And here's an exercise on the difference between Volume SAC Rate (also known as RMV) and Pressure SAC Rate, and how to actually use Pressure Sac rate for the cylinder that you are breathing.

Example:

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

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

Now instead of a single AL80 with an 11 liters/bar tank factor rating, use a twinset (double 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*ATA divided-by 22 ~~liters~~/bar = **1 bar/min*ATA Pressure Sac Rate**.

So the point is: your Volume Sac Rate of 22 liters/min is 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.**

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Below are some example pressure Surface Consumption Rate (SCR) values for a variety of common cylinders with a given arbitrary volume SCR (also known as volume SAC rate or RMV):

Given a arbitrary nominal volume SCR of 22 liters/min per ATA (that's 0.78 cuft/min per ATA in US Imperial Units, a reasonable & achievable volume SCR for most novice divers):

Cylinder Size | Pressure SCR

11L/bar tank (AL80): **2bar/min per ATA**;

12L/bar tank (Steel HP100): 1.8bar/min per ATA;

13L/bar tank (AL100): 1.7bar/min per ATA;

15L/bar tank (Steel HP119): 1.5bar/min per ATA;

16L/bar tank (Steel HP130): 1.4bar/min per ATA;

11L Twins (Double AL80's): **1bar/min per ATA**;

12L Twins (Double HP100's): 0.9bar/min per ATA;

16L Twins (Double HP130's): 0.7bar/min per ATA.

Given a arbitrary nominal volume SCR of 15 liters/min per ATA (0.53 cuft/min per ATA in US Imperial Units):

11L/bar tank (AL80): 1.4bar/min per ATA;

12L/bar tank (Steel HP100): 1.3bar/min per ATA;

13L/bar tank (AL100): 1.2bar/min per ATA;

15L/bar tank (Steel HP119): **1bar/min per ATA**;

16L/bar tank (Steel HP130): 0.9bar/min per ATA;

11L Twins (Double AL80's): 0.7bar/min per ATA;

12L Twins (Double HP100's): 0.6bar/min per ATA;

16L Twins (Double HP130's): 0.5bar/min per ATA.

Given a arbitrary nominal volume SCR of 11 liters/min per ATA (0.39 cuft/min per ATA in US Imperial Units):

11L/bar tank (AL80): **1bar/min per ATA**;

12L/bar tank (Steel HP100): 0.9bar/min per ATA;

13L/bar tank (AL100): 0.8bar/min per ATA;

15L/bar tank (Steel HP119): 0.73bar/min per ATA;

16L/bar tank (Steel HP130): 0.68bar/min per ATA;

11L Twins (Double AL80's): 0.5bar/min per ATA;

12L Twins (Double HP100's): 0.45bar/min per ATA;

16L Twins (Double HP130's): 0.3bar/min per ATA.

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If you know your nominal bar/min consumption rate at depth for the particular cylinder used --you already know by rote what your SPG is going to read after a five or ten minute interval of time. So if I started the dive with a full cylinder at 200bar, and have a depth consumption rate of 8bar/min, I know I will consume 80 bar in ten minutes and the SPG will read 120bar remaining pressure. If I then come up multi-level shallower to a new depth where my consumption rate is 4bar/min, I know in another ten minute interval I'll consume 40bar and the SPG will show 80bar remaining. By normal habit and experience over many dives --you just automatically begin to do this iterative subtraction & SPG tracking process. When you finally reach Min Gas Reserve pressure and confirm it on your SPG, you then start your ending ascent to safety stops & the surface. . . IOW, the SPG is telling you, confirming what you already know and figured in terms of remaining gas pressure.