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 If one is logging their SAC for every dive this teaching procedure is obsolete.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If one is logging their SAC for every dive this teaching procedure is obsolete.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

 

[34109411]

maybe not the right place but anyway. While diving yesterday with my mares quad i notice an icon on a secondary screen showing 0.35 CF/m i assume this was my sac rate ? i look in the mares user manuel didn't find anything.

Is there a formula to convert this into PSI per minute ? ( if it's what is all about).

80 cf at 30 feet.

Thank you

 

[tursiops]

maybe not the right place but anyway. While diving yesterday with my mares quad i notice an icon on a secondary screen showing 0.35 CF/m i assume this was my sac rate ? i look in the mares user manuel didn't find anything.

Is there a formula to convert this into PSI per minute ? ( if it's what is all about).

80 cf at 30 feet.

Thank you

To convert cf/min to psi/min, multiply by psi/cf for the cylinder you are using. For an AL80, that is 3000/77.4 = 38.8, so call it 40. Your 0.35 cf/min would be 14 psi/min.

However, I find nothing in the Mares Quad manual about it displaying SAC in cf/min (or in ANY units), and if it is able to do this then it would already know your tank information, which you must have given it. I find nothing in the manual about that, either.
Also, it is unlikely to display "CF/m" since "m" is the abbreviation for meters, not minutes.
So, I have no idea what you are talking about!

 

[scubadada]

Hi @34109411

I assume you dive a Quad Air. The short answer is your computer calculates your gas consumption in cubic feet/min when set in imperial units.

Setting gas integration is discussed in the user's manual in section 2.2.1.6 on page 9. You set the tank volume in step 2 and the operating pressure in step 3. With this information, along with psi drop, avg depth, and time, the computer calculates the RMV.

The main screen display is discussed in section 3.3 on page 13. The bottom row generally displays tank pressure and TTR (time to reserve, Mares version of air or gas time remaining). Pushing either bottom button cycles TTR through other parameters, the first is your gas consumption.

Do you download your computer? In the download, does it give you your average gas consumption/RMV, for the entire dive?

 

[tursiops]

maybe not the right place but anyway. While diving yesterday with my mares quad i notice an icon on a secondary screen showing 0.35 CF/m i assume this was my sac rate ? i look in the mares user manuel didn't find anything.

Is there a formula to convert this into PSI per minute ? ( if it's what is all about).

80 cf at 30 feet.

Thank you

To convert cf/min to psi/min, multiply by psi/cf for the cylinder you are using. For an AL80, that is 3000/77.4 = 38.8, so call it 40. Your 0.35 cf/min would be 14 psi/min.

However, I find nothing in the Mares Quad manual about it displaying SAC in cf/min (or in ANY units), and if it is able to do this then it would already know your tank information, which you must have given it. I find nothing in the manual about that, either.
Also, it is unlikely to display "CF/m" since "m" is the abbreviation for meters, not minutes.
So, I have no idea what you are talking about.!

Hi @34109411

I assume you dive a Quad Air. The short answer is your computer calculates your gas consumption in cubic feet/min when set in imperial units.

Setting gas integration is discussed in the user's manual in section 2.2.1.6 on page 9. You set the tank volume in step 2 and the operating pressure in step 3. With this information, along with psi drop, avg depth, and time, the computer calculates the RMV.

The main screen display is discussed in section 3.3 on page 13. The bottom row generally displays tank pressure and TTR (time to reserve, Mares version of air or gas time remaining). Pushing either bottom button cycles TTR through other parameters, the first is your gas consumption.

Do you download your computer? In the download, does it give you your average gas consumption/RMV, for the entire dive?

Thanks, Craig. Almost answers all my questions! He does NOT have a Quad, but rather a Quad Air. Duh.
He is apparently set to the default tank, which is a 3000 psi AL80.
The units are cu ft/min. His 0.35 cu ft/min is presumably referenced to 1 ATM, but I do not see that spelled out in the manual.
What he is seeing is apparently the "instantaneous" gas consumption, but the manual does not state over what time interval it is measured.

 

[scubadada]

Thanks, Craig. Almost answers all my questions! He does NOT have a Quad, but rather a Quad Air. Duh.
He is apparently set to the default tank, which is a 3000 psi AL80.
The units are cu ft/min. His 0.35 cu ft/min is presumably referenced to 1 ATM, but I do not see that spelled out in the manual.
What he is seeing is apparently the "instantaneous" gas consumption, but the manual does not state over what time interval it is measured.

Hi Mel,

I made the same assumptions that you did. The manual is lacking in detail.

The information needed to calculate the RMV is in the on-board dive log, but the average RMV for the dive is not given.

 

[34109411]

Thanks, Craig. Almost answers all my questions! He does NOT have a Quad, but rather a Quad Air. Duh.
He is apparently set to the default tank, which is a 3000 psi AL80.
The units are cu ft/min. His 0.35 cu ft/min is presumably referenced to 1 ATM, but I do not see that spelled out in the manual.
What he is seeing is apparently the "instantaneous" gas consumption, but the manual does not state over what time interval it is measured.

let me try to take a pic next time maybe i didnt see correctly. Yes it s the mares quad air.