RMV math…

[L13]

@L13

You can measure SAC in pressure/time at the surface, you are then already at 1 atm. Or you can measure SAC at depth and correct by dividing by the atmospheres. Both come out in pressure/time/atm. Calculating SAC for a dive is more realistic than doing it on your couch.

RMV in volume/time is also at 1 atm, could be done at the surface, have only seen it for dives.

Exactly. Personally, I use exactly these definitions.

Except, we have @Gareth J (with similar # of dives as you, not sure how else to casually measure expertise on SB):

I don't understand how SAC can be confused?
SAC - Surface Air Consumption - that is self explanatory.

For air planning, the SAC is modified dependent on the circumstance.
Being lazy. I tend to use 15 l/min for normal circumstance, 50 l/min for bailout to the first stop. Maybe 20 l/min if it is a high work environment (working against current etc).

He is using vol/min units for SAC, not pressure/min. And yet, I understand exactly what he is saying(because he specified units).

 

[Gareth J]

I am still struggling to understand how surface air consumption (SAC), could be the gas consumption rate at depth.
Surface Air Consumption, is what it says on the tin. The rate of gas consumption at the surface in the units you define, be they litres per minute, or cubic feet per minute, US (dry) gallons per minute, US (liquid) gallons per minute, imperial gallons per minute, etc.
It is the volume of gas consumed at the surface in the units specified over a specified time.

 

[scubadada]

...we have @Gareth J (with similar # of dives as you, not sure how else to casually measure expertise on SB):

He is using vol/min units for SAC, not pressure/min. And yet, I understand exactly what he is saying(because he specified units).

I have a few over 2250 dives, not sure about my friend. Most important thing is communicating with others, units should do it.

My Oceanic download software displays both SAC in psi/min and RMV in cu ft/min because I am able to enter the cylinder characteristics.

Shearwater only gives SAC in psi/mim, you must calculate your RMV in cu ft/min, Teric manual p54

There you go.

 

[Gareth J]

I also don't entirely understand what use a Surface Air Consumption (SAC) rate defined in pressure per minute is.

It gives no useful or transferable information. A Surface Air Consumption (SAC) rate of 2 bar per minute is a useless figure. Is it for a 7 litre cylinder, a 10 litre cylinder, a 12 litre cylinder, a set of 12 litre twins (doubles)? It is a nonsense figure.

A figure giving the Surface Air Consumption (SAC) as a volume pre minute (or any other unit of volume and time), is a number that can be used for any cylinder size or configuration.

 

[L13]

I am still struggling to understand how surface air consumption (SAC), could be the gas consumption rate at depth.
Surface Air Consumption, is what it says on the tin. The rate of gas consumption at the surface in the units you define, be they litres per minute, or cubic feet per minute, US (dry) gallons per minute, US (liquid) gallons per minute, imperial gallons per minute, etc.
It is the volume of gas consumed at the surface in the units specified over a specified time.

For some people's understanding of SAC: (Surface Air) is a unit, equivalent to a cuft at STP or a liter at STP. At 2 ATA I consume 2x the (Surface Air) units/min that I consume when I am at the surface.

And, as you see above, some like @scubadada don't think the units for SAC is volume/time/ata, and should be pressure/time/ata on the used cylinder.

 

[scubadada]

I am still struggling to understand how surface air consumption (SAC), could be the gas consumption rate at depth.
Surface Air Consumption, is what it says on the tin. The rate of gas consumption at the surface in the units you define, be they litres per minute, or cubic feet per minute, US (dry) gallons per minute, US (liquid) gallons per minute, imperial gallons per minute, etc.
It is the volume of gas consumed at the surface in the units specified over a specified time.

There has always been confusion here. Both SAC and RMV are surface consumption. Use whatever you want, the units should make it clear to anyone you are talking to. To make gas requirement calculations, you multiply your RMV times the depth, in atmospheres.

 

[Gareth J]

If you provide a buddy (European?), or team (European?) with your SAC, in litres per minute., and the proposed site depth. They can calculate the dive time for any cylinder set that is available.

Perhaps more relevantly, if we are looking at buddy diving. They can calculate the turn pressure on their cylinders to ensure that there is sufficient gas volume available in case you need to exit on their cylinder set. i.e. if I have a higher SAC than they do, they can calculate the appropriate turn pressure!!

 

[L13]

I also don't entirely understand what use a Surface Air Consumption (SAC) rate defined in pressure per minute is.

It gives no useful or transferable information. A Surface Air Consumption (SAC) rate of 2 bar per minute is a useless figure. Is it for a 7 litre cylinder, a 10 litre cylinder, a 12 litre cylinder, a set of 12 litre twins (doubles)? It is a nonsense figure.

A figure giving the Surface Air Consumption (SAC) as a volume pre minute (or any other unit of volume and time), is a number that can be used for any cylinder size or configuration.

If my SAC on a 12 litre is 2 bar/min, then I can look at my depth, say 30m (~4 ATA), and know that my pressure should drop at 8 bar/min. If It is changing 10 bar/min I am working harder than normal, etc.

 

[Gareth J]

I can understand RMV being misleading, i.e it could relate to the surface, or to a specified depth.
But SAC is not ambiguous, it is a rate of gas consumption at the surface in the units specified.

 

[L13]

If you provide a buddy (European?), or team (European?) with your SAC, in litres per minute., and the proposed site depth. They can calculate the dive time for any cylinder set that is available.

Perhaps more relevantly, if we are looking at buddy diving. They can calculate the turn pressure on their cylinders to ensure that there is sufficient gas volume available in case you need to exit on their cylinder set. i.e. if I have a higher SAC than they do, they can calculate the appropriate turn pressure!!

If I was diving with you, I would give my SAC in l/min. If I was diving with Scubadada, I would give my RMV in (probably) cuft/min. With other Americans I would maybe use SAC in cuft/min.