Question Definitions for SAC and RMV?

[Wibble]

It is bar usage from the bottle at surface that the computer is telling you, not at current pressure.

To me this is a silly question, liter / min or bar from bottle / min is the same thing just different units meaning the same thing. Like 32 fahrenheit is the same as 0 celsius.

Shearwater displays sac in bar so they remove a potential user mistake if you enter the wrong cylinder volume which would cause the computer to give you wrong sac rate if it was displayed in liters / minute.

I now see the difference.

I am talking about Surface Air Consumption, the volume of gas you consume at one Bar/Atmosphere in one minute.

You are talking about one manufacturer's implementation of their partial information; the rate of pressure reduction in a cylinder of unknown capacity at depth. This really isn’t anything like Surface Air Consumption

 

[mboden77]

I now see the difference.

I am talking about Surface Air Consumption, the volume of gas you consume at one Bar/Atmosphere in one minute.

You are talking about one manufacturer's implementation of their partial information; the rate of pressure reduction in a cylinder of unknown capacity at depth. This really isn’t anything like Surface Air Consumption

It is the same thing expressed differently, your SAC will be the same if you see it in BAR / min or Liter / Min.

If it wasn't anything like Surface Air Consumption then this forumla would not work:

SAC in pressure * Cylinder Size = Your preferred SAC.

I would think all manufacturers use the pressure value which is what they actually meassure with the transmitter then use this to calculate your gtr. Then some computers let you input your cylinder size so they can display the value also in your preferred unit.

Garmin definitely is like this, It will display in bar unless you input your cylinder size.
Their gtr will be correct even if you have a wrong cylinder size programmed since they calculate gtr using the pressure value and the conversion to liter / min is only for your viewing pleasure.

I do agree that I would like to see the value in liter / min but I understand why Shearwater went they way they did and I am fine with that. When I used my garmin I forgot to update the cylinder size several times when I switched between 12L and 15L cylinders.

 

[Wibble]

They’re not the same thing.

One’s SAC is expressed volume/min, generally in terms of the working/bottom phase and decompression phase (i.e. relaxed consumption) of a dive. This is then used with a stress factor for all other calculations to plan the correct volume of gas for your:

• Minimum gas required to safely ascend
• Decompression gas volumes for safety
• Bailout gas volumes for stressed ascents
• Stage bottle volumes for stressed exits in overheads
• Sharing gasses with other divers with different SAC rates
• Etc…

How you derive your SAC will ultimately require a measurement of gas pressure change and depth over a period of time, generally longer than one minute to average usage and produce a measurable pressure change at that depth. This would be easily determined if using AI on all cylinders, but is just as simple with a SPG.