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Doghouse
07-24-2007, 11:39
I have used this page for a long time, but noticed that there was not a good path to it.
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<DIV>so Thanks to Scuba Toys for the great descriptive information:</DIV>
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<DIV>http://www.scubatoys.com/education/sac.asp (http://www.scubatoys.com/educati&#111;n/sac.asp)</DIV>
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<DIV>As a side note, I started at an SAC of 25 and now am down to low 19 upper 18 psi/min. </DIV>
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<DIV>This has been great for planning dives and learning how to dive. </DIV>
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cummings66
07-24-2007, 11:49
Keep in mind that SAC unlike SCR is relevant to a certain size tank only. If you used my tanks HP100 or HP120's your SAC rate would not be the same thing it is now.
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<DIV>Convert your SAC into SCR and you can then apply it to my tanks as well. So when you see high numbers like yours it's SAC, if I tell you I have a .35 it's SCR.</DIV>
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<DIV>Additional education.</DIV>

tc_rain
07-24-2007, 13:11
I posted this in a another thread but in case you missed it.
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<DIV>http://www.spearfishing.org/bruces_tips/java/sac.html</DIV>

skdvr
07-24-2007, 15:27
so how do you calculate SAC to SCR?
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<DIV>Phil</DIV>

cummings66
07-24-2007, 22:27
Take your tank volume and divide it by the pressure, for an AL80 that's 77.4 cf at 3000 psi. That gives you .0258. Now say you've used 1500 psi, that half the tank or 1500x.0258 or 38.7 cf.
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<DIV>Now that we know how to calculate cf we can then apply it to the SAC. Suppose you breathed 23 psi per minute.By doing the math you can figure that you use ~.6 cf per minute at the surface. I.e. .0258x23 gives you .5934 cf.</DIV>
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<DIV>You apply the same conversion factors to other tanks, i.emy HP120 is 120cf at 3500 psi for .034 conversion factor. That means I use 17.64 psi per minute instead of 23 psi per minute.</DIV>
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<DIV>Some people make it easier for themselves under water and figure on 100 psi. So 3.4 cf per 100 psi HP120, 2.5 cf per 100 psi AL80. This is all based at the surface, we know that at99 feet we have roughly 4 ata so the .6cf measurement is now 2.4 cf per minute.</DIV>
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<DIV>That math by the way is what you use to figure the turn pressure. Suppose you and your buddy together breathe under an air sharing situation 2 cf per minute (common value). We're not risking anything here so we plan on doing a normal 30 fpm ascent and also 3 minute safety stop. How do we figure out how much air we'll require?</DIV>
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<DIV>First assume 1 minute to sort it out and share air. 90 feet is 3.7 ata and at 2cf per minute and one minute we'll use 7.5cf sorting things out. Now, how long will it take to get to the stop is the first question. Say for math's sake we're at 90 feet and want to stop at 15 feet. That 75 feet up we need to go. 75/30 gives us about 2.5 minutes going up to the stop, 3 minutes there and then up. We need to know the average depth, I'll assume half the depth or 45 feet. That's (45/33)+1 ata or 2.4 ata. At that pressure we'll consume 2.4ata X 2cf per minute X 2.5 minutes or 12 cf of air is needed to get to the safety stop. Add the time at depth for a total of 19.5 cf of air to the safety stop, now figure the safety stop. That's 15 feet for 3 minutes. 1.5 ata X 2cf X 3 minutes for 9 cf of air giving us a total volume of 28.5 cf of air needed to do a safe ascent from 90 feet and include our safety stop at 15 feet and then ascend.</DIV>
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<DIV>28.5 cf of air is how much psi? From above we know our conversion factor for the tank sizes we use, so the answer for an al80 is 1,104 psi so our turn pressure is 1104 psi and the tank will be empty at the surface, or possibly empty. You can add 500 psi if you so wish, I prefer 300 psi so I'd choose a turn pressure of 1400 psi for that dive on an AL80 tank. My HP120 is 838 psi or adding 500 gives me 1138 turn pressure. Simple math and easy to figure based on any size tank a diver has.</DIV>
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<DIV>That means a responsible diver will leave the bottom of a 90 foot dive on an AL80 with somewhere around 1400 psi of air, that's nearly half the air in your tank. Now you know why people say don't dive deep on an AL80 tank. Short dives result.</DIV>
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<DIV>You can adjust the 2 cf of air used however you wish, I find I run less than 1 cf and my buddies would probably run higher because they're newer and have less experience. I figure for most of my buddies 2cf works out, there are some I'd figure 1.5 cf of air on though.</DIV>
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<DIV>Clear as mud? PS, you now see why many divers believe spare air is not worth the effort. 3 cf of air vs the nearly 30 needed? We're a bit short aren't we. An AL30 to AL40 would be the best size pony for deeper dives in the 90 to 100 foot range. That gives up some leeway because obviously were not both using the pony tank. One person will consume half that or roughly 15 cf to ascend, making a 19cf pony a bit tight.</DIV>
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Doghouse
07-25-2007, 13:15
Very good points.
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<DIV>I see how converting to SCR would be helpful. I had not considered it as I only own 1 tank and rarely go beyond 60'.</DIV>
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cummings66
07-25-2007, 16:10
I find that because I dive with a lot of other buddies and they have tanks that range from steel 72's to steel 130's that being able to do the cf version of air usage is more useful.
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<DIV>However, even on a dive to 60 it's nice to know the air plan. 60 feet deep means 1.5 minutes to the SS. 1.9 ata average depth. Gives about 7.6 cf of gas required to the SS. 9 cf of air at the SS for around 16.6 cf of air needed to come up safely from even 60 feet. On an AL80 that's 643 psi + the 300 I add due to gage's not always being accurate down that low for 943 psi, I'd round to needing 1000 psi on an AL80 coming back from 60 feet.</DIV>
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<DIV>By the way, using those numbers doesn't mean you end the dive with 800 psi of air if nothing goes wrong, what you do is slowly come up changing the time you stay at that depth to match the air needed for an emergency until you get to the point where it's safe no matter what. You could easily end up with 500 psi on top and were safe the entire dive. As long as you have 350 psi at the safety stop you're good to go no matter what happens, and that pressure is lower than almost anybody would run a tank to.</DIV>
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<DIV>So, in conclusion don't fret the high numbers of PSI you're leaving the bottom with becauseyou'll still be able to use them up as you ascend. Just watch the depth and have some built in figures that you know in your head. I use each ata pressure of 4, 3, and 2 and I know that I need to leave each depth at the psi planned on prior. Of course as you get shallower you spend more and more time at the depth if you choose which still gives you a long dive. In other words on an AL80 we left the bottom with 1400 psi, using my SCR I'll have a bit more than 1300 psi by the time I get to 60 feet leaving me with 300 psi at 60 feet to play there with or almost 8 cf of air. That's about 3 minutes of play at that depth before I have to come up or I could continue and save it to play with at a shallower depth. For me I normally play from 20 to 30 feet at the end of a dive.</DIV>
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Mmm Scuba
08-17-2007, 03:36
So what is a good SAC rate?

