Average Gas Consumption

What is your average RMV?

  • less than 0.3 cu ft/min, 8.5 l/min

    Votes: 12 1.4%

  • 0.3-0.39 cu ft/min, 8.5-11.2 l/min

    Votes: 99 11.7%

  • 0.4-0.49 cu ft/min, 11.3-14.1 l/min

    Votes: 226 26.7%

  • 0.5-0.59 cu ft/min, 14.2-16.9 l/min

    Votes: 254 30.0%

  • 0.6-0.69 cu ft/min, 17.0-19.7 l/min

    Votes: 125 14.7%

  • 0.7-0.79 cu ft/min, 19.8-22.5 l/min

    Votes: 88 10.4%

  • 0.8-0.89 cu ft/min, 22.6-25.4 l/min

    Votes: 18 2.1%

  • 0.9-0.99 cu ft/min, 25.5-28.2 l/min

    Votes: 11 1.3%

  • greater than or equal to 1.0 cu ft/min, 28.3 l/min

    Votes: 15 1.8%
  • Total voters
    848

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ATJ:
Please reread my post. I don't think you have followed what I was saying.

I certainly wasn't saying that a 12L tank holds the same air as a 10L tank. I was talking about GTR calculations.
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I see what you’re saying, but why complicate yourself with RMV? Just simplify your mind and math by sticking with SAC in bar/min. And record what type of tank you use as reference if you really want to know your correct RMV?
 
Bigger tank should give you higher GTR since your SAC (in bar/min) would be lower when you use a bigger tank. Your SAC of 12L tank would be lower than your SAC of 10L tank. So the correct tank size as reference is still needed.

I can drop my SAC by half when I use 2 AL80, However the RMV (L/min) should remain the same, but now you have twice more volume to breathe from.
 
Dan:
I see what you’re saying, but why complicate yourself with RMV? Just simplify your mind and math by sticking with SAC in bar/min. And record what type of tank you use as reference if you really want to know your correct RMV?
Click to expand...
That is exactly what I already do and I wasn't suggesting anything different.

I was pointing out that if Shearwater (or any other dive computer) manufacturer allowed you to put in the tank size, that it wouldn't cause GTR calculations to be wrong if you entered the wrong tank size as was suggested by the person to whom I was replying.
 
Dan:
Bigger tank should give you higher GTR since your SAC (in bar/min) would be lower when you use a bigger tank. Your SAC of 12L tank would be lower than your SAC of 10L tank. So the correct tank size as reference is still needed.

I can drop my SAC by half when I use 2 AL80, However the RMV (L/min) should remain the same, but now you have twice more volume to breathe from.
Click to expand...
GTR = Gas Time Remaining. It is based on your current gas consumption and the remaining gas in the tank, and assumes you will maintain your current depth and current gas consumption.

Yes, a larger tank will give you a longer GTR. That is obvious.

What I am saying is that if you entered the tank size into the dive computer incorrectly, the GTR won't change as it is based on the ACTUAL tank size not the tank size the DC might think. It is the ACTUAL tank size that determines the ACTUAL current gas consumption and GTR will be based off that.

Yes, if you put in the wrong tank size and the computer displays your RMV, the displayed RMV will be wrong, the rate at which you are draining the tank is dependent only on your current breathing rate (and depth) and the ACTUAL size of the tank.
 
Tank volume is not involved in calculating GTR. You need to use your SAC (in bar/min) for that same tank though when you want to calculate GTR, as your 12L tank SAC in lower than your 10L SAC.
 
Dan:
Tank volume is not involved in calculating GTR. You need to use your SAC (in bar/min) for that same tank though when you want to calculate GTR, as your 12L tank SAC in lower than your 10L SAC.
Click to expand...
Seriously?

This is what I was responding to....
TooCold:
Which I think is why Shearwater says they won't add tank volume settings to their DC's. What if your trusted DC was showing you an underestimate of your SAC rate? What if they used that rate for other calculations and showed you an overestimate of GTR, for example?
Click to expand...
And pointing out that if you add tank volume to the dive computer AND it was used to calculate GTR it would not change anything.
 
ATJ:
Seriously
Click to expand...
What’s wrong with my statement? I simply said tank volume is not needed for DC to calculate GTR since its AI gets your SAC from that same tank.

In the other hand, I can’t say my SAC is 1 bar/min without saying from what tank size I breathe from. My SAC would be 0.5 bar/min if I breathe from 2 of that same tanks. RMV (in L/min) should remain the same regardless you breathe from 12L tank or 2x12L tank, my RMV should be 12L/min in this example whether I breathe it from a 12L tank or from 2x12L tanks.
 
So, looks like the thread went off on the SAC/RMV tangent yesterday. My simpleton explanation:

SAC is pressure/time/atm. It is cylinder dependent. GTR or ATR uses SAC to calculate time remaining.

RMV is volume/time/atm. It is cylinder independent. The SAC can be converted to RMV by multiplying by the tank factor (volume of gas/working pressure for the cylinder)

In imperial units, SAC is psi/min/atm and RMV is cu ft/min/atm. In metric units, SAC is bar/min/atm and RMV is liters/min/atm. The atm is usually left out in stating the units, but that is exactly why you must know your average depth to calculate either SAC or RMV

My Oceanic logging software has a drop-down list of cylinders to choose from. It calculates both SAC and RMV, as shown in my earlier post Average Gas Consumption Shearwater dive computer or cloud application calculates only SAC. You must multiply by the tank factor to get your RMV. I don't know why Shearwater won't allow you to enter the cylinder used on the dive to automatically calculate the RMV, at least on the Cloud app.
 
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