Air consumption

[cmcarver]

I am curious as to how long everyone can make a AL80 last. Went diving with 2 people last weekend in cold water(70* Surface, 42* Below 30') and averaged 45' Dives. Vis sucked at 3-5' above 30' and 15' under. I used 1400psi of 3100psi on a 27 minute dive when the others used 2000 and 2600psi. Therorecticly I could get 50 minutes out of a AL80 at 45' but don't seem to get the chance to find out.

 

[String]

Work out your surface air consumption for dives, from there you can extrapolate an average for any tank size and depth.

We're told to assume 25litres/minute for air planning although dont know anyone that exceeds that.

As for cold water vs warm, my air consumption abroad is roughly 35% lower than the same depth here in the UK.

I have no idea how to do it for imperial units but im sure your basic OW course covered it.

 

[simbrooks]

Work out your SAC (surface air consumption) as vol/time/pressure ie in your case you used 1400psi/3000psi*77.4cuft=36.1cuft at depth, you were down 27 mins and unless you got the average from a computer i would assume that 45ft was your max depth, which might mean 35-40ft average, but lets take 45ft anyway just for kicks which is 2.36atm, so you are looking at 36.1cuft/27mins/2.36atm = 0.56 cuft/min on the surface. If we took 38ft (more likely average for a 45ft max) it would be 36.1/27/2.15=0.62cuft/min surface which is still pretty good for a newbie - that is about my average at the moment, but getting better!!

So reversing this, assuming you need 3mins at 15ft, and a 30ft/min ascent your rock bottom time (ie min to ascend in an emergency) would be about 1.5 mins ascent (average pressure = 1.7 atm) plus 3 at 15ft (1.5atm) would be about 7.1 mins of surface time which would be 8.8cuft for two divers with emergency SAC (total) of 2cuft/min (vol required is essentially nothing), so lets say a reserve of 600psi to get you up and back on the boat with a resonable pressure. You have 2400psi to use on the dive which equates to about 62cuft which at the surface would be 100 mins, at 2 atm (33ft) average would be 50 mins, at 3 atm average (66ft - although i would refigure the rock bottom on this one, but for kicks anyway) would be about 33mins etc. So to work that one out you have time=vol/sac/pressure ie 50 mins = 62cuft/0.62cuft/min/2atm - nice how those numbers worked out isnt it

Does that make sense?

 

[mjh]

Way to much math going on here. Based on always hitting shore or boat w/500psi. Cold water dive avg about 55min, this usally involves swiming against some minor current. Tropics w/boat on anchor and swimming around 1:10-1:15. Coz drift diving +1:20.

 

[simbrooks]

Fairly simple math really all to do with vol, pressure and time, nothing more, it is useful to help plan deeper dives, 40ft is hardly a hassle, but if you wanted to know how long you might stay down or guage your air consumption improvement then it is a useful tool. I just overelaborated a bit on the description of how/why the numbers are found, i could have written it in two lines.

Essentially it is not possible to tell cmcarver how long an AL80 can last in terms of minutes only as that changes with depth and your SAC.

 

[cmcarver]

That first post is great. Gave me info I was looking. But where did you get the 77.4 in your first example. So lets see if this is right SAC= VOL/TIME/PRESS VOL=USEDpsi/STARTpsi*CUBICft TIME=MINUTES DOWN PRESS=(AVERAGEdepth/33feet)+1
Actually max was 55' and I figured 45 on average but probably less.

 

[String]

Doing some very rough maths, rounding and converting it seems an AL80 tank is roughly the same as a 11.1l cylinder pressurised to 206 bar.

This would contain about 2287 litres of air.

Assuming a surface rate of 25 litres/minute that would last 2287 / 25 == 91 minutes on the surface.

40ft is a very shallow 12m which is very roughly a pressure of 2.2 bar.

Converting down assuming that surface rate youd expect 41.5 minutes total out of a cylinder at that depth.

As stated above, that SAC rate is very high and is used as an absolute maximum. If you assume a "normal" cold water rate of about 16 SLM you would get 142 minutes surface and expect 64 minutes at 40ft depth.

If you use a clear warm water, no current consumption of nearer 12 SLM you're looking at 86 minutes at 40ft depth.


Of course, all these conversions assume the tank is totally emptied which of course it isnt.

I think the americans use 500psi as a reserve pressure so i'll try to factor that in:

(my maths may go to pot here as im not used to any imperial conversions).


If its a 3000psi tank than 500psi is 1/6 total amount. To simplify here assume linear relationship with pressures, reduce those times by 1/6

A vacuum cleaner would therefore expect 34.5 minutes at that depth before reaching reserve pressure, a normal cold water diver about 53 minutes and a warm tropical diver would expect 71 minutes.



Hope thats correct, bit of a maths rush job and i hate imperial units. Metric makes it SO much easier to calculate

 

[simbrooks]

Standard AL80's are actually about 77.4cuft at 3000psi (rated pressure) although people can fill them a little higher (3100psi ~ 80cuft), but mostly 3000-3200 is a reasonable fill from most shops.

Other than that little bit of explaination, glad that you understood what i wrote. Those formulae are pretty close:
V used=used psi (end-start)/rated psi*rated vol and pressure=depth/33+1, but in this example that would be average depth. Most computers will work your average out for you, others also have a way of keeping track and averaging their profile.

So in your case we would be looking at the 0.56cuft/min SAC which would give 110 surface minutes, 55mins at 1atm etc. Again work our your minimum pressure to start ascending assuming 2cuft SAC for the ascent profile.

 

[Al Mialkovsky]

Lots of people enjoy the math used to compute SAC rates and then factor them to figure dives at various depths. I never have bothered myself. I don't see the value in them. Temperatures are just a big of a factor as SAC rates. Your work rate is a factor also so the SAC rates get totally skewed.

 

[simbrooks]

A vacuum cleaner would therefore expect 34.5 minutes at that depth before reaching reserve pressure, a normal cold water diver about 53 minutes and a warm tropical diver would expect 71 minutes.

It doesnt sound way off the mark, but even so, dont you mean hoover? I love the way Americans get confused by that term.

Hope thats correct, bit of a maths rush job and i hate imperial units. Metric makes it SO much easier to calculate

But Americans dont understand a simple system like the metric one - its far too easy. The unfortunate thing is since coming here and learning this convoluted system again (which i tried to forget in my youth) i have thrown out many of my old metric understanding and visualisation (ie checking stuff in orders of magnitude by what seems reasonable etc) and now have this imperial stuff back in my head again, to the point where if i went back to the UK i am not sure i'd be able to get the metric system straight again after brainwashing with imperial every day. Some things are eaiser with imperial though, particularly the ~120rule for NDL calcs in your head, doesnt work for metres as well!