SAC Rate

[EFX]

Hey, when you get here get a bottle of "jack". The bottle capacity is in metric. Have a drink. There. Problem solved.

 

[Kevrumbo]

One of the many reasons I prefer the Euro standard for designating tank size: water volume and service (fill) pressure

It's easier to fundamentally explain and conceptualize the rated volume and service pressure of a scuba cylinder based on the standard European/Asian surface atmosphere reference convention of 1 bar: The common AL80 tank has a metric cylinder rating factor of 11 liters/bar, or in other words, at the surface of 1 bar, if you pour water into the cylinder, the measured liquid volume it can contain is 11 liters.

However when pressurized instead with breathing gas to any value up to its recommended Service Rating (207 bar for the 11L per bar Alu cylinder in this example ), a cylinder carries an equivalent volume of free gas much greater than its water capacity, because the gas is compressed to several hundred times the standard surface atmospheric pressure of 1 bar (as opposed to water which is incompressible). So if you have a gas pressure reading of 200 bar in your AL80 tank, you have a total available free gas volume of 200 bar multiplied-by 11 liters/bar or 2200 liters.

 

[Kevrumbo]

Using the formula from TDI's Intro to Tech manual, p. 46...
[(psi used/working pressure) x cylinder capacity] / {[(depth/33)+1] x minutes}

If I go from an AL80 tank with a working pressure of 3000 to a steel HP80 with a working pressure of 3400 and everything else remaining the same, mathematically, my SAC rate goes down because of reducing the numerator based on the higher working pressure of the HP tank.

. . .

I think this is a better arithmetic example to understand, using AL80 cylinders to calculate:

Example:
Given a Volume Sac Rate of 22 liters/min per ATA, divide it by the Tank Factor Rating of the Cylinder in use. For an AL80 cylinder with a metric tank factor rating of 11 liters/bar:

22 liters/min per ATA divided-by 11 liters/bar = 2 bar/min per ATA Pressure Sac Rate

Now instead of a single AL80 with an 11 liters/bar tank factor rating, use a twinset (double manifolded AL80's) for a total of 22 liters/bar tank factor rating, and let's see what happens to the Pressure Sac Rate:

22 liters/min per ATA divided-by 22 liters/bar = 1 bar/min per ATA Pressure Sac Rate.

So the point is: your given Volume Sac Rate of 22 liters/min per ATA is an arbitrary constant across all tank sizes & capacities, but it is your Pressure Sac Rate that will change with the sizes & capacities (i.e. tank rating factors) of the cylinders that you actually use. . .

(Note: your “given” arbitrary Volume Sac Rate can be anything you choose from “panic physically stressed” like 30liters/min per ATA for example, to a low “drift diving with the current” relaxed state of maybe something like 12liters/min per ATA as an example)