SAC vs. SCR???

[NorCalDawg]

Okay, so the only measure of air consumption i've ever seen here on scubaboard is SAC, however, the only measure of consumption i have been taught in a class is SCR. I know the calculations for SCR and that it gives you your PSI/minute, so why use SAC to get cu. ft per minute instead?

 

[Charlie99]

Cubic feet per minute are independent of which tank is used, so is a more universal measure.

If you change tanks, you will have to convert your SCR in psi/min to an SAC in cubic feet, then back to SCR in psi/minute on the new tank.

Different people and agencies use different defintions for RMV, SAC, and SCR. Fortunately, as long as you say what units (i.e psi/min, liter/min, cfm) you are using, the definition is clear, even though different people may have a different name for it.

For the common AL80 tank, SCR to SAC conversion is pretty simple using either the "tank factor" of 2.6cu ft/100psi, or the inverse of that -- 39psi/cu ft. Since SAC/SCR/RMV is a general range rather than a specific repeatable number, I round off to 40psi/cu ft to simply the mental arithmetic.

 

[Mark Vlahos]

SAC = Strategic Air Command

SCR = Silicon Controlled Rectifier

The first one was an extremely complicated branch of the United States Military, one of its missions was as the first line of defense in a nuclear war involving the United States. The second is one component used in large scale incandescent light dimming applications, commonly used in entertainment.

Frankly they really don't compare very well...




OK, that was not what you really asked, but I just felt frivolous.

Mark Vlahos

 

[frankc420]

What Charlie said

 

[ClayJar]

Tank-independent SAC numbers start showing how useful they are when you start using several different size tanks. I regularly use 80- and 100-cf tanks, and I always carry a 19-cf pony, except when I swap it for something larger. When you're working out a dive plan, it's a lot easier to think in cubic feet than to think in multiple pressure scales at once.

On a somewhat related but more forward-looking note, if you log your SAC for each dive in your logbook, you'll have a number you can use for comparison throughout your diving. If you log a tank-dependent number, you'll have to do conversions if you decide to see how your past consumption compares to what you're getting in that drysuit with those doubled HP-130s with the AL80 stages.

 

[TeddyDiver]

When you're working out a dive plan, it's a lot easier to think in cubic feet than to think in multiple pressure scales at once.

And a much more easier to think metric
Tanks: 10, 12, 15 litres, volume (and capacity at 1bar)
Pressure: BAR, almost 1ATM
Depth: meter, each 10m deeper equals one more bar ambient pressure.

Dive to 25m 50min, 12ltr tank, starting 205bar, end 55bar

Air used (205-55)*12=1800ltr
1800/50=36ltr/min
36ltr/3,5=10,3ltr/min at surface:14:

 

[ClayJar]

And a much more easier to think metric

People who insist on saying everyone should work in metric are sadly misguided.

We should all be working in SI.

 

[TeddyDiver]

People who insist on saying everyone should work in metric are sadly misguided.

We should all be working in SI.

But how many know what we are talking about then? And what if they really check up your link No more funny ','', cfm, oz

 

[Charlie99]

But how many know what we are talking about then? And what if they really check up your link No more funny ','', cfm, oz

SI doesn't have any Bar either. Pascal, megaPascals, kiloPascals yes. But no bar, which are equal to 100 kPa.

 

[ClayJar]

(The fun bit is that in chemical engineering, we'd often use things like "kilopounds" and the like. If I remember correctly, the speed of light is approximately 1.8 megafurlongs per microfortnight, too.)