SAC vs RMV, revisited

[Miyaru]

It's amazing how lengthy this discussion can get. No matter what words and/or units you put to it, the exact same issue is in the Open Water course:

if a full tank of compressed air would last 40 minutes at 10m, how long would it last at 30m?​

The method behind the calculation is to take it to the surface (multiply with the pressure at the first mentioned depth), calculate the time, followed by taking it to the new depth (divide by the pressure at the second mentioned depth). The goal stays the same along the way from Open Water down to trimix depths: how long will a tank last?

And that time is (fortunately) expressed the same way in both the metric and the imperial system.

Complicating things by expressing gas consumption in pressure difference per minute, which depends on the tank size, is a choice. RMV, SAC, pounds-per-square-inch-per-minute....

The principle of taking the calculation to the surface from one depth and recalculating at a different depth, is still the same. If you're doing a long technical dive with a team, gas-matching can be easy if someone tells you how much (s)he breathes in volume-per-minute at the surface.

 

[scubadada]

...if a full tank of compressed air would last 40 minutes at 10m, how long would it last at 30m?...

Just a little bit longer than your NDL on air

 

[Storker]

Just a little bit longer than your NDL on air

...giving you zero reserves for the ascent or for sharing gas with your buddy

 

[Kensei]

It's amazing how lengthy this discussion can get. No matter what words and/or units you put to it, the exact same issue is in the Open Water course:

if a full tank of compressed air would last 40 minutes at 10m, how long would it last at 30m?​

The method behind the calculation is to take it to the surface (multiply with the pressure at the first mentioned depth), calculate the time, followed by taking it to the new depth (divide by the pressure at the second mentioned depth). The goal stays the same along the way from Open Water down to trimix depths: how long will a tank last?

And that time is (fortunately) expressed the same way in both the metric and the imperial system.

Complicating things by expressing gas consumption in pressure difference per minute, which depends on the tank size, is a choice. RMV, SAC, pounds-per-square-inch-per-minute....

The principle of taking the calculation to the surface from one depth and recalculating at a different depth, is still the same. If you're doing a long technical dive with a team, gas-matching can be easy if someone tells you how much (s)he breathes in volume-per-minute at the surface.

Nice summary. Adding to this, there apparently is no standard for the naming of these formulas. For example, Suunto calculates SAC using the vol. per minute formula. Shearwater calculates SAC using the pressure per minute formula. Two people could be discussing SAC and not realize they aren't discussing the same calculation. It occurred in the group I dive with recently.

 

[Manatee Diver]

...giving you zero reserves for the ascent or for sharing gas with your buddy

Then put a little of the spare air in your wing, and shoot out of the water like a Polaris Missile aimed at Moscow.

The bends are for the weak.

 

[tursiops]

For example, Suunto calculates SAC using the vol. per minute formula.

Can you explain how it does this? All it can measure is the pressure in the cylinder.

 

[Hatul]

While you are technically correct that we should be using the term surface RMV for vol/minute and SAC the surface consumption in pressure change/minute, as long as you specify the units there's no confusion, and most divers now use SAC to mean both. When you put that into equations or in a context where the units are not provided you have to be more precise, but if a diver tells me his SAC is 0.6 cu ft/min I know what is meant and there's really no confusion. If he says SAC of 30 psi/min it's also clear. SAC is easier to say than surface RMV.

 

[scubadada]

Can you explain how it does this? All it can measure is the pressure in the cylinder.

You enter the cylinder data through DM5 for the computer to use in the calculation. If you change cylinders, you have to reenter the data

 

[tursiops]

You enter the cylinder data through DM5 for the computer to use in the calculation. If you change cylinders, you have to reenter the data

So the Suunto does what the Shearwater does (measures pressure-drop/minute), but takes it a little further if you give it the additional information. Which Shearwater says they don't allow for since it introduces errors due to user error. Is there evidence of such user error by Suunto users?

 

[tursiops]

While you are technically correct that we should be using the term surface RMV for vol/minute and SAC the surface consumption in pressure change/minute, as long as you specify the units there's no confusion, and most divers now use SAC to mean both. When you put that into equations or in a context where the units are not provided you have to be more precise, but if a diver tells me his SAC is 0.6 cu ft/min I know what is meant and there's really no confusion. If he says SAC of 30 psi/min it's also clear. SAC is easier to say than surface RMV.

What if the divers says his SAC is 18?
Does he mean 18 liters/minute, or 18 psi/minute? They are very different consumption rates... in fact the latter indicates nothing about a consumption rate unless you say the tank size in addition.
Using your example, 30 psi/minute might be a very low consumption rate or a very high rate, depending on the tank(s) being used, so, No, you don't really know what he means.
My points -- as in post #1 -- are just two:
(1) volume/minute, at the surface, is generally a more valuable number for talking to others, and for planning purposes, and
(2) making it clear if you are using metric or imperial units is also a good way to eliminate confusion.

I make no point about the names of these things, because that just raises (mostly) silly arguments.