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Discussion Starter · #1 · (Edited)
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
 

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Wreck and Cave Diver
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SAC Rates?

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
 

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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.
 

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aka Chimp 1 or Mavis...
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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.
 

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Wreck and Cave Diver
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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
 

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A short fat well off crap cave diver. Likes wrecks
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I like Andys idea. I have never done it like that but its neet for cave diving.

For OW diving I have several SACs. There is my normal bottom time SAC my deco/ ascent sac and my "oh s#it" SAC.


For me 15 on the bottom 10 for deco and 30 for oh s#it


These were worked out the long math way. (Gas used devided by time) devided by depth in ATMs+1


so 24ltrs (twin 12s) X starting presure 235bar = 5640ltrs


Finish presure 70 bar X 24 = 1680 ltrs

Gas used 5640 - 1680 = 3960ltrs


3960 devided by time to first gas switch (say 40min) = 99ltrs per min

To get this a a Surface Air Consumption figure you then devide by depth in atms. 40m = 5atms (depth/10 + 1)

99/5 = 19.8 SAC

When I dived OC id check my SAC every dive and make notes on conditions work load etc. Id plan on 15 but often get 12 or 13 SAC. Id still plan on 15

for deco you need to do a bit of creative math bassed on time at stop depths and averages but sufice as to say SAC on deco is significantly less than for the working phase.


ATB

Mark Chase
 

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Utrinque Paratus
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this might help


Traditional SCR
The amount of gas (liters) a person breathes during one minute while being on the surface is referred to as SCR (Surface Consumption Rate). 20 liter SCR is normal

SCR Formula
SCR = Consumed BAR x Tank volume ÷ average ATA ÷ time
The amount of gas (liters) consumed at depth is referred to as DCR (Depth Consumption Rate)

DCR Formula
DCR = SCR x average ATA x time

Example
SCR & DCR
You consume 30 BAR out of your double 12l during 10 min @ 30m depth, What is the your SCR?

30BAR x 24l ÷ 4 ATA ÷10min = 18 liter SCR

You plan to dive to 48m. You already know your SCR but how many liters can you expect to consume per minute during this deeper dive? What is your DCR @ 48m depth?

18 SCR x 5.8 ATA x 1 min = 105 liter DCR

How many BAR gas will you consume every 5 minute @ 48m?

18 SCR ÷ 24 l x 5.8 ATA x 5 min = 22 Bar every 5 minute

Borrowed from GUE paperwork

Graham
 

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Discussion Starter · #13 ·
Thanks to all for your help - I posted my Q on various websites and now have all the info I req

Cheers - Matt S
 

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The worlds slowest sailor.
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sac rate guide.

8ltr min- i love my buddy,what a nice reef and i.m glad i went to bed early and had no curry or beer.

20ltr min-blimey its dark,did i charge my torch(yes i did but its still on the boat)how deep???? bloody german buddys. belch!!!
 

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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?
 

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Open Wide
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I just plug my Suunto into my laptop, enter the start and end pressures and it does the rest.
 

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aka Chimp 1 or Mavis...
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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?
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.
 

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A short fat well off crap cave diver. Likes wrecks
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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


Check the avtar Andy, looks like a UK cave diver. Buddies are too dangerous ;)

ATB

Mark Chase
 
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