meaning of SAC?

[Charlie99]

The only thing you're missing is that different agencies have different definitions of SAC and RMV.

Some define SAC as a measure of gas consumption in psi/min or bar/min. Obviously, THIS definition of SAC causes SAC to depend upon the tank size.

Others define SAC in cubic feet or liters per minute. Using that definition, tank size has nothing to do with it.

As long as one specifies the units, then there isn't any confusion.

 

[The Kraken]

One last question . . .

If you're going on a dive do you ask your dive buddy "What size PSI tank are you diving?" or do you ask "What size cubic foot/liter tank are you diving?".

People don't measure how much they breathe by PSI, they measure how much they breathe by volume.

the K

 

[Charlie99]

Arguing about what definition to use doesn't serve a useful purpose, particularly when all parties understand the issues at hand, but just choose to express them differently.

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Liters per minute or cubic feet per minute is a standardized measurement that is easily understood and is independent of tank. Kind of like the reason we normalize air consumption to 1ata conditions rather than having to specify consumption rate at a whole bunch of different depths.

When we get in the water though, my SPG is calibrated in PSI and that's how I monitor my actual gas consumption at depth.

As to what I ask my dive buddy, of course I ask both what size tank he has (at what rated fill pressure) and what his actual starting fill pressure really is. Both items are needed. Same for the metric world. I both need to know that it's an 11 liter tank, and also that he is starting off at 200bar. Unless I have both bits of data, I don't have the whole picture.

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Just to confuse things a bit more, when planning gas consumption for a dive, I find the most useful unit is a non-standard one ---- the "surface equivalent minute". Rather than figuring that my AL80 has 3000psi or 77.4cu ft (or 2500psi/65 cu ft before reserve), I figure that my AL80 has 160 "surface equivalent minutes" in it. Of course, that is only true for a low 0.4cfm SAC, and for an unknown diver I'll assume that he has only 90 "surface equivalent minutes" in his tank (which corresponds to 0.75cfm/21lpm).

There are many ways to calculate, plan, and monitor gas consumption. Use whatever works for you.

 

[The Kraken]

Charlie, I like your SEM !!!

the K

 

[Cave Diver]

Isn't the real issue of this debate whether or not the consumption RATE is dependent on cylinder size?

I have to agree with Kracken, there are LOTS of variables with SAC or RMV. But it's my understanding the size of the tank will determine how MUCH gas you have to consume, but not the RATE of consumption. Or am I missing something very simple? (which isn't hard for me )

JR

Yes, the figure you calculate for SAC rate is dependant on tank size.

Lets see if this helps. A diver has a respitory minute volume (RMV) of .50

To figure his SAC rate for an AL80 and an AL100 you use: SAC = (CWP/CV) x RMV

SAC(for the AL80) = (3000/80) x .50 ###SAC(for the AL100) = (3000/100) x .50
SAC = (37.5) x .5 ############### SAC = (30) x .5
SAC = 18.75 psig per minute ######## SAC = 15 psi per minute

If you want to know how much gas (in psi) you will consume on a 20 minute dive to 100 ft for each tank use: AC = SAC x ATA x T

AC = 18.75 x ((100/33)+1) x 20 #### AC = 15 x ((100/33)+1) x 20
AC = 18.75 x 4.03 x 20 ########## AC = 15 x 4.03 x 20
AC = 1511.25 psi ############## AC = 1209 psi

To find out cft of gas you will use for that same dive use: AC = RMV x ATA x T
AC = .50 x ((100/33)+1) x 20
AC = .50 x 4.03 x 20
AC = 40.3 cft of gas

So a diver with an RMV of .50 will consume 40.3 cft of gas on a 20 minute dive to 100 ft regardless of which tank size is used. However, because of the difference in tank size, they will use 1500 psi with the AL80 and 1200 psi with the AL100. For this reason, measuring gas consumption in psi does not give an accurate reflection of breathing rate. It is also the reason why dissimilar tank turn pressures are calculated based on tank volume then converted to psi.

