Gas consumption lunchtime doodles

[jeckyll]

So I was sitting around at lunch and decided that I should start to thing not only in terms of rock bottom gas but also about how much gas is necessary to complete a dive. In the past I've done 'on the fly' calculations of (3000 - RB) /2 = Gas till turnaround on an AL80.
Since I recently switched to doubles, knowing in advance how much gas I have after dive one will let me make some decisions about what is possible for dive 2 (all above RB for the depth of course)

I'm hoping that if I made a mistake anywhere someone will point it out

I purposely decided to do this without a calculator & just round up to make things easy to do on a scratch piece of paper at the dive site.

So let say I want to do a dive to 100 fsw on 32% and spend 30 minutes of BT (descent and actual time at the bottom). On my ascent I'll do stops as follows: 1 @ 50, 1 @ 40, 1 @ 30, 3 @ 20, 3 @ 10 with an ascent rate of 30 fpm

Bottom gas used (includes descent which I'm using as if it was at full depth)

30 min at 4 atm at 0.6 cf/min/atm = 72cf

Ascent gas used:

To make this easier I'll say that I need 3 1/2 minutes of travel time for my total ascent without stops = avg depth of 50 fsw or about 2.5 atm. I'll round the time up to 4 minutes to make it easy

4 min at 2.5 atm at 0.6 cf/min/atm = 6 cf

for my stops I'll combing the 50, 40 & 30 stops as if they were 1 stop for 3 minutes at 40 ~ 2.5 atm

3 min at 2.5 atm at 0.6 cf/min/atm ~ 4.5 cf

for the longer stops at 20 & 10 I'll use 6 minutes at 15 fsw ~ 1.5 atm

6 min at 1.5 atm at 0.6 cf/min/atm ~ 5.5 cf

Total gas required = 72 + 6 + 4.5 +5.5 = 88 cf

on double AL80's that would mean I'd have 154 -88 = 66 cf remaining.

I realize that this is pretty rough on the math side, but does that sound like a reasonable approach for 'in the field' gas calculation?

Thanks in advance.

Bjorn

 

[TSandM]

Remember that your travel time is included in your stop time. So you actually have travel from 100 to 50, which is rounded up to two minutes, and then your stops include your travel from that point onward.

If I were going to do it, I'd use 20 minutes from 100 to 50, average 75 which is 3.25 ATA, and I'd round down to 3 because I rounded the time up. So 3 x .6 x 2 = 3.6 round to 4.

Then you have nine minutes at an average depth of 23 feet or 1.7 ATA (call it 2). 9 x 2 x .6 = 10.8, call it 11.

I come up with 87 cu ft, which is almost identical to yours but went a bit quicker, I think, and it's all within the accuracy of the approximations.

 

[amascuba]

For Minimum Gas on a 100ft. dive you would need 40ft3 of gas reserve. That's assuming that you and your OOG buddy will have a SCR of 1ft3 each.
In an emergancy it wouldn't be all that uncommon for an elevated SCR of 1ft3/min.

4.03ata(100ft) x 2ft3(SCRx2) x 1min.(time to address the emergancy at depth) = 8.06ft3(gas consumed)

30ft/min ascent to 50ft for the first stop, we'll round up to 2min. with an average depth of 70ft.

3.12(70ft) x 2ft3(SCRx2) x 2min.(time to ascend from 100ft to 50ft) = 12.48ft3(gas consumed)
2.51(50ft) x 2ft3(SCRx2) x 1min.(30sec. stop, 30sec. move) = 5.02ft3(gas consumed)
2.21(40ft) x 2ft3(SCRx2) x 1min.(30sec. stop, 30sec. move) = 4.42ft3(gas consumed)
1.91(30ft) x 2ft3(SCRx2) x 1min.(30sec. stop, 30sec. move) = 3.82ft3(gas consumed)
1.61(20ft) x 2ft3(SCRx2) x 1min.(30sec. stop, 30sec. move) = 3.22ft3(gas consumed)
1.30(10ft) x 2ft3(SCRx2) x 1min.(30sec. stop, 30sec. move) = 2.60ft3(gas consumed)

Minumum Gas = 39.62ft3 (Round up to 40ft3)


So 72 + 40 = 112ft3 of gas needed for your dive. Now depending on the dive profile would determine the turn pressure, whether you are using all usable, 1/2 usable, or 1/3 usable.

154-112 = 42ft3. That's a short shallow dive at best unless you bring another set of doubles or a stage bottle.

 

[jeckyll]

Lynne: I'd been doing the stops on top of any travel time. If the travel time is included it certainly makes the math easier

How did you average out to 9 minutes at 23 feet for the stops from 40fsw to 10? I'm not quite getting there...

Thanks

Bjorn

 

[amascuba]

disregard my post.

 

[TSandM]

I did (1x50)+(1x40)+(1x30)+(3x20)+(3x10), then divided by nine total minutes to get the weighted average depth for the ascent.

James, there's a difference between the calculations for an uneventful ascent and minimum gas. Minimum gas is a reserve, but you don't actually use that much doing a normal ascent, so when you're trying to figure out if you can get two dives out of your doubles, you can use the normal ascent numbers.

 

[amascuba]

James, there's a difference between the calculations for an uneventful ascent and minimum gas. Minimum gas is a reserve, but you don't actually use that much doing a normal ascent, so when you're trying to figure out if you can get two dives out of your doubles, you can use the normal ascent numbers.

My brain finally caught up with that after the fact.

 

[jeckyll]

amascuba: All I'm trying to work out is how much gas I'm going to be using on the dive in question.

If I'm doing the dive in doubles and my buddy has an emergency requiring us to share gas on the way up, chances of doing a 2nd dive that day are probably slim. I think you were working out different numbers

Bjorn

<edit:> nevermind we are all on the same page

 

[jeckyll]

I did (1x50)+(1x40)+(1x30)+(3x20)+(3x10), then divided by nine total minutes to get the weighted average depth for the ascent.

.....

ahhh I didn't count the 50 foot stop, I thought that was included in the other number

I'd averaged only the 40 -> 10 foot stops

Makes much more sense now.

Thanks again!

 

[Charlie99]

I get pretty much the same answer, but with a slightly modified method. I figure everything out in "surface equivalent minutes" and only apply SAC at the end.

30 minutes at 100' is 30@4ata = 120 minutes (or more properly, 120 ata-minutes)
ascent to 50' is 2 min x 3.xata = 7 min
(if you do a standard set of stops, then the gas or surface equiv minutes for them is always the same, so you won't have to repeat this section again ..... I assumed stops included travel time.)
1@50', 1@40', 1@30' = 3@40'. 40'=12m=2.2ata. 3x2.2 =7 ata-minutes
3@20+3@10=6@15'= 6@1.5ata = 9 ata-minutes.

Total minutes 120 bottom, 7 ascent to stops, 16 in your standard stops.
= 143 minutes of breathing on the surface

Now you just multiply by whatever SAC you assume to get estimated cu ft used.
0.6 * 150 minutes = 90 cu ft.


===============================

The reason I find "surface equivalent minutes" easier is that I know I have 130 to 160 minutes available to me (at 0.5 to 0.4cfm) in the first 65 cu ft of an AL80, so I usually don't bother ever converting over to cubic feet.

Charlie Allen

p.s. Now to continue your lunchtime calculations, figure out how much gas you want to start your second dive with. Knowing that you need 23 ata-minutes or 14 cu ft to ascend, now you can calulate out the psi you should be leaving the bottom on your 1st dive in order to have enough gas to start the 2nd dive.