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The term "SAC" is commonly used as a volumetric measure ... which is really your RMV ... but as I said, for real dive planning you need to know how to calculate both (which is simple if you understand the relationship between the two).
When you are determining your turn pressure ... or rock bottom pressure ... you need to know it in psi. It won't do you much good to understand only your volumetric consumption rate unless you're good at doing mental conversions from CFM to PSI underwater, for the cylinder you are using ... and if at depth, narcosis might make this somewhat difficult. However, if you know your SAC, in psi, and know your depth, you can make some pretty quick, and reasonably accurate, estimates of how fast you're going to burn thru what you have in your cylinder simply going on the information you're reading off your SPG. You should also have planned, in advance, the psi at which you will either turn the dive or start your ascent (depending on the type of dive you are doing). This is, of course, completely dependent on your consumption rate and the cylinder you are using ... which is why a SAC calculation, in psi, is important.
RMV means Respiratory Minute Volume ... the term alone should tell you what sort of measurement units are involved. You need this information when planning a dive in order to know ... prior to the dive ... whether or not you've got enough gas to do the dive you're planning.
They are equivalent, but expressed in different units for a reason.
You can express your SAC rate however you choose (to my concern) ... the important thing is to understand why you are doing so, and how to apply it to your dive planning.
... Bob (Grateful Diver)
When you are determining your turn pressure ... or rock bottom pressure ... you need to know it in psi. It won't do you much good to understand only your volumetric consumption rate unless you're good at doing mental conversions from CFM to PSI underwater, for the cylinder you are using ... and if at depth, narcosis might make this somewhat difficult. However, if you know your SAC, in psi, and know your depth, you can make some pretty quick, and reasonably accurate, estimates of how fast you're going to burn thru what you have in your cylinder simply going on the information you're reading off your SPG. You should also have planned, in advance, the psi at which you will either turn the dive or start your ascent (depending on the type of dive you are doing). This is, of course, completely dependent on your consumption rate and the cylinder you are using ... which is why a SAC calculation, in psi, is important.
RMV means Respiratory Minute Volume ... the term alone should tell you what sort of measurement units are involved. You need this information when planning a dive in order to know ... prior to the dive ... whether or not you've got enough gas to do the dive you're planning.
They are equivalent, but expressed in different units for a reason.
You can express your SAC rate however you choose (to my concern) ... the important thing is to understand why you are doing so, and how to apply it to your dive planning.
... Bob (Grateful Diver)
