RMV math…

[UofMI_Divegeek]

Regardless of acronym volume/time is tank size agnostic. A cubic foot/liter is the same in any cylinder. I don't know why anyone would use pressure/time as an end point metric.

Underwater, the SPG measures pressure, not volume. So, many divers who use SAC type calculations in their dive planning will be focusing on pressure since that is their real time underwater indicator of available breathing gas.

 

[Kendall Raine]

Underwater, the SPG measures pressure, not volume. So, many divers who use SAC type calculations in their dive planning will be focusing on pressure since that is their real time underwater indicator of available breathing gas.

Of course, but the next step is to convert from pressure change to volume consumption. That's why the pressure change is an intermediate step, not an end point as a gas planning metric. Otherwise the calculation is only useful for one particular tank volume.

 

[tursiops]

Regardless of acronym volume/time is tank size agnostic. A cubic foot/liter is the same in any cylinder. I don't know why anyone would use pressure/time as an end point metric.

During a dive, pressure/min is the only data directly available to you. Any volume considerations requires an in-water computation.

 

[tursiops]

Of course, but the next step is to convert from pressure change to volume consumption. That's why the pressure change is an intermediate step, not an end point as a gas planning metric. Otherwise the calculation is only useful for one particular tank volume.

You are focused on planning, but diving is also a consideration.

 

[L13]

During a dive, pressure/min is the only data directly available to you. Any volume considerations requires an in-water computation.

which would be trivial, IF i could only convince my community to go metric.

 

[Wibble]

During a dive, pressure/min is the only data directly available to you. Any volume considerations requires an in-water computation.

And you MUST know your tank volumes in the water. Not to mention knowing your working SAC, elevated SAC and extreme SAC.

From that you work any adjustments to turn points; whether the dive is matching the plan.

 

[Wibble]

which would be trivial, IF i could only convince my community to go metric.

As a metric person, it’s the mad tank volumes which are ridiculous. Using wet volumes is so much more intuitive. Ali80s are 11 litres. With a SAC of 22 you know the pressure drops by 2 bar x depth in ATA (divide depth in metres by 10 and add one).

OK, I get what you mean!!!

You Americans must be amazing at mental arithmetic compared with us metrical peoples!

 

[lowwall]

You Americans must be amazing at mental arithmetic compared with us metrical peoples!

It's 12 inches to the foot, 3 feet to the yard, 2 yards to the fathom, 110 fathoms to the furlong and 8 furlongs to the mile*.

What could be simpler?

* That's the statute mile. The nautical mile is a bit longer at 6076 feet 1 and 193/500 inches instead of an even 5280 feet.

 

[inquis]

which would be trivial, IF i could only convince my community to go metric.

I tried it, metric was actually harder, believe it or not. I dive double LP85s. I'll stick with my super easy imperial tank factor of 6 instead of the metric factor of 26 or even 13 (then doubled). Even for the ubiquitous AL80, I find the imperial factor of about 2.5 (more easily, cutting the 5x result in half) to be easier than messing about with the metric factor of 11. I also like my imperial 0.5 cuft/min SAC (I'm really good at dividing by 2) as opposed to the substantially more difficult metric 14 liter/min.

TBH, I'll typically just convert pressures directly rather than psi->volume followed by volume->psi. For instance, I've memorized that PSI(LP85) ~ 0.9 PSI(AL80) because I've done it so many times. (Although, I suppose that rule works for bar as well.)

 

[VikingDives]

I don't know why anyone would use pressure/time as an end point metric.

Come to America and let me introduce you to a generation of divers who don't even know how to compare pressure/time to each other...