Calculating SAC?

[stuartv]

If you can find it you should pick up Steve Lewis' book "The Six Skills and Other Discussions: Creative Solutions for Technical Divers".

I found it on Amazon. But, dang it, there's no Kindle edition! Who reads paper any more?!?

 

[Dhboner]

I gather you are talking to me? I spent two years majoring in Mechanical Engineering in college before I switched to Applied Math. I was definitely not lost.

But, I really do appreciate you taking the time to write that post. I would also go out on a limb, though, and say that no "rate" can be reported in PSI. However, it could be reported in PSI per minute. Or liters per minute. Moles per second. Pounds per hour. Stone per fortnight. Or any other units you want that are suitable for the measurements being taken. :-D

CptTightPants21 went to great lengths to give you an incredibly detailed answer in what was clearly an attempt to help you with your question. You might want to just hit "like" and leave it alone ;-)

 

[eelnoraa]

A simple and quite accurate way: with a computer with average depth. note your initial pressure, note your pressure after your dive and dive time. you should have enought information to calculate average PSI use your average depth. Then knowing your tank factor, you should be able to calculatae SAC.

Once you know these number, next time, before you look at your guage, guestimate the reading and check it to see if they match

 

[tursiops]

CptTightPants21 went to great lengths to give you an incredibly detailed answer in what was clearly an attempt to help you with your question. You might want to just hit "like" and leave it alone ;-)

Really? "The official SAC rate is a number reported in PSI" is hardly a correct statement.

 

[stuartv]

CptTightPants21 went to great lengths to give you an incredibly detailed answer in what was clearly an attempt to help you with your question. You might want to just hit "like" and leave it alone ;-)

Right. Even though I didn't appreciate the "I'm sure you're probably lost" comment, it seems to be the norm around here to automatically assume that everyone new to diving has no better than an 8th grade education - and got straight Cs in getting there, so I didn't let it get me wound up.

But, I do hope that anyone who comes along later and reads this thread because they were wondering the same thing as I was will not read that post and leave thinking that a number of pounds per square inch is a rate of air consumption. As we say, that just ain't right.

100 psi =3.788 cuft of air in a low pressure tank, but 100 psi = 2.905 cuft in a high pressure tank.

Also, maybe low pressure tank and high pressure tank mean more to you experienced folks than they do to me, but since this is the New Divers forum I have to ask. Because, to me, that statement just says that one tank is rated to hold a gas at a higher pressure than the other. When I used to play paintball, there were low pressure tanks that only held 3000 psi, and there were 4500 psi tanks (aka high pressure tanks) - but you could get either in the same range of sizes.

If two tanks are, say, 12L in volume, and one is rated to hold, say 200 bar, and one is rated to hold 300 bar, then isn't 100 psi of air going to be the same cubic feet at STP (Standard Temperature and Pressure), regardless of which tank it came out of? I don't see why being rated for higher pressure would make any difference to that.

Maybe what that really means is that a 100 psi drop from 4500 psi to 4400 psi represents a 3.788 cubic feet at STP and a 100 psi drop from 3000 to 2900 represents 2.905 cubic feet (at STP)? I have not done the math to verify that, so I'm just asking.

I apologize because I'm sure I'm sounding like a dick. But, I don't believe in dumbing stuff down in general, and especially not when the result is that the information is actually not correct (like saying that SAC is psi). And I don't especially like being spoken to like I'm stupid or even woefully uneducated, really. "I'm sure your probably lost" is borderline insulting. I understand that it wasn't intended that way - it's just the Way of ScubaBoard - so I'm taking it in the spririt in which I believe it was intended. And I really, really do, sincerely appreciate the CptTightPants was willing to take the time to write all that up.

 

[CptTightPants21]

Really? "The official SAC rate is a number reported in PSI" is hardly a correct statement.

What Is a Surface Air Consumption (SAC) Rate?

No really.

 

[gopbroek]

As I understand it SAC rate is measured in PSI (imperial) per minute and qualified by the size of bottle. The useful measurement is RMV rate which is the actual volume of gas per minute consumed at the surface (1 ATA). This value can then be easily scaled for depth; 0.5 CF/Min at the surface would be around 2.0 CF/Min at 100 Ft.

Ripped from http://scuba.about.com/od/Theory/ss/Air-Consumption-Rates-For-Scuba-Diving-Sac-Rates-Rmv-Rates-Easy-Calculations_4.htm
“A surface air consumption rate, or SAC Rate, is a measurement of the amount of air a diver uses in one minute on the surface. SAC Rates are given in units of pressure; either in psi (imperial, pounds per a square inch) or bar (metric).”