I feel the problem with this discussion is that fact that different agencies have different definitions of the term "SAC/SCR." The definition I was taught of SAC through IANTD measures psig/min. Other agencies apparently teach a definition based on volume.

Most of us agree on the concept, just not the terminolgy.

 

[Cave Diver]

Just to confuse things a bit more, when planning gas consumption for a dive, I find the most useful unit is a non-standard one ---- the "surface equivalent minute". Rather than figuring that my AL80 has 3000psi or 77.4cu ft (or 2500psi/65 cu ft before reserve), I figure that my AL80 has 160 "surface equivalent minutes" in it. Of course, that is only true for a low 0.4cfm SAC, and for an unknown diver I'll assume that he has only 90 "surface equivalent minutes" in his tank (which corresponds to 0.75cfm/21lpm).

There are many ways to calculate, plan, and monitor gas consumption. Use whatever works for you.

This was also part of my deco procedure class and I come up with 200 and 106 SEC minutes respectively for your example. Do you mind posting your calculations?

 

[Packhorse]

I have always calculated SAC or SCR based on volume not pressure. To use pressure IMO is pointless and confusing when talking about diffrent size tanks.
Of course the metric SAC/SCR is very easy to work out.
All you need to know is volume of air consumed (bar used x tank size) / time/average pressure (depth in meters/10 + 1)
So a 30 minute dive with a average depth of 20m using a 10 liter tank and you have used 170 bar = 1700 liters of air
1700/ 30 min = 56.6 liters per minute
56.6 / 3 ata=18.9 litre per minute SAC

 

[Scuba]

This was also part of my deco procedure class and I come up with 200 and 106 SEC minutes respectively for your example. Do you mind posting your calculations?

I believe he is excluding a 500 psi tank reserve from calculations, and using remaining 2500psi/65 cu ft..

 

[Cave Diver]

I believe he is excluding a 500 psi tank reserve from calculations, and using remaining 2500psi/65 cu ft..

Yep, that would do it. Thanks.

 

[joe rock]

Yes, the figure you calculate for SAC rate is dependant on tank size.

Lets see if this helps. A diver has a respitory minute volume (RMV) of .50

To figure his SAC rate for an AL80 and an AL100 you use: SAC = (CWP/CV) x RMV

SAC(for the AL80) = (3000/80) x .50 ###SAC(for the AL100) = (3000/100) x .50
SAC = (37.5) x .5 ############### SAC = (30) x .5
SAC = 18.75 psig per minute ######## SAC = 15 psi per minute

If you want to know how much gas (in psi) you will consume on a 20 minute dive to 100 ft for each tank use: AC = SAC x ATA x T

AC = 18.75 x ((100/33)+1) x 20 #### AC = 15 x ((100/33)+1) x 20
AC = 18.75 x 4.03 x 20 ########## AC = 15 x 4.03 x 20
AC = 1511.25 psi ############## AC = 1209 psi

To find out cft of gas you will use for that same dive use: AC = RMV x ATA x T
AC = .50 x ((100/33)+1) x 20
AC = .50 x 4.03 x 20
AC = 40.3 cft of gas

So a diver with an RMV of .50 will consume 40.3 cft of gas on a 20 minute dive to 100 ft regardless of which tank size is used. However, because of the difference in tank size, they will use 1500 psi with the AL80 and 1200 psi with the AL100. For this reason, measuring gas consumption in psi does not give an accurate reflection of breathing rate. It is also the reason why dissimilar tank turn pressures are calculated based on tank volume then converted to psi.

I feel the problem with this discussion is that fact that different agencies have different definitions of the term "SAC/SCR." The definition I was taught of SAC through IANTD measures psig/min. Other agencies apparently teach a definition based on volume.

Most of us agree on the concept, just not the terminolgy.

Thanks Cave Diver, I do understand that. It seems there are two different things going on in this thread (although I could be missing something). One is the calculation and what you're calculating. The other is what effects consumption rate. No question the size of the tank is critical for the calcuation of either measure, but I don't think the size of a tank effects consumption rate. If for whatever reason I have a SAC of .5 on a dive, wouldn't it be .5 wheter I used an AL 80 or a steel 100?