RMV math…

[scubadada]

I (we) use bar (pressure), minutes (time)
SAC - Surface Air Consumption.

RMV seems to be more a (North) American thing.

Aha, you use only SAC, no name for pressure/time/atm

 

[scubadada]

Everyone, see Question - Definitions for SAC and RMV?

 

[Gareth J]

Aha

Also we think volume

All cylinders are measured in litres. i.e 7, 10, 12, 15.
Gas pressure is measured in bar i.e 230bar.

A 10 litre cylinder contains 2300 litres of gas (@230bar).

If you are following the basic (no-stop) rule of 50bar reserve. - that's 180 x10 = 1800 litres of available gas leaving a 50 bar reserve.
If you are being more technical . 2330/3 = 766 litres (76 bar) exit pressure (reserve), 155 bar turn pressure. (1530 litres of gas available for the dive).

SAC of 15 litres a minute.... At 30 m = 60 litres a minute, 1530/60 - 25 minute dive, turn on 12 minutes.

A bit artificial I know, I would use larger cylinders at 30m! - But it makes the maths easy!

 

[inquis]

You calculate turn pressures based available gas.

I agree, but that is not what you said earlier. You said your buddy computes their turn pressure based on your estimate of your SAC. In that case, if you haven't hit your turn pressure (e.g., because you use larger tanks) but are breathing faster than originally expected, the reserve your buddy is holding for you will be too small.

 

[rfwoodvt]

...and Subsurface calls its calculation of volume/minute (after you've put in the requisite info) SAC, instead of the more usual RMV.

So that brings up the question what does that SAC value translate into on the subsurface app?

Is it True SAC in terms of PSI Per minute or Is it liters or cubic feet per minute the volume per minute?

Along that line of thinking is there any way to configure subsurface to provide RM volume terms?

 

[inquis]

what does that SAC value translate into on the subsurface app?

Subsurface (and the majority of respondents in a recent SB poll) uses the term "SAC" for the volume/min quantity that is applicable across differing cylinder sizes. Whether that <volume> is cubic ft or liters depends on the imperial/metric unit setting. If you want a cylinder-specific pressure/min quantity (for which you apparently use the term SAC), you need to convert yourself.

is there any way to configure subsurface to provide RM volume terms?

That what you already get, they just call it "SAC".

 

[Kendall Raine]

The interesting part is that it varies with depth, which leaves you more gas to enjoy yourself on a shallow dive than Thirds allows. It also shows you a depth where Thirds would be insufficient. (Hint, under some common assumptions, that's anything deeper than 75 ft on an AL80 tank.)

Completely agree. Rock Bottom is the way to go.

Also, varies with depth and circumstances of the dive (i.e. overhead-wrecks) or any other situation where you need to swim at depth for any length of time before ascending.

 

[rsingler]

Rock Bottom?

Why Surface with 500 PSI?

(Author Update) This old thread discusses the fallacy of "dive planning" which consists merely of "Return to the boat with 500 psi." It gives you a tool to do deeper dives safely as you increase your diving experience after getting your OW certificate... Every boat trip, the crew will brief the...

scubaboard.com

 

[60plus]

My feeling is that knowing your sac or rmv is pretty pointless. There are so many factors that change your consumption below the surface. It is far better to have a general idea of how long a certain size cylinder will last in a given set of circumstances and how that will change as circumstances deviate from expectation.

 

[rsingler]

My feeling is that knowing your sac or rmv is pretty pointless.

Not to needlessly quarrel, but then how do you gas plan for a complex dive?