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Respiratory Minute Volume November 7, 2009

Posted by Chris Sullivan in Technical Diving.
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It’s interesting that there is no agreement among agencies as to the exact definition of Respiratory Minute Volume (RMV) and Surface Air Consumption (SAC).

My original certification agency, NASDS, defined SAC as the surface equivalent number of PSI per minute on a dive, which is (PSI Consumed/Total Dive Time)/((33+Depth)/33).

IANTD does the same thing, and defines RMV as the SAC/(Working Pressure/Rated Cylinder Volume).

DSAT defines SAC as the same as IANTD and NASDS defines RMV, and defines RMV is the (Tidal Volume minus Respiratory dead air space) times breaths/minute. I added the parenthesis to what I read in the book (p42 Tec Deep Diver Manual) because otherwise it doesn’t make any sense. Oddly the Encyclopedia of Recreational Diving doesn’t seem to mention either term. Wikipedia’s definition of RMV is similar to DSAT but doesn’t factor Respiratory Dead Air Space.

In the SDI solo diving course SAC is defined as the volume per minute consumed at rest on the surface. They recommend breathing from a tank while sitting around watching TV or like activity to measure this. For me, consuming about 325 PSI of a 3000 PSI working pressure tank with a rated volume of 77.4 cubic feet in 30 minutes, this was about .28 cubic feet per minute. They call what IANTD and NASDS call RMV the SRMV, or Surface Respiratory Minute Volume. They then go on to recommend that before each dive the RMV by multiplying by the number of absolute atmospheres of pressure and also by a “Dive Factor”, which takes into account effort and should be at least 1.5 for an easy dive, and perhaps more than 3 for a high effort, cold or stressful dive.

Disconcertingly, their sister agency, TDI, recommends determining SAC using a swim at depth, the very thing that SDI says doesn’t work, and implies that RMV is just another term for SAC.

0.28 seems a bit low. I think having the reg in my mouth called attention to my breathing and slowed it down some. At 33 feet a tank would last over 2 hours at that rate, although my record is about an hour and 20 minutes on a reef dive (and 1h 40 in really shallow water) on a single 80 so maybe it’s not all that far fetched. I know when it’s cold and I’m working a bit hard the rate goes way, way up.

It would be nice to have a consensus between agencies on this topic. Maybe ISO will define it some day.





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