SAC Rate

[bfisher]

I see it talked about and I think I know what it is. How do you come up with a figure? Not that I'm an air hog or that it's important. I just want some more edumacation.

Thank you all.

 

[wedivebc]

I see it talked about and I think I know what it is. How do you come up with a figure? Not that I'm an air hog or that it's important. I just want some more edumacation.

Thank you all.

to calculate it you need to know how much gas you use at a give depth for a given time. If you swim for 5 minutes at 100ft and your al 80 goes from 2000 psi to 1500 psi you have used 500psi of air.
We'll say your tank is 80cuft /3000psi (tank factor changes with every different size)
80/3000 = .027 cuft/psi
therefore you used 500*.027 = 13.5 cuft in 5 minutes
13.5/5 = 2.7 cuft used in 1 minute and because you were at 100ft which is 4 ata you need to convert it to surface pressure
2.7/4 = .675 cuft/minute at the surface.

hope that helps.

 

[spectrum]

Not that I'm an air hog or that it's important.

With that many dives I bet it could have been important at least a few times.

If you dive with a computer you can use the bottom time, average depth and start/finish pressures with cylinder data to get an overall number.

I track it in my log spreadsheet and the varriations almost always coincide with the ease of the dive.

Pete

 

[bfisher]

OK, I had to read it twice to get it. My mind just wasn't "right here" the first time. Now that I've got it digested I have another question. What would I use the figures for.

Spectrum has me thinking that I don't plan dives in enough detail. And with all due respect, I am strictly a recreational diver that does all my diving in warm waters, mostly in the Caribbean. Always travel and dive with a group and usually dive op DM's.

I have dove the quarries up here in Pa. a few times. Like four times in three years. I care not for the murky water or it's thermocline, thankyou very much. But I can still learn things from you guys.

 

[Peter Guy]

What would I use the figures {Sac rate} for?

Please see TSandM's thread on does DIR training have a value for the "recreational" diver.

 

[TSandM]

If you're doing mostly warm water, DM-led dives, you don't need a whole lot of dive planning.

Things I personally think you should know: Rock bottom, or the amount of gas required to get you and a buddy to the surface from however deep you've gotten yourselves. To know this, you need to know your SAC rate and your buddy's (or at least a reasonable estimate thereof), and calculate the time required to get to the surface at a safe ascent rate, with whatever stops you think are important to you in the process. If you're down at 100 feet without enough gas to get two of you to the surface, neither of you is much help to the other, and in the event of a major equipment failure, you're going to be SOL.

The second thing is to figure out whether the dive the DM proposes is something YOU can do on the tank supplied. If you know your SAC rate, you can figure out how long you're likely to be able to stay at 100 feet on an Al80, and whether that's similar to the proposed time from the DM. If you know before you submerge that your gas consumption is such that you can't do the Cozumel Devil's Throat dive, it's better than finding out a third of the way into the ascent that you don't have enough gas to get to the surface.

In my opinion, everybody who dives should know those two things.

 

[Skull]

So is your SAC rate the same for a shore dive as with a 100ft wreck dive? Is it the same in july at a particular dive site as in december? Is it the same as the 10th dive on a vacation as it is the first time you dive after a 8 week layoff?

Conditions are what dictate your SAC rate ...for the vast majority of recreational divers.

I look at my profiles after everydive by downloading it on my PC from my DC ...I am astounded by the variations always due to conditions...

Just a reality check!!

 

[bfisher]

I thank everybody who has posted. I guess I'm just gonna have to figure some of this stuff out. I do plan on a Bonaire trip in a onth and it'll all be shore diving, with a group from here at home. I know who my buddy will be and he's a good Dive Master. We get very comaprable air times and I know there won't be any problems.

In most cases, as in Cozumel, I rarely get back on the boat with less than 1000lbs of air (AL-80), even on the Devil's Throat dive. So I don't consider air consumption as a problem for me. I've even considered dropping to an AL-63 just to trim things down, but further thought changed my mind. That AL-80 doesn't weigh much under water and if not me, then who else just might have use of some of that extra air in my tank? You never know.

I've also given some thought to what would happen in the case of a wreck dive or something if I should get into any type of entanglement or other such situation. It sure wouldn't hurt to have a little extra air, wouldn't you say?

So basically what I'm saying is that, although I'm noted as being a little adventuresome, I don't feel a need to prove anything other than that I'm having a good time and wish to continue to do so. Actually, the longer I dive the more conservative I'm beginning to get. Guess that camera has something to do with it.

Again, I thank you all for your responses. It has been helpful.