“A Respiratory Minute Volume Rate (RMV Rate) is a measurement of the
volume of breathing gas that a diver consumes in one minute on the surface. RMV Rate are expressed in either cubic feet per a minute (imperial) or liters per a minute (metric).”
That being said most people (myself included) use the RMV rate as their ‘SAC’ rate. I use Subsurface Dive log with an AI computer and it calculates the ‘SAC’ for me but the number displayed is actually the RMV rate (cubic ft. / min), which I am happy with.

 

[CptTightPants21]

Right. Even though I didn't appreciate the "I'm sure you're probably lost" comment, it seems to be the norm around here to automatically assume that everyone new to diving has no better than an 8th grade education - and got straight Cs in getting there, so I didn't let it get me wound up.

But, I do hope that anyone who comes along later and reads this thread because they were wondering the same thing as I was will not read that post and leave thinking that a number of pounds per square inch is a rate of air consumption. As we say, that just ain't right.



Also, maybe low pressure tank and high pressure tank mean more to you experienced folks than they do to me, but since this is the New Divers forum I have to ask. Because, to me, that statement just says that one tank is rated to hold a gas at a higher pressure than the other. When I used to play paintball, there were low pressure tanks that only held 3000 psi, and there were 4500 psi tanks (aka high pressure tanks) - but you could get either in the same range of sizes.

If two tanks are, say, 12L in volume, and one is rated to hold, say 200 bar, and one is rated to hold 300 bar, then isn't 100 psi of air going to be the same cubic feet at STP (Standard Temperature and Pressure), regardless of which tank it came out of? I don't see why being rated for higher pressure would make any difference to that.

Maybe what that really means is that a 100 psi drop from 4500 psi to 4400 psi represents a 3.788 cubic feet at STP and a 100 psi drop from 3000 to 2900 represents 2.905 cubic feet (at STP)? I have not done the math to verify that, so I'm just asking.

I apologize because I'm sure I'm sounding like a dick. But, I don't believe in dumbing stuff down in general, and especially not when the result is that the information is actually not correct (like saying that SAC is psi). And I don't especially like being spoken to like I'm stupid or even woefully uneducated, really. "I'm sure your probably lost" is borderline insulting. I understand that it wasn't intended that way - it's just the Way of ScubaBoard - so I'm taking it in the spririt in which I believe it was intended. And I really, really do, sincerely appreciate the CptTightPants was willing to take the time to write all that up.

Perhaps it was presumptuous of me to to think you would not get it. I apologize. As someone who has gone over this 2-3 times with 5-6 divers, I can say from experience that many do not get it when they are first starting out.

A HP 80 and a LP 80 both read 800 PSI...which tank has more air or do they both have the same?

 

[tomfcrist]

Well stuartv, I've noticed a pattern in your posts. You ask a solid question, and get some solid feedback. You then immediately go into prick mode and get defensive. Hey dude...you asked the fuxing question. Honestly if you have an applied math degree with an engineering background you should be able to figure out you sac/RMV fairly easily.

When he said sac is a psi measurement, it was mostly correct. Sac in scuba is measured in psi per minute. When you were talking liters per minute, you were talking RMV... Not sac, so when cpttightpants assumed you were "lost" that is the basis for his determination.

Just FYI, you already have a good number of very knowledgable and experienced divers on this board just about fed up with you. It's kinda hard to get advise and help from this forum when you piss off the folks that actually have the info you need.

I probably sound like a dick right now, and if that's what it takes to get you to throttle back and think before posting...I guess being a dick isn't such a bad thing. Seriously, quit getting so pissed off every time someone tells you how to do something/why something is wrong. You come off as already having made your mind up and the hell with anyone who tells you you are wrong.

Just trying to help YOU out Stu.

 

[gopbroek]

Perhaps it was presumptuous of me to to think you would not get it. I apologize. As someone who has gone over this 2-3 times with 5-6 divers, I can say from experience that many do not get it when they are first starting out.

A HP 80 and a LP 80 both read 800 PSI...which tank has more air or do they both have the same?

OK I will bite (and probably will be wrong): An HP 80 (I think) runs a pressure of about 3450 PSIG to hold 82 Cu/Ft of air (@ 1 ATA) and a LP80 holds 78 Cu/FT @ 2400 PSI so it is not an apple to apple comparison due to the 5 % greater capacity of the HP. I still think the actual internal physical volume of the HP is smaller so I am going to guess that given equal pressures the LP80 will have a greater volume of air in it.