SAC vs RMV, revisited

[scubadada]

So the Suunto does what the Shearwater does (measures pressure-drop/minute), but takes it a little further if you give it the additional information. Which Shearwater says they don't allow for since it introduces errors due to user error. Is there evidence of such user error by Suunto users?

I don't dive a Suunto, only helped somebody set one up, once. My Oceanic VT3 only measures gas use. I have to enter the cylinder size and working pressure in the download application so that it can calculate RMV from SAC. An easy dropdown menu makes this very easy SAC vs RMV, revisited My Teric gives me only the SAC, I have to multiply by the tank factor to get my RMV

Here are the pertinent sections of the Suunto D5 owner's manual. I assume the user must enter the current cylinder size and working pressure to get the correct RMV. Someone who dives an AI Suunto can confirm this. There is always room for the user to enter incorrect or noncurrent cylinder data. So they would be diving with incorrect consumption data but would still have accurate NDL and gas pressure, not terribly likely to harm.

 

[Storker]

So the Suunto does what the Shearwater does (measures pressure-drop/minute), but takes it a little further if you give it the additional information. Which Shearwater says they don't allow for since it introduces errors due to user error. Is there evidence of such user error by Suunto users?

I'm using Suunto DM3 to download my dives from my PDC. Rather obviously, its SAC (or whatever you want to call it) rates don't use the ideal gas law. I'd hazard a guess that they're using the vdW equation with the stock constants (without having checked that, but their numbers fit neither the ideal gas law nor real-world compressibility data, but are closer to the the latter than to the former).

 

[EFX]

So the Suunto does what the Shearwater does (measures pressure-drop/minute), but takes it a little further if you give it the additional information. Which Shearwater says they don't allow for since it introduces errors due to user error. Is there evidence of such user error by Suunto users?

It's odd that Shearwater doesn't allow entering the tank factor (or type). You've got to enter in the %O2 for nitrox. Isn't that error prone? They could ask you for rated tank volume and pressure. The calculation from pressure/min to volume/min is trivial. After all, it is a computer.

 

[tursiops]

It's odd that Shearwater doesn't allow entering the tank factor (or type). You've got to enter in the %O2 for nitrox. Isn't that error prone? They could ask you for rated tank volume and pressure. The calculation from pressure/min to volume/min is trivial. After all, it is a computer.

FFO2 is critical if you are diving Nitrox; it is a safety item. Vol/min instead of pressure/minute is just a convenience, with no safety degradation if you do it afterwards.

 

[Blackcrusader]

Obviously you’re forming gills since you’re apparently hardly breathing!

I've been told the same thing. I had a DM call me a mutant after ending a 55 minute MOD 20m dive with 140 bar left on an AL80 with 210 bar start. The average depth was only like 12.5m

There is another good thread here Improving my SAC rate

This is one of my favourite dive logs though. AL 80 210 bar start end dive with 50 bar after 93 minutes.

 

[inquis]

People don't have this difficulty with their car's fuel consumption. By giving units (e.g., miles per gallon, km per liter) no one is confused. Convert as needed or compute range for whatever sized tank (or remaining fuel in that tank).

Diving is no different in my view. ETA: normalize by the things that vary (time and ambient pressure).

 

[inquis]

No. No, no, no. He's breathing 0.5 cu.ft/min.

While certainly correct, that rate isn't terribly useful and is extremely misleading on its own. He is also breathing at a rate of 2 cu.ft/min/atm, which is a significantly more useful metric.

 

[BlueTrin]

While certainly correct, that rate isn't terribly useful and is extremely misleading on its own. He is also breathing at a rate of 2 cu.ft/min/atm, which is a significantly more useful metric.

I think @Storker point wasn’t that it was an useful metric: I think he meant to say that you have to be precise in the wording you use, which you did in your reply by adding the pressure in your unit.

 

[Storker]

He is also breathing at a rate of 2 cu.ft/min/atm, which is a significantly more useful metric.

Quite.

.5 cubic feet is .5 cubic feet. 2 cubic feet is 2 cubic feet. At 4 ATA, they are the same gas consumption. Unless you specify at which depth, the volume units are meaningless.

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

 

[Bob DBF]

He is also breathing at a rate of 2 cu.ft/min/atm, which is a significantly more useful metric.

Wouldn't it be .5 cuft/min/atm which would work out to 2cuft/min at depth?