SAC vs RMV, revisited

[BlueTrin]

My bad. I got the units mixed up. Like I said it’s weird way of measuring how much distance you gain per volume of gas.

I think both can make sense depending of the context.

Litres / 100km allows you to know quickly how much you litres you need by mutiplying by your trip distance ?

 

[Storker]

t’s weird way of measuring how much distance you gain per volume of gas.

Sure.

But it's a perfectly cromulent way of measuring how much fuel you burn to gain a given distance.

 

[inquis]

Which is one of the fundamental flaws with the Imperial unit convention (cu.ft/min).

I don't understand why you called out the Imperial convention. The use of metric units (liter/min) is equally flawed is it not?

 

[inquis]

I wonder if the pre-existing convention of RMV units from the medical field is to blame. The "surface" in SAC makes it (mostly) clear it is at (or has been normalized to) 1 atm. (The "per atm" unit can be dropped by definition.) Perhaps SRMV would be better?

I do have high hopes that dive computers may be converging to SAC using pressure in the numerator, RMV using volume, and both being normalized to 1 atm. Fingers crossed, at least.

 

[tursiops]

I don't understand why you called out the Imperial convention. The use of metric units (liter/min) is equally flawed is it not?

I think he is used to it, so it is OK.

 

[Manatee Diver]

I wonder if the pre-existing convention of RMV units from the medical field is to blame.

The pre-existing meaning is why I dislike RMV, as not only is it a different meaning but because it has similar units it can be confused with scuba RMV/SAC.

I think that SAC is a better term as it is scuba specific, and it gets across that it needs to be adjusted for pressure. But I get the feeling that I am just screaming into the wind as the scuba community has decided that SAC is pressure/min/atm while RMV is volume/min/atm. And it would take a hard 180 to just go back to SAC being either depending on which unit you give.

 

[tursiops]

the scuba community has decided that SAC is pressure/min/atm while RMV is volume/min/atm

Why do you think this is now decided?
Yes, NOAA and Navy would agree with you, but our scuba training agencies maybe not. PADI says SAC can be either pressure or vol/time, at one atm, but RMV is always vol/time. TDI only talks about SAC (in the index in one of its manuals under RMV it says "see SAC"), and as far as i can tell only as vol/min. Various dive logging programs and computers seem to use either SAC for both pressure/time and vol/time, but reserve (if they use it) RMV for vol/time.

At least SAC says "surface" in its name, and RMV says "volume" in its name.
There have been suggestion to call Pressure/min SACP, and vol/min SACV, but divers (unless they are ex-Navy) don't like unpronounceable acronyms.

What do I use? SAC, and I give the units, like psi/min if I'm talking about what my Shearwater says or cuft/min if I'm gas matching with my cave buddy. I don't need to say "surface" because SAC is at the surface. I'll even tell my Dutch DM in Bonaire that my SAC is 14 liters/min if I have to.

 

[clownfishsydney]

True!

BTW, European mpg convention in metric is a little weird. They use km/100L.

Are you sure? We use litres per 100 km.

 

[Blackcrusader]

What do I use? SAC, and I give the units, like psi/min if I'm talking about what my Shearwater says or cuft/min if I'm gas matching with my cave buddy. I don't need to say "surface" because SAC is at the surface. I'll even tell my Dutch DM in Bonaire that my SAC is 14 liters/min if I have to.

I've only seen RMV used on forums in real life everyone asks me what my SAC rate it.
When I tell them they often don't believe it and they think I made an error.

 

[Storker]

Are you sure? We use litres per 100 km.

No matter whether he's sure, he's wrong. See post #114.