Deep Diving - Safe bottom air pressures

[yeohcheeweng]

I read that for an AL80 cylinder the rock bottom pressures should be around:

30 ft - 700 psi
70 ft - 1000 psi
100 ft - 1300 psi
130 ft - 1800 psi

10m - 50 bars
20m - 70 bars
30m - 90 bars
40m - 130 bars

What are the figures for 50m & 60m? Is there a formula underlying the calculated values?

Thank you for any response.

[bwerb]

Uh...you shouldn't be diving to 50m - 164 feet or 60m - 197 feet with a single AL80, it's not near enough gas.

[Thalassamania]

It is, in point of fact, perfectly possible to do a bounce to 197 on a single 80 and surface with plenty of gas left over. There may be plenty of reasons for a given individual to not make such a dive in a given situation, but to make a blanket statement like, "Uh...you shouldn't be diving to 50m - 164 feet or 60m - 197 feet with a single AL80, it's not near enough gas." leaves me wondering about the many times I had to make just that dive with just that gear.

[mikemill]

You need to figure out how much (volume) air you need to get from that depth to the surface making all needed stops. Once you know the volume of air needed you can then calculate the tank pressure.

The volume needed is a function of depth, SAC rate, and ascent rate. So using an ascent rate of 30 ft/min from 30 ft, and a SAC rate of 0.6 cuft/min you would use about 0.87 cuft. To get this number first figure out your average depth which assuming a constant ascent rate would be half the depth (in this case 15ft). Then you need to figure out what the atmospheric pressure at that depth is by dividing by 33 and and 1 (1 atm per 33 ft plus the 1 atm at the surface). Then you need to figure out how long it'll take you to get to the surface which is simply the depth divided by the ascent rate (our example is 1 min). Finally take the pressure multiplied by your SAC multiplied by the time to ascent and you get the volume of gas used.

Now since you still want to do your safety stop you use the same function to figure out how much gas you'll use during the stop and just add the results. I got 2.62 cuft for the stop and a total of 3.49cuft.

Remember that you'll want to have enough gas for your buddy in case something happens. If they have the same SAC you can just double it or you can rerun the calculation and add them together. For this example we'll just double it for a total of 6.98 cuft.

To go from this volume to the tank pressure do the following: (volume needed / total volume of tank @ working pressure) * working pressure. So for an AL80 (assuming a volume of 80cuft) with a WP of 3000 PSI we get: 261.75 PSI.

Now of course this all assumes you can go all the way down to 0 PSI but in reality you can't. You really could only take it down to atmospheric pressure (14 PSI at the surface) and you may need some additional pressure for the regulator to work. So if you wanted to make sure you ended at 500 PSI you'd get a rock bottom of 761.75 PSI.

[mikemill]

Uh...you shouldn't be diving to 50m - 164 feet or 60m - 197 feet with a single AL80, it's not near enough gas.

Running the calculation for a person with a 0.5 SAC they could do it (abit without much of a safety margin).

Using the following:
SAC: 0.5 cuft/min
Decent Rate: 15 ft/min
Ascent Rate: 30 ft/min
Bottom Time: 5 min
Depth: 160 ft
Safety stop: 3 mins @ 15 ft

Gas used during the...
decent: 39.29 cuft (correct number would be 18.26 cuft)
bottom: 14.62 cuft
ascent: 9.13 cuft
stop: 2.18 cuft

Total gas used: 65.22 cuft (corrected: 44.2)

End pressure: 554.17 psi (corrected: 1342.61)


Granted, I wouldn't do it...

Edit: Going back over the numbers it seems as though my spreadsheet is doubling the amount of gas needed during the ascent and decent but I haven't figure out why yet.

Edit 2: I think I found the mistake: Over engineered the formula

[Teamcasa]

It is, in point of fact, perfectly possible to do a bounce to 197 on a single 80 and surface with plenty of gas left over. There may be plenty of reasons for a given individual to not make such a dive in a given situation, but to make a blanket statement like, "Uh...you shouldn't be diving to 50m - 164 feet or 60m - 197 feet with a single AL80, it's not near enough gas." leaves me wondering about the many times I had to make just that dive with just that gear.

