RMV math…

[L13]

Using standard definitions is most important when communicating with others. This topic has been beaten to death on SB

SAC is pressure/time/ata, psi/min/ata or bar/min/ata and is cylinder dependent
RMV is volume/time/ata cu ft/min/ata or l/min/ata and is cylinder independent

RMV is used to make gas requirement calculations regardless of the cylinder(s) used

Many authoritative references disagree with you on SAC (many others agree with you). What you declare as a standard is not a universally accepted standard, so it should not be assumed. Like you, I would like it to have one meaning, but it doesn't.

If you specify units on your SAC, it doesn't really mater because the definitions is implicit in the units. You should specify your units for RMV too, even though you might be able to assume it from the magnitude of the number.

SAC, as you define it, only really makes sense if you are using imperial units. For metric units, the pressure to volume conversion is easy enough that it is easier just to use RMV (if you know your TF, then RMV is easier for imperial as well).

 

[scubadada]

Many authoritative references disagree with you on SAC (many others agree with you). What you declare as a standard is not a universally accepted standard, so it should not be assumed. Like you, I would like it to have one meaning, but it doesn't.

If you specify units on your SAC, it doesn't really mater because the definitions is implicit in the units. You should specify your units for RMV too, even though you might be able to assume it from the magnitude of the number.

SAC, as you define it, only really makes sense if you are using imperial units. For metric units, the pressure to volume conversion is easy enough that it is easier just to use RMV (if you know your TF, then RMV is easier for imperial as well).

They are the definitions that I use and use in communication with others.

Do as you please.

 

[L13]

They are the definitions that I use and use in communication with others.

Do as you please.

They are my preferred definitions as well.

But you can't make everyone agree with your definitions just by declaring a definition. If they don't, throwing a fit won't improve communication.

 

[inquis]

SAC, as you define it, only really makes sense if you are using imperial units.

I must be misunderstanding you. Knowing my current SAC in bar/min/ata is pretty useful if my SPG is showing bars.

 

[L13]

I must be misunderstanding you. Knowing my current SAC in bar/min/ata is pretty useful if my SPG is showing bars.

It is useful.

I guess I was overstating the relative ease of going from SAC to RMV with metric (and also with TF if you know it).

When thinking about my consumption using metric, I routinely do the extra step with little extra effort. I should get more comfortable with TF so I can do the same in imperial, since that is what I usually dive.

 

[Gareth J]

Many authoritative references disagree with you on SAC (many others agree with you). What you declare as a standard is not a universally accepted standard, so it should not be assumed. Like you, I would like it to have one meaning, but it doesn't.

If you specify units on your SAC, it doesn't really mater because the definitions is implicit in the units. You should specify your units for RMV too, even though you might be able to assume it from the magnitude of the number.

SAC, as you define it, only really makes sense if you are using imperial units. For metric units, the pressure to volume conversion is easy enough that it is easier just to use RMV (if you know your TF, then RMV is easier for imperial as well).

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).

 

[inquis]

I don't understand how SAC can be confused?

It's an open question whether you are consuming liters, cubic ft, psi, or bars.

 

[Gareth J]

It's an open question whether you are consuming liters, cubic ft, psi, or bars.

I can understand confusion over units. But not that it is a Surface Air Consumption number.

I have to say, it could make the maths far easier for you American cousins, if you adopted metric units.

 

[L13]

I can understand confusion over units. But not that it is a Surface Air Consumption number.

I have to say, it could make the maths far easier for you American cousins, if you adopted metric units.

Is SAC the air consumption rate on the surface (effectively lung volume change * breaths per min)? Or is SAC the air consumption rate at depth as measure in surface units of air(effectively lung volume change * breaths per min * ATA)? Both "make sense" from the letters SAC. Should the units be vol/min or pressure/min(on a particular tank)?

I have seen both definitions, with both types of units, proposed by "authoritative" sources. Everyone seems to have their own preferred authority and refuse to accept the competing authority as more authoritative.

I resolve the issue by accepting the definition of whoever I am talking to and asking questions as necessary if it isn't obvious which they are using. As long as the units are specified, it is easy to determine which definition is being used by someone from the magnitudes involved. The goal after all is to communicate.

 

[scubadada]

@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.