SAC Rates

[SJT]

Originally Posted by GLOC

Unless you weren't breathing for part of the dive , yes you can use the average depth from the computer. It is the easiest way to work out Avg Depth for a complete dive unless of course you are using a deco gas when things become a little more complicated to work out your bottom gas SAC.

Thanks!

[Finless]

I've mentioned it before so ............ I have a little spreadsheet to assist in working out SAC rates - either planning or for the 'calculating of' after a dive.

It is for OC, up to two mix, square profile diving ............. or any profile that may be cut into mini squares.

I know some people haven't died because they said they hadn't BUT, of course, they may have been lieing? Should anyone use a copy they will die ........... probably!

I particularly like the planner because you can run bottom times and deco times and confirm you have sufficient reserves (given the same SAC rate through the dive). If only using one gas then leave gas two at zero and ..........

Obviously, it is impossible to be spot on but it has worked for me.

I don't want any money just give me your women, mwahahahaha ........... Ok, adulation then? Of FFS just have it and never say a thing!! PM me an email address and I'll send it.

[Richard Mason]

Will an air integrated computer provide this information?

I just find that there are way too many variables to take into account to get an accurate idea that is readily reproducible, especially as I tend to dive triangular profiles anyway. You not only have to take constant changes in ambient pressure into account, but also changes in personal work rate too. My feeling is that if you try to do a calculation where there are two unknown, or at least poorly understood variables to worry about, the end result is going to be subject to some pretty significant limitations.

The best I could come up with was to strap my rig on, complete with mask (so no nose breathing) and do a very brisk walk around my yard for 10 mins, not getting completely breathless but, with a 15L on my back, working reasonably hard and blowing a bit and noting the change in cylinder pressure then doing the calculation. That way, I'm guaranteed that the test is run at constant ambient pressure and at a constant rate of work on my part.

I also did the same thing, only taking a gentle stroll, to simulate a relaxed bimble in still water.

I've tried the other in-water methods and found such a wide variation that it's not something I'd trust as being reproducible.

[Finless]

Originally Posted by Richard Mason

Will an air integrated computer provide this information?

I just find that there are way too many variables to take into account to get an accurate idea that is readily reproducible, especially as I tend to dive triangular profiles anyway. You not only have to take constant changes in ambient pressure into account, but also changes in personal work rate too. My feeling is that if you try to do a calculation where there are two unknown, or at least poorly understood variables to worry about, the end result is going to be subject to some pretty significant limitations.

The best I could come up with was to strap my rig on, complete with mask (so no nose breathing) and do a very brisk walk around my yard for 10 mins, not getting completely breathless but, with a 15L on my back, working reasonably hard and blowing a bit and noting the change in cylinder pressure then doing the calculation. That way, I'm guaranteed that the test is run at constant ambient pressure and at a constant rate of work on my part.

I also did the same thing, only taking a gentle stroll, to simulate a relaxed bimble in still water.

I've tried the other in-water methods and found such a wide variation that it's not something I'd trust as being reproducible.

The SAC rate is never accurate for planning as you can't guarantee your SAC on the next dive.

With non square profiles I find the best guestimate is to break the dive down into sections and pick your average depth:-

30 mtrs to 20 mtrs over 20 mins would average out at 20 mins at 25 mtrs (assuming steady ascent rate). If your profile doesn't fit a 'steady ascent rate' and can't be broken down to smaller sections then adjust the depth to the average ............. it comes out close enough in the end.

Alternatively, note your cylinder pressures during a section of a dive and work a SAC on that section and allow some margin for other sections of the dive if you are more/less stressed ............ err on the side of caution and build in a margin.

[mala]

Originally Posted by SJT

I have a quick question on this topic:

When calculating your OC SAC after a "normal" incident free dive, is it correct/accurate to use your average depth for the entire dive (as shown on a computer) for the depth part of the calculation? (i.e. if my Suunto tells me the average depth for a dive was 20m, can I calculate my SAC by working out my air consumption in litres per minute and diving this by 3)?

Is this too simplistic?

yes.

if your work rate was similar at all depths.
if you had a large amount of time at shallow depths doing nothing(deco for example) this will cause your true sac to be underestimated.

[mala]

Originally Posted by mala

yes.

if your work rate was similar at all depths.
if you had a large amount of time at shallow depths doing nothing(deco for example) this will cause your true sac to be underestimated.

yes to both questions that is.

god i wish i could edit.

[Harlequin]

Originally Posted by Harlequin

you can also use the calculator I put on my 232 bar website if you like.

Guys, just been tidying up the 232 Bar website's database and after getting rid of all the crap worked an average on 850 odd SAC calculations and it came out to 19 Litres a minute. OK, not pure science but it's better than actually doing 850 dives and doing the calculation.

If I leave the records in the DB with no ATA then it comes out at a modest 18 litres a minute.