While I agree in actual fact that a bounce dive can be made, I get the impression the OP is asking about turn pressures, indicating he plans to spend a little time at those depths. In that case, an AL80 is not a smart choice for plus 100 ft, much less 164+ depths.

[elan]

You also have to multiply the SAC rate by 2 or 3 as RBT should take into account emergency situations where the rate increases.

[bwerb]

The concept of Rock Bottom is the minimum amount of gas to get both yourself and your buddy...you are diving with a buddy right...to the surface in case of an emergency.

In order to calculate it you first need to calculate your SAC or surface air consumption rate...how much air you breathe at 1 ATM in a minute.

You then need to calculate how many ATM's deep you are. (one ATM = 33 feet)

So for a dive to 50 M (164 feet/33 feet = 1 ATM = 6 ATMs) at a really good SAC of .5 we're looking at 6 cubic feet per minute for the buddy team.

So, problem occurs, we give two minutes to solve the problem 12 cubic feet then using an ascent rate of 30 feet per minute to ascend or 6 minutes to round up...with an average depth of 82 feet or 4 ATM's for the ascent, we're looking at 4 cubic feet per minute average.

So...(12 cubic feet + (4 cubic feet/minute x 6 minutes) = 36 cubic feet Rock Bottom...but...in an emergency people's breathing rate can often double...so we're up to 8 cubic feet per minute...and add a three minute stop and we're into the "oops" really quickly.

Here's a metric rock bottom calculator for you to try your own inputs with: www.leonswebsite.net/resources/XLDiveTools.xls

[bwerb]

It is, in point of fact, perfectly possible to do a bounce to 197 on a single 80 and surface with plenty of gas left over. There may be plenty of reasons for a given individual to not make such a dive in a given situation, but to make a blanket statement like, "Uh...you shouldn't be diving to 50m - 164 feet or 60m - 197 feet with a single AL80, it's not near enough gas." leaves me wondering about the many times I had to make just that dive with just that gear.

Sorry guys...I was typing a long response and pulled the trigger on my initial reply a tad prematurely. A bounce dive is one thing...doing it as a planned buddy dive of any duration and you really don't have much gas left for anything other than a reading on your depth gauge.

[dumpsterDiver]

Sorry guys...I was typing a long response and pulled the trigger on my initial reply a tad prematurely. A bounce dive is one thing...doing it as a planned buddy dive of any duration and you really don't have much gas left for anything other than a reading on your depth gauge.

Yesterday I did a wreck dive with a single tank (and 13 cu-ft pony) with about 140 cu-ft in it at 3000 psi. Dropped in swam down to about 140 and then cruised for a few minutes as we drifted toward a wreck. As we got to the wreck, my buddy shot a large fish and drifted off in the 2 kt current.

I saw another fish, grabed onto the wreck at the deck at 160 ft, stalked a fish for maybe a minute, speared it, fought it a little, allowed myself to sink down into the ship to the bottom where I was out of the current (at about 190 ft). I then pulled out my knofe and killed the fish (at that point I remebered I forgot my ascent reel), I then removed fish from the shaft, placed it in my bag, re-loaded the spear, strung the line and loaded one band of gun for sharks. I remember feeling pretty narced and a little out of breath, when wrestling with the fish (I've not been over 140 feet for 8 months probably). I remember breathing faster than I should and could detect a little resistance in the reg, so i must have been sucking pretty hard.

I then ascended at a reasonable pace (my computer had died on the descent), which i did not realize until I was at a depth of about 100 feet. When I first realized the computer was dead, i checked the time and it was 8:25 am.

I hate ascending without a computer (or any depth guage) but I came up following my little bubbles and kinda hurried toward the surface because I wanted to send up a smb, which only had 25 of line on it, so I didn't stop deeper. My watch read 9:36 when I reached 25 feet. I did 12 minutes of hang and hoped that was a reasonable time and ascended.

I still had half my air left (so i used about 70 cu-ft).