SAC rate & RMV

[TSandM]

Yes. If you always dive the same size and pressure tanks, knowing your psi/min consumption at the surface translates quickly and neatly into a predicted psi/min at any given depth.

Tech divers get into this stuff big time, and it was almost funny when I did my cave class in Mexico, because my buddy and I, who had painfully learned all the calculations to turn psi consumed on a dive into a measure of cu ft/min at the surface, kept getting yelled at by the instructor, who wanted to know what our SAC rate was in PSI/MINUTE, because, as he said, what we were going to know in the cave was psi, not cu ft!

Metric divers have life so much easier . . .

 

[Doc Intrepid]

'Surface Air Consumption' value does not insist that the measurements be taken at the surface. The formula includes a variable D = depth at which your measurements were made. You could calculate SAC at any depth you select to record your measurements. (Its kinda handy if you can stick to a given ATA - 33', 66', 99', etc. but ultimately the factor is really only meaningful to you yourself.)

Respiratory Minute Volume provides a consumption rate calculated on the total volume of gas in your cylinders. With different cylinders, your RMV may vary minutely - your "consumption of gas" is expressed as a volume measurement (in cubic feet) in relation to how much total gas volume you have available.

SAC = Total Gas Consumed (in psig) -:- [Depth (ATA) x Time (minutes)]

(Depth and Time variables are the depth at which measurements were taken, and the duration over which the measurements were made. Record beginning psig, swim at one depth fairly vigorously for a period of time, then record ending psig and the total time. The real caveat is that you want to hold the same depth throughout the period of measurement...)

You next convert SAC to Respiratory Minute Volume (RMV) to quantify a "cubic feet of gas consumed per minute at X depth" measurement. You're looking for your personal rate of gas consumption at a given ATA. (Obviously the calc includes depth; not so obviously it also includes which tanks you're using)

RMV = SAC -:- [rated cylinder working pressure -:- rated cylinder volume]

To find RMV:
1. Divide the rated cylinder working pressure by the rated volume;
2. Divide your SAC by this value (from step 1) to find your RMV.

(These formula may be found in many Advanced Nitrox course books, the ones above happened to come from an IANTD text.)

RMV comes in handy to plan how much gas to have available for a given dive, given expected conditions.

SAC comes in handy while you're actually DOING the dive, and you encounter conditions quite a bit different than those you expected!

 

[texdiveguy]

Some handy tools......... SAC Calculator

 

[Peter Guy]

Hmmm, RMV - SAC -- Maybe I should just call it a Rose --- which by any other name......

 

[Charlie99]

A lot of confusion is because there are TWO definitions of SAC.

Some people define SAC such that it is in units of psi/minute., and varies with the type of tank.

Some people define SAC such that the units are cubic feet per minute (or more correctly cfm/ata). This is the consumption at depth, normalized to 1ata.

This second definition is the same as one common definition of RMV.

Another bit of confusion comes because RMV is most often expressed in liters per minute rather than cfm and some will claim that RMV must be in lpm while SAC is cfm.

If you specify whatever unit you are using, then there won't be any confusion.

If you have a psi/min measurement and want to convert to cfm or vice versa, just use the psi/cu-ft for your tank. For example, an AL80 holds 77.4cu ft at 3000psi. 3000psi/77.4cu-ft = 38.75 psi/cu feet. For SAC calculation purposes, it just rounding that off to 40psi/cu ft is accurate enough. Some divers use the reciprocal of this, or 0.025 cu feet / psi. Since this is an awkward number and we normally only measure pressure to 100psi, you can multiply this by 100 to get 2.5 cu ft/100psi. In this format it is called a tank factor.