Question Determining SAC Rate

[Edward3c]

Yes, you can certainly do it that way, and it is probably how most people calculate RMV. However, it is very useful to know RMV under different situations: Being calm and still for deco gas planning, normal movement for 95% of dives, and under work/duress for high energy environments or bailout gas planning.

You might go through all three types on a single dive if you have some current to swim against at some point, and then do a nice 5 min safety stop at the end. The differences will get lost using average depth and total gas used. That may not matter at all (or might even be better) for no deco dives, but may be important for gas planning. As a newer diver, it's probably useful into know how much of a difference there is between working and resting.

I've done quite a few dives with individuals going gas planning to the nth degree, against my rough and ready overall consumption rate. Never had to cut the dive short, but I've had to ascend early because my buddy didn't have enough gas to finish the planned dive with mandatory stops.

 

[FishWatcher747]

Calculating gas usage by a timed swim at a particular depth is a holdover from the days people used tables or were using computers that didn't calculate average depth. If one is logging their SAC for every dive this teaching procedure is obsolete.

 

[inquis]

If one is logging their SAC for every dive this teaching procedure is obsolete.

It's still useful for insight into consumption outside of the average effort over the entire dive. Granted, that's likely more detail than the average rec diver cares about.

 

[scubadada]

...However, it is very useful to know RMV under different situations...

If you calculate your RMV on all dives, you will quickly learn the effect of exertion, being cold, or being anxious has on your gas consumption.

You can examine your SAC over the entire dive on the Shearwater Cloud graph. I recently did a dive where there was a 10 minute episode of rather intense exertion in the middle of an otherwise easy dive. At the peak of activity, my RMV was nearly double my average gas consumption.

Average Gas Consumption

Speaking of gas consumption, I had an interesting dive on Tuesday while visiting Jupiter, FL. We dived over two sites, High Ledge and Capt. Kirle's. We started on top of the reef and at about 20 min, we did a 10 min swim back to the main ledge against a strong current. We finished the dive at...

scubaboard.com

Like many divers, I use twice my average RMV when calculating gas needs for emergency situations, such as the capacity of my pony bottle.