Help in understanding SAC rate

[ron63]

I need a little help in understanding how knowing my SAC rate will help me with estimating my air consumption. No need to explain how to figure your SAC rate as I know how to do that and know my SAC rate. If I understand it right with a SAC rate of .5 an 80CuFt tank with a starting PSI of 3300 should give someone 160 minutes of air at the surface, 80 minutes of air at 33 feet, 53 minutes of air at 66 feet, and 40 minutes of air at 99 feet. Is that correct? Is that all there is to it or am I missing something? In those famous words of Charlie Brown...AGGGHHHHHH!

Thanks,

Ron Shephard

 

[DNK]

Thats about it.
Don't forget to save some for the trip up and for the boat

 

[PfcAJ]

And for your buddy!

 

[HowardE]

Here. http://www.spearfishing.org/bruces_tips/java/sac.html

 

[ron63]

Thanks. The SAC calculator really makes computation easy. There was a difference of about .04 in my figuring and the SAC calculator's figures. It was a little higher but probably more accurate.

Ron

 

[spectrum]

Thanks. The SAC calculator really makes computation easy. There was a difference of about .04 in my figuring and the SAC calculator's figures. It was a little higher but probably more accurate.

Ron

It will vary from dive to dive so you want to look at it over time. After a while you will see a trend based on the dive, gear and conditions. You can then apply the appropriate value to plan a similar future dive.

Pete

 

[DiveDiva_Rach]

This was written by a member of SDNZ and may be of use to you:

http://www.scubadive.net.nz/bb/viewtopic.php?t=135

I hope it helps

DD
Win a dive trip to the Poor Knights Islands! Enter SDNZ's March UW Photo Contest!

 

[Meng_Tze]

It will vary from dive to dive so you want to look at it over time. After a while you will see a trend based on the dive, gear and conditions. You can then apply the appropriate value to plan a similar future dive.

Pete

A very good point spectrum!

My advise is to plan more conservatively (including reserves and such) for the first 10 dives or so and meticuously write down what you did, where you were and what your consumption was. This will then serve as a baseline for future dives.

This is what I write down:
site and condistions
wet/drysuit
any special equipment that could influence drag or work load

then a table on a slate

time: depth (distance): gas reading:

this will give you all info to calculate everything. It is cumbersome initially, but will pay back in spades in the future when you can just look up a profile and plan your dive pretty accurately.

 

[TSandM]

If you combine a knowledge of your SAC rate with a concept called tank factor, you can very easily predict your gas utilization, and check it on the fly.

Tank factor's easy: How many cubic feet are in 100 psi in your tank? For example, for my double 85's the TF is 6.4. That tells me that every 100 psi I use is 6.4 cubic feet of gas.

Now, if my SAC rate is .4 (which it is) and I'm at 60 feet (3 ATA), I'm using 1.2 cu ft per minute. Every five minutes, I'm using 6 cu ft. So I should be using close to 100 psi every five minutes. If I check my gauge, I should see a 100 psi drop. If it's less than that, either I'm REALLY relaxed or maybe, for example, my left post is turned off (I'm diving doubles) or the isolator is closed. If it's more than that, it's very possible I have a reel in my hand . . . or there's current I haven't noticed, or I'm uncomfortable or nervous. But at any rate, in that circumstance, the gas I have won't last as long as I'd planned it would. That's a good thing to know.

This sounds ridiculously complicated, but I did a nice recreational dive today, and every time I checked my gauge, I asked myself what I expected to see, and the resulting readout was right on. It's very reassuring.

 

[H2Andy]

Tank factor's easy: How many cubic feet are in 100 psi in your tank? For example, for my double 85's the TF is 6.4. That tells me that every 100 psi I use is 6.4 cubic feet of gas.

yes, and you figure out tank factor (for lack of a better name) by knowing how many cubic feet per psi your tank holds and multiplying by 100

for example, an AL80 will hold 80 cubic feet (let's cheat) at 3,000 psi

that is .0266 cubic feet per psi

or 2.66 cubic feet per 100 psi

my LP85's hold 85 cubic feet at 2640 psi each

that is .0321 cubic feet per psi each

or 3.21 cubic feet per 100 psi each

for a total of 6.42 cubic feet per 100 psi when doubled