to expand...

The SAC is in the calcs - the cf/min. I used 1 in these calcs, as it seemed an easy, relatively safe number (for me at least). When diving with a buddy who simply doesn't know, what do you think would be reasonable?

Scubaroo, thanks for the suggestion. I think all of my buddies are more than capable of doing this calculation - and it's almost moot because none of us dive anything other than Al 80's atm - but this page might make its way around some more, so I'll go ahead and add that.

Edit: new section added with general formula and tank calculation

At what pressure does a regulator stop delivering air? Can a regulator deliver gas below it's set IP? Never drained a tank that low to find out.

So would you have to add the IP of your regulator to rockbottom, because anything below that IP just isn't available? ie about 140psi? Or would you have to add 14.7psi, which is ambient pressure at the surface, because you couldn't go below that?

Not being smart or anything, but if a reg won't deliver gas below it's IP, 140psi can make a big difference to your calculations.