RMV vs SAC rate

[FreeFlyFreak]

Old school

Up until the part about recording it in an app on my phone

 

[Divectionist]

I use Dive Log to download dives from the Perdix AI and enter the tank size there. The SAC value then turns into RMV for the logged dive.

I use RMV to plan my dives and then simply monitor my SAC, which I keep on the home screen of the Perdix, for the value that corresponds. I know to adjust my dive plan when, for whatever reason, my SAC is higher than anticipated.

It's all very easy in the metric world hehe.

 

[mnjhuz76]

Since the SAC on the Perdix merely is a better guess, because it completely ignores some high pressure effects, I don't rely on it too much anyway.

It's interesting how much influence Z-Values have on calculations - impressive to see when comparing to subsurface's calculated RMV/SAC.

 

[Divectionist]

Since the SAC on the Perdix merely is a better guess, because it completely ignores some high pressure effects, I don't rely on it too much anyway.

Could you please elaborate?

 

[tursiops]

Could you please elaborate?

Shearwater measures pressure changes. To get to volume changes "accurately" you should assume it is a Van der Wal gas, not a perfect gas. This makes a small difference in volume calculations at high pressures.

 

[Divectionist]

Shearwater measures pressure changes. To get to volume changes "accurately" you should assume it is a Van der Wal gas, not a perfect gas. This makes a small difference in volume calculations at high pressures.

Ah, the difference in compression - I can live with an error that small.

 

[tursiops]

Ah, the difference in compression - I can live with an error that small.

Yup.

 

[kelemvor]

You guys seem to be misunderestimating sac vs rmv. Here's a writeup RMV vs. SAC: What’s the Difference? - Cave Diver Harry

Here are some of the things I’ve had tech Instructor candidates tell me when I asked them to explain the difference between the two:

• One is metric; the other imperial
• You measure SAC in psi and RMV in cubic feet
• RMV is for CCR diving; SAC is for open circuit

Of course, every one of these statements is wrong. Dead wrong.

 

[scubadada]

SAC and RMV have been discussed many times on SB. SAC is reported in pressure/time, psi/min or bar/min. RMV is reported in volume/time, cubic feet/min or liters/min. Both are at the surface. SAC is cylinder dependent, RMV is not

I have been using an Oceanic VT3 since 2010 and download to Oceanic's simplistic OceanLog. You choose the cylinder you have used from a dropdown menu of the cylinders you use and the size and working pressure are filled in. The application then gives you both the SAC and the RMV. If one had only the SAC, the RMV can easily be calculated by multiplying by the cylinder size/working pressure. Using the example below, 10.4 psi/min x 77.4 cu ft/3000 psi = 0.27 cu ft/min.

Now onto Shearwater. I have been using a Teric just since last May. The Teric gives you only the SAC in the onboard log. The RMV can easily be calculated as above. Though not really necessary, it would not be difficult for Shearwater to add RMV. You are already required to enter the gas(es) you will be using. You are required to activate the transmitter(s) you are using. You are required to activate the one transmitter you would like to calculate GTR and SAC. It would not be difficult to add one additional screen to choose the cylinder you are using, corresponding to the transmitter calculating GTR and SAC, by tank size and working pressure. For convenience, this could be a list of up to 5 cylinders/doubles that you most commonly use, similar to the gas list. You would choose one per dive. If this were to happen, you could probably choose to display SAC or RMV on your home screen and the onboard log could calculate both.