Question Determining SAC Rate

[scubadada]

Whether you call it SAC or RMV, as long as you give the units, people will understand what you are talking about

Pressure/time/atm = psi/min/atm or bar/min/atm, cylinder dependent
Volume/time/atm = cu ft/min/atm or liters/min/atm, cylinder independent

 

[tursiops]

I can tell from the context of the sentence what someone means.

And if they will provide the units of their SAC/RMV, even better. The numbers for cf/min and psi/min are sufficiently different that there is usually no confusion, but the numbers for l/min and psi/min can easily be confused, as can bars/min and cuft/min.

 

[jborg]

I have had a couple students begin classes telling me that they knew their SAC rates because they were calculated by their air integrated dive computers. In each case, they reported rates better than I could do. Then we dived, and neither had rates as good as mine. I offer no explanation--just an observation.

Well. An hour in the water doing exercises with an instructor is not the same as an hour looking at pretty fish (or the local equivalent, as it may be). Though if the starting numbers are unrealistic, there may be other things in play as well, of course.

 

[tursiops]

Well. An hour in the water doing exercises with an instructor is not the same as an hour looking at pretty fish (or the local equivalent, as it may be). Though if the starting numbers are unrealistic, there may be other things in play as well, of course.

One difference is the usual drop in tank pressure as the tank cools off early in the dive. Shearwater accounts for this by not starting the SAC calculation immediately, but just using starting and ending pressure will provide a SAC estimate that is too high.

 

[jborg]

One difference is the usual drop in tank pressure as the tank cools off early in the dive. Shearwater accounts for this by not starting the SAC calculation immediately, but just using starting and ending pressure will provide a SAC estimate that is too low.

Too high, surely?

 

[tursiops]

Too high, surely?

Uhh, yeah. Thanks. Corrected.

 

[jborg]

There's also the sister phenomenon for us euro divers with 300 bar tanks, where we're breathing the Z factor for the first part of the dive, which inflates the computed SAC rate unless the dive program takes it into account.

In any case, I know (by measurement) that I have at least 35% higher SAC when doing valve drills with someone watching me than when just swimming around. And that's as an average for the entire dive, when a fair amount of it might be spent looking the other students doing their part...

 

[scubadada]

Well. An hour in the water doing exercises with an instructor is not the same as an hour looking at pretty fish (or the local equivalent, as it may be). Though if the starting numbers are unrealistic, there may be other things in play as well, of course.

Yes, that is why you develop your average RMV. That takes into account the usual factors on your dives. You will quickly pick up the variables, exertion, being cold, anxiety issues... You can use your average RMV to plan for subsequent dives by accounting for unknown stressors. This includes surfacing on a pony.

 

[scubadada]

There's also the sister phenomenon for us euro divers with 300 bar tanks, where we're breathing the Z factor for the first part of the dive, which inflates the computed SAC rate unless the dive program takes it into account.

In any case, I know (by measurement) that I have at least 35% higher SAC when doing valve drills with someone watching me than when just swimming around. And that's as an average for the entire dive, when a fair amount of it might be spent looking the other students doing their part...

Deviations from the ideal gas law are considerable at 300 bar, almost trivial at 3000 psi, 207 bar.

 

[DiveLikeAMuppet]

The method is to swim for 10 minutes at a fixed depth, record tank pressure before / after and that will give you your RMV rate. It will never be quite precise as you tend to swim a bit more when you measure your RMVrate. Alternative you could stay at a fixed depth and do nothing for 10 minutes to get your "deco" RMV rate.

The point is to get an approximation, you will round it anyway quite a bit to get something easy to calculate underwater.

Eagle freedom units don't help here but you can most likely flip a coin - your RMV will be either close to 15 lpm (litres per minute) or 20 lpm (* for the bottom portion of the dive and perhaps either 10 or 12 lpm for deco.

Let's assume it's 20 lpm. RMV is not very useful, so let's calculate what would be your SAC rate - how much pressure in bars would you consume every 5 minutes out of your cylinder on the surface.

11 litres cylinder (ali80): 20/11 * 5 = 9 bars every five minutes on the surface, so le'ts round it to 10 bar
12 litres cylinder: 20/12 * 5 = 8 bars every five minutes on the surface, still round to 10 bar
15 litres cylinder: 20/15 * 5 = 6.5 bars every five minutes on the surface, so about 6 bar?
2x12 litres cylinder (doubles/twinset): 24/20 * 5 = 4.15 bars every five minutes on the surface so let's round to 4 bar

If I assume that I'm diving at around 30 meters (4 atmospheres pressure) on a 2x12; doubles, I should be consuming about 4 atm * 4 bar (surface) = about 16 bar every five minutes. Underwater I would round it to either 15 bar every 5 minutes (looking at pretty fish) or 20 bar (swimming a bit). So if you start your dive with 220 bar pressure, 10 minutes (= 2 five minute intervals) into the dive you should see somewhere between 190 bar (220 - 2 intervals * 15) and 180 bar on your SPG. If you see more - cool, your SAC rate is probably lower and maybe you could switch to 15lpm. If it's a bit higher - maybe you are a bit nervous on the dive, maybe there's a bit of current.

There isn't much of point in trying to be too precise, because each dive is a bit different. You need to find a set of numbers that are easy to use underwater - mental math is not fun when narked or taskloaded .

(* if your SAC/RMV (can' rate is more than 20 lpm, go work on your cardio/buoyancy/fin kicks/underwater comfort)