The Spearfishing link gave me .528 for my last dive.

Buoyant1
08-17-2007, 06:55
So what is a good SAC rate?

The Spearfishing link gave me .528 for my last dive.


Is that an accurate tool?

I just checked my last dive and I got .460

What is a good average rate?

ScaredSilly
08-17-2007, 12:24
IMHO, there is no such thing as a good SAC. It is was it is and is different with each person. Just breathe and breathe easy. Most people find that the more they dive that their SAC rate will go down.

Also I prefer to report my SAC in cuft/min. Makes things easier.

cummings66
08-20-2007, 18:27
Is that an accurate tool?

I just checked my last dive and I got .460


In your case it was off, double it.

Seriously, if you have a good average depth and enter the numbers it's accurate. The problem is most people have no clue as to what the average depth of their dive was.

If you did and it said that then you have a good consumption rate.

cummings66
08-20-2007, 18:30
Also I prefer to report my SAC in cuft/min. Makes things easier.

The age old arguement, I know you mean RMV when you said you like it in cf/min. SAC is really psi per minute but you kind of look at the numbers and you can tell what they meant. If somebody says my SAC is .6 you know what they mean. If they said my sac is 13 and I dive an HP120 you know what they mean. A true SAC number would most likely be any number greater than 2, and in which case it's only useful if you know what tank they used.

I prefer to have cf instead of psi because it applies to all tanks and not just one size.

BobbyWombat
08-23-2007, 12:35
In reading this forum, the question that comes to my mind is "what would your SAC/RMV increase to in an emergency?" I started a seperate thread on this so see what others thought.

WV Diver
08-23-2007, 12:49
My SAC rate averages around 6/PW.

No Misses
08-23-2007, 13:22
The Aeris ACI software calculates my SAC for me (if I enter tank size and Working pressure). Most of my dives are in the .5 to .65 range. I have had one that hit 1.2. This just goes to show, it depends on how hard you are working.

cummings66
08-23-2007, 16:05
I can't say in an emergency, if it wasn't happening to me I don't think mine would get higher than my work rate which is 1 RMV.

Flatliner
08-23-2007, 16:18
My Suunto Cobra does my calc for me in surface cubic feet per min. I like that because it allows me to compare myself with my 11y/o. Unfortunately that usually leads to either sub-clinical depression or a vow to buy doubles.

fire diver
08-23-2007, 16:27
In reading this forum, the question that comes to my mind is "what would your SAC/RMV increase to in an emergency?" I started a seperate thread on this so see what others thought.


Plan on at least double or triple for emergencies. My SAC (RMV for you purists....) is usually in the .38 - .42 range. I plan using .50 and plan emergency gas consumption at 1.0

If I had a lot of hard exertion, I would probably plan for a 2.0

FD