SAC & Cylinders

[Diver Dan 28]

Will Someone please explain why using a different cylinder changes your sac rate?
The formula I have is SAC = [(psi/time) X 33] / (depth+33) if this give me my Sac rate why will it change with a different cylinder? I understand my consumption rates change the deeper I go but I don't understand what the capacity has to do with it in the big picture.

 

[PfcAJ]

That formula is incomplete. You're ending up with psi/min, and what you want is cubic feet per minute.

Convert your psi/min to cuft and thats your SAC rate.

SAC doesn't change with different tanks, psi/min will, and that's because 100psi in an al63 is a whole lot less gas than 100psi in double LP120s. But .6cuft/min is always .6cuft/min.

 

[rx7diver]

Will Someone please explain why using a different cylinder changes your sac rate?

Diver Dan 28,

If a larger cylinder has a greater cross-section, then it will be less hydrodynamic. Also if you need to inflate your BC more (because a cylinder is more negatively buoyant) then the BC will be less hydrodynamic.

Hope this helps.

Safe Diving,

rx7diver

 

[Kevrumbo]

I think this is where the confusion is:

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. . .

______
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.

 

[Colliam7]

Will Someone please explain why using a different cylinder changes your sac rate? The formula I have is SAC = [(psi/time) X 33] / (depth+33) if this give me my Sac rate why will it change with a different cylinder? I understand my consumption rates change the deeper I go but I don't understand what the capacity has to do with it in the big picture.

Part of the challenge - I think - is the terminology involved. When most people think of their SAC rate, they are thinking of their gas consumption, in terms of cubic feet per minute. And, that does NOT change with different cylinders. But, technically, SAC units of measurement are psi per minute - the formula in your post is correct - and that does change with cylinder. A 100 psi change in a low pressure steel 72 cf cylinder, with a working pressure of 2250 psi, reflects a different amount of gas used (~3.20 cf), than a 100 psi change in a high pressure steel 120 cf cylinder with a working pressure of 3500 psi (3.43 cf), than a 100 psi change in an AL80 (nominal capacity 77.4 cf) with a working pressure of 3000 psi (2.58 cf).

What you are probably interested in is your consumption, measured in cubic feet per minute, and that is referred to as the Respiratory Minute Volume (RMV). Unfortunately, most of us a a bit cavalier in our use of 'SAC rate', and use it to describe consumption in cubic feet per minute, when we should use 'RMV'.

There are some good threads on this subject on SB, such as: SAC vs RMV.

 

[Storker]

What you are probably interested in is your consumption, measured in cubic feet per minute, and that is referred to as the Respiratory Minute Volume (RMV). Unfortunately, most of us a a bit cavalier in our use of 'SAC rate', and use it to describe consumption in cubic feet per minute, when we should use 'RMV'.

As I'm from the metric side of the pond, my mind boggles that something like bar/min can have any meaning at all. I know folks who dive 10Ls, 12Ls, 15Ls, D7s, D8.5s and even D12s. Bar/min just isn't meaningful. I've never considered anything but surface liters per minute to have any meaning if we're talking about gas consumption. If that's RMV and not SAC, I really don't care. I'll just use 'gas consumption' and avoid the terminology issue completely

 

[Neilwood]

As I'm from the metric side of the pond, my mind boggles that something like bar/min can have any meaning at all. I know folks who dive 10Ls, 12Ls, 15Ls, D7s, D8.5s and even D12s. Bar/min just isn't meaningful. I've never considered anything but surface liters per minute to have any meaning if we're talking about gas consumption. If that's RMV and not SAC, I really don't care. I'll just use 'gas consumption' and avoid the terminology issue completely

Agreed!

If you are calculating something that changes according to cylinder volume, I don't have a clue how that could be useful other than if you only ever dive that tank. Surface L/Min means that irrespective of number of tanks or configuration you know what you use. No need for tank factors or any other nonsense.

Easy example - 12l filled to 200 bar= 2400l @1atm.
Surface consumption rate of 30l=80 mins
Therefore at 10m I have 40 mins, 20m 26 mins etc.

Easy to work a new tank - 10l filled to 230bar=2300l@1atm.
Surface consumption rate of 30l= 76 mins.

If everyone went metric - no issues.

 

[MXGratefulDiver]

Will Someone please explain why using a different cylinder changes your sac rate?
The formula I have is SAC = [(psi/time) X 33] / (depth+33) if this give me my Sac rate why will it change with a different cylinder? I understand my consumption rates change the deeper I go but I don't understand what the capacity has to do with it in the big picture.

Using a different cylinder doesn't change the rate at which you consume air ... but what it does change is how your air consumption is measured. Air consumption is in volume ... in imperial measure, that's cubic feet. But we don't look at a gauge that's measured in cubic feet, or volume at all. We look at a gauge that shows us how much air, in pressure, is in our cylinder. When you watch your cylinder pressure drop by a specific amount, the actual volume consumed will vary depending on the size and working pressure of the cylinder. That's what you're actually looking at.

As an example, for every 100 psi in your cylinder, an aluminum 80 will provide 2.6 cubic feet of air, while a low-pressure 95 will provide 3.6 cubic feet ... almost one-third more. If you were to measure your air consumption in volume, rather than pressure, then it would be independent of the cylinder size. But since it's a pressure gauge we look at when we dive, we tend to use pressure as the unit of measure, and that's what makes it dependent on cylinder size and working pressure.

A good analogy is the gas gauge in your car. If you take two cars with exactly the same miles per gallon (your SAC rate), and put different size tanks in them, the gas gauge will show different values as you drive. If both cars get 25 miles per gallon and you drive 100 miles, both cars will use 4 gallons of gas. But if one car has a 20-gallon gas tank and the other has a 10-gallon gas tank, the former will show that it still has 3/4ths of a tank of gas, while the latter has only a half-tank. They didn't change their gas consumption rate at all ... what changed was the relative measure of how much gas they actually consumed, because the size of their tanks is different.

It works pretty much the same way for calculating gas consumption when you're diving. You might find some of the information in this article useful in helping you understand how it works ... NWGratefulDiver.com

... Bob (Grateful Diver)

 

[Storker]

If everyone went metric [...]

Uh-oh...

 

[scubadada]

Part of the challenge - I think - is the terminology involved. When most people think of their SAC rate, they are thinking of their gas consumption, in terms of cubic feet per minute. And, that does NOT change with different cylinders. But, technically, SAC units of measurement are psi per minute - the formula in your post is correct - and that does change with cylinder. A 100 psi change in a low pressure steel 72 cf cylinder, with a working pressure of 2250 psi, reflects a different amount of gas used (~3.20 cf), than a 100 psi change in a high pressure steel 120 cf cylinder with a working pressure of 3500 psi (3.43 cf), than a 100 psi change in an AL80 (nominal capacity 77.4 cf) with a working pressure of 3000 psi (2.58 cf).

What you are probably interested in is your consumption, measured in cubic feet per minute, and that is referred to as the Respiratory Minute Volume (RMV). Unfortunately, most of us a a bit cavalier in our use of 'SAC rate', and use it to describe consumption in cubic feet per minute, when we should use 'RMV'.

There are some good threads on this subject on SB, such as: SAC vs RMV.

I call it Surface Respiratory Minute Volume, SRMV, reflecting it is the gas volume at 1 atm and not at depth, cf/min