question on RMV
- Thread starter Tobagoman
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OP
Tobagoman:
I was wrong again on the above. Now that I'm home and not at work trying to do two things at once, I wrote out the calculations and indeed it is cf/min.
I calculated my SAC from a dive at 33 fsw for a 10 minute period on a 80 cf cylinder filled with 3000 psi. I swam in a large circle with moderate effort. I breathed down 420 psi in that time period. Plugging in the data for 2 ATA it gave me an SAC of 21 psig/min.
plugging 21 psig/min into the formula for RMV (with the 80 cf cylinder) gave 0.56 cf/min. on the surface
I think the way you use this number is that the 0.56 cf/min is for any cylinder size (that is why you use RMV instead of SAC, since SAC is calculated for whatever tank you used to get the original data and RMV allows you to get a rate across cylinders).
So if I'm breathing from a 45 cf cylinder at 15 fsw, using the RMV of 0.56 cf/min, I multiply the RMV by the ATA of 1.45 and get a modified RMV of 0.812 cf/min at 15 fsw. Thus, the 45 cf in the cylinder is divided by the mod RMV of 0.812 cf/min to give 55.4 minutes of breathable gas in the cylinder at this depth.
If I was breathing a 100cf cylinder at 15 fsw it would be 100cf/ 0.812 cf/min = 123 minnutes to breath down the cylinder.
Does everyone agree with this reasoning?
I realize that physical or thermal stress will add to the breathing rate.
Thanks again for putting up with my ranting and original misinformation!
Your math sounds about right without getting a calculator out to completely check it. My point above was that people use RMV and SAC interchangeably, but most people quote the same kind of figure in cuft/min on the surface. You can ratio that to your tank size, depth and exertion rate. What you have to remember with you little 45 is that you arent going to be working, hence the 0.58 is a bit high (as you calc'd that from a little work), expect it to be around 0.3-0.4 max if you are doing nothing but decoing. You breathing down rate assumes its all at depth, of course you have to add in safety factors for ascents, buddy breathing, stops etc which means your 100cuft would be more like 83cuft at 3 ATA as indicated in my previous attempt at the math 
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A gas management synopsis ...
- SAC is the amount of gas you breathe at the surface in pressure (psi or bar) per minute. It is specific to the cylinder you are breathing from.
- You can calculate your SAC rate from any dive if you know the average depth by using the formula (depth/33+1) to calculate the pressure in ATA at that depth, dividing the total gas consumed (in psi) by the ATA, and dividing by the total minutes you consumed the gas.
- RMV is the amount of gas you breathe at the surface in volume (cubic feet or liters) per minute. It is independent of the cylinder you are breathing from.
- SAC will vary, depending on the cylinder you are using. Within certain limits (imposed by the amount of exertion you are doing on a specific dive) your RMV will