SAC & RMV Differences

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acreichman
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SAC & RMV Differences

Post by acreichman » Tue Dec 03, 2019 1:37 pm

I know some similar questions have been asked in these forums but I have a couple of questions I haven't been able to find an answer to.

1. Is the only difference between SAC and RMV (besides the units) that SAC rate is at the surface and RMV is at depth? If not, what are the differences between SAC & RMV?

2. In MacDive, the units for SAC rate are psi/min and RMV is ft^3/min. When I learned SAC rate, it was in ft^3/min. Is there a way to change the unit into ft^3/min?

3. I don't always use the same size tank so I need a measure of volume used per minute at the surface to plan my dives. Based on my understanding of SAC & RMV above, I don't think that particular metric is available in MacDive. Is there something I'm missing here? How do the rest of you plan dives without a measure of volume used at the surface?

Thanks very much for your help.

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nick
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Re: SAC & RMV Differences

Post by nick » Tue Dec 03, 2019 1:40 pm

1. Is the only difference between SAC and RMV (besides the units) that SAC rate is at the surface and RMV is at depth? If not, what are the differences between SAC & RMV?
No. Both factor depth, but RMV also factors the tank.
2. In MacDive, the units for SAC rate are psi/min and RMV is ft^3/min. When I learned SAC rate, it was in ft^3/min. Is there a way to change the unit into ft^3/min?
No, the units reflect what they have calculated.
3. I don't always use the same size tank so I need a measure of volume used per minute at the surface to plan my dives. Based on my understanding of SAC & RMV above, I don't think that particular metric is available in MacDive. Is there something I'm missing here? How do the rest of you plan dives without a measure of volume used at the surface?
RMV factors the tank size/volume.

acreichman
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Joined: Fri Feb 08, 2019 1:37 am
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Re: SAC & RMV Differences

Post by acreichman » Thu Dec 05, 2019 1:47 am

Sorry, I'm still a little confused. If I use 500psi over 10 minutes at 2atm, my SAC rate should 50/2 or 25psi/(min*atm) but I can't use that number to compare different dives using different tanks. When calculating RMV, is my 50 psi/min then divided by 2atm before using tank info to convert to cubic feet? When I use different tanks at a variety of depths, which one should I be using to see if my air consumption is improving over time?

Thanks again for responding so quickly.

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nick
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Re: SAC & RMV Differences

Post by nick » Thu Dec 05, 2019 1:54 am

The RMV.

elisecoen
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Re: SAC & RMV Differences

Post by elisecoen » Sun Aug 30, 2020 3:58 am

RMV and SAC (and SCR) are essentially the same thing. RMV is used in volumetric terms, e.g. litres per minute while SAC is used in terms of pressure units, e.g. bar per minutes. RMV then has the advantage that it is independent of tank size as you always adjust for tank volume. So a diver with a nominal RMV of 20 litres per minute will never get surprised if they change from a 12 litre to a 10 litre tank. The disadvantage is that the calculations require one extra step. SAC has the advantage that you can easily do calculations on the fly, but the disadvantage is that it is dependent on tank volume. If you use 2 bar per minute from a 12 litre tank, you will use more from a 10 litre tank.
I refer to both RMV and SAC in surface terms, and indicate consumption rates at depth by the terms dSCR and dRMV.

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