Anyone not measure their SAC?

[MargaritaMike]

I LOVE ScubaBoard!!!! @soggybadger asked a question "Anyone not measure their SAC?" He got several responses and then he said

Thanks for the replies fellas. I took that as no's.

There has now been 3 pages of discussion of what SAC is, what RMV is, why someone would use these, the importance (or non-importance) of confusing them, the need for standard terminology, what the implications to reserve air are, why we need to know reserve air, the fact that 10 out of 11 divers in a bar don't even know what SAC is (I really liked that one), links to articles on SAC/RMV (thanks, Bob), links to YouTube, etc. etc. etc.
I realize that everyone who has posted (including myself) had something they thought was germane to SAC measurement. There has been some great info regarding the calculation and use of SAC, RMV, and other acronyms, but the real answer to the OP's question (which is kind of posed backwards) is "No, there are a LOT of people who DO measure their SAC in one form or another".
Have a GREAT day!

Cheers - M²

 

[stuartv]

I LOVE ScubaBoard!!!! @soggybadger asked a question "Anyone not measure their SAC?" He got several responses and then he said

There has now been 3 pages of discussion of what SAC is, what RMV is, why someone would use these, the importance (or non-importance) of confusing them, the need for standard terminology, what the implications to reserve air are, why we need to know reserve air, the fact that 10 out of 11 divers in a bar don't even know what SAC is (I really liked that one), links to articles on SAC/RMV (thanks, Bob), links to YouTube, etc. etc. etc.
I realize that everyone who has posted (including myself) had something they thought was germane to SAC measurement. There has been some great info regarding the calculation and use of SAC, RMV, and other acronyms, but the real answer to the OP's question (which is kind of posed backwards) is "No, there are a LOT of people who DO measure their SAC in one form or another".
Have a GREAT day!

Cheers - M²

I think they all knew what SAC is. They didn't know what THEIR SAC is.

 

[kelemvor]

Hi folks. Just wondering if anyone on here doesn't record their air consumption. I don't log dives at the moment and haven't bothered to record my gas use per dive. This is not a problem as I always seem to have plenty for my dives and redundancy. If and when I get a bit more technical I will log but at the rec level I just don't see the point. Thanks.

My dive computer does it for me. I think you were counting that as no? I think it's an important thing to know but I'm not in the habit of manually doing what the dive computer does unless I have reason to suspect an error.

 

[stuartv]

I LOVE ScubaBoard!!!! @soggybadger asked a question "Anyone not measure their SAC?" He got several responses and then he said

There has now been 3 pages of discussion of what SAC is, what RMV is

It did seem like this part of the OP invited additional commentary beyond just the simple Aye or Nay.

at the rec level I just don't see the point.

 

[cneal]

I never have calculated SAC. With the difference between the first dive of a trip and the 20th, trying to plan a dive to a number would be delusional.

Gas management is checking air everytime I touch the camera shutter. When I'm down to where I will be back at the boat or surface with a reasonable amount left I turn the dive. Since I rarely go deep and never into deco it has always kept me out of trouble.

Time to duck.

 

[kelemvor]

It did seem like this part of the OP invited additional commentary beyond just the simple Aye or Nay.

I didn't even think to address that in my response. The reason I use the SAC that my DC tracks for me in recreational diving is simply because most of my (single tank) dives are ended because of gas consumption rather than nitrogen loading. Someday, I'd like for the opposite to be true. I guess the fact that I always dive nitrox makes this goal tougher than it would be for some.

 

[stuartv]

I didn't even think to address that in my response. The reason I use the SAC that my DC tracks for me in recreational diving is simply because most of my (single tank) dives are ended because of gas consumption rather than nitrogen loading. Someday, I'd like for the opposite to be true. I guess the fact that I always dive nitrox makes this goal tougher than it would be for some.

Out of curiosity, what is your RMV and what tanks do you use? And what is your typical dive depth?

 

[MargaritaMike]

I think they all knew what SAC is. They didn't know what THEIR SAC is.

Point well made, however:

NONE of them knew their SAC/RMV or even how to figure it out.

the fact that they didn't know how to figure it out implies they REALLY DIDN'T KNOW what SAC/RMV are. With that being stated, I have to admit that until recently (within the past year), I didn't know what the acronyms were. I knew my general consumption rate on a given tank and knew how long I could stay at a given depth on a given day doing a given activity. I don't think I ever (until recently) did any actual computations. My Perdix AI is what really started me thinking about it. But now that I'm thinking about it, I don't have to think about it because my computer thinks for me.

Cheers - M²

 

[Kevrumbo]

. . .Thus, when I say RMV, I always mean volume per unit of time. If I mean pressure per unit of time, I will say SAC. But, if I'm just talking about generic consumption - as in, for comparison purposes, I will generally say SAC. Obviously, in almost all contexts, if you are comparing consumption you would be assuming the same size cylinder, so SAC or RMV would always give the same answer. If SAC is higher, then RMV is higher. If SAC is lower, then RMV is lower.

Not necessarily true. . . The correct statement is:

RMV is consistent across all cylinder sizes including manifolded double cylinders -also known as manifolded twinsets- but pressure SAC is not. See below.

Al my planning is done on a SAC of 15L/min.

When measured, it goes from 10 to 18 with a, average @ 13.

Since I carry 95% of the time a 15L tank, the calculations are easy: 1bar/minute at the surface.

Freewillow uses a planning RMV or volume SAC of 15 liters/min per ATA, with a 15 liters/bar tank (@stuartv , that's similar to your hp120 cylinder):

15 liters/min per ATA ÷ 15 liters/bar = 1 bar/min per ATA pressure SAC.

If Freewillow instead utilizes a manifolded twinset of 15 liters/bar cylinders (or 30 liters/bar total):

15 liters/min per ATA ÷ 30 liters/bar = 0.5 bar/min per ATA pressure SAC.

Therefore you can have the same RMV or volume SAC rate across all cylinder sizes as well as twinsets, but not necessarily the same pressure SAC rate.

Incidentally @stuartv , if you use your Perdix AI with double manifolded hp120's, and set it to Bar (Metric System), the pressure reading will also indicate an approx value in cubic feet remaining:

Proof:

1 cubic foot = 28.3 liters;

30 liters/bar tank ÷ 28.3 liters/cubic foot = 1.06 cubic foot/bar tank or approx 1 cubic foot per bar.

So a full 240 bar reading on a digital or analog SPG will also indicate 240 cubic feet of gas available.

In other words -in this instance only- there is an apparent one-to-one correspondence between bar pressure units and cubic feet volume units.

 

[kelemvor]

My dive computer does it for me. I think you were counting that as no? I think it's an important thing to know but I'm not in the habit of manually doing what the dive computer does unless I have reason to suspect an error.

Actually, I just thought about this. The dive computer does not calculate SAC for me. It merely records depth/pressure/time and some other data. It's subsurface that calculates the SAC. Not that it really matters.