SAC Rates

[MDS]

I'm looking for any good resources/texts/info/advice on working out SAC rates.

Your SAC rates help you decide which size twinset to go for so I am told.

All help appreciated!

Ta - Matt

[Andy Kerslake]

Matt

hmm I don't think your SAC rate should decide any size of twinset, it is far easier/simpler to use the same size set as you buddy - this make gas planning much more straightforward.

But working out your SAC should be done over a few dives so you can come up with an average litres per min.

A better way of translating sac rate into something useable underwater is to come up with a number of bar per 5 minutes, this helps significantly in working out how long your gas supply is going to last.

So lets take an example. I am going to dive to 10m using a 10l cylinder, and my normal SAC rate is 20LPM.

10m is 2ata so I will be using 40LPM each minute for 5 mins. this is 200l of gas divided by cylinder size equals 20bar.

So I will use 20bar every 5 minutes at 10 metres, so if I have 150bar of useable gas it should last 37.5 minutes.

The same maths apply to any depth or any size cylinder, you just need to have a good handle or your SAC rate over a variety of conditions.

Andy

[Digger]

You want to know how to work out your SAC on a dive, or how to work it out in theory?

On a dive get a computer, check your start pressure (as in the moment you leave the surface) go for a dive, get out and check the pressure when you hit the surface again. Multiply number of bar by cylinder(s) size. This is the amount of gas you used in litres.

Now get the average depth from your computer. Convert to atmospheres (divide by ten and add one) then divide the amount of gas in litres by the average atmospheres.

Then divide that by the number of minutes the dive took (from point A to point B).

This gives you the equivalent number of litres on the surface, your Surface Air Consumption (SAC). Measured in litres per minute.

If you don't have a computer with average depth, then go on a dive. Pick a moment where you are going to be at the same depth for a while, check you gas, swim about a bit, check your gas again. Write everything down, as you'll forget it otherwise.

The easy way to do it is to dive with someone who knows their SAC very well, and see whether you're muhc over or under. This is what I do, and it works remarkably well, what with SAC being anything but an exact science.

All of this could be bollocks, but it makes sense to me.

Digs.

[hilrosepaul]

Try this one:

232 Bar - SAC Calculator

Paul

[Andy Kerslake]

Originally Posted by hilrosepaul

Try this one:

232 Bar - SAC Calculator

Paul

Just a tad difficult to use underwater

[GLOC]

Originally Posted by Andy Kerslake

Just a tad difficult to use underwater

Sharp, as ever Andy

Haven't seen you on here for a while though Andy.

[DIVE BUDDHA]

Originally Posted by hilrosepaul

Try this one:

232 Bar - SAC Calculator

Paul

thats a very intresting little app thanx for that paul !
i always work on 20 litres a minute on gas planning which allows me about 4 litres a minute over !!

[SallyUK]

Thanks Andy - that makes it easier to understand.

SallyUK

[Digger]

Of course if you are diving solo it is more difficult to find out someone else's and see how your rate compares to them. In that case you should use the calculator or have a guess and plan a dive. Start at 10 lpm and work up one litre per minute at a time. When you stop running out of gas on dives you've got it.

Digs.

[Andy Kerslake]

Diggers approach is okay, but still requires too much work underwater. You can always convert back to liters per minute when you finish.

Assuming you are diving at approximately the same depth all you need to do is measure consumption based on time, because LPM is irrelevant underwater and the only guage you have measures in bar.

Simply check your contents guage at a regular time interval, if shallow then perhaps every 10 minutes (this will depend on either using single/doubles) if deeper then 5 minutes (or shorter) may be more appropriate.

All you need to do is then take bar consumed per time period x Cyl size in Litres = total litres consumed, and then divide by the time period.

If I am managing gas I do not think of a SAC rate when underwater I just use gas per time period, once you get used to this it has the added advantage that when you look at yout guage you know what it should read, because you will be able to say at this depth I use 20bar per 5 minutes. It does take some practice but once you get used to it - it really does work.

In addition it is easy to adjust for working (which we should not be doing) or cold etc, all you do is adjust the time available. If I started using 25bar per 5 minutes I know my consumption has gone up so I will have less bottom time, if I use less I could have more time, subject to deco limits etc.

Best

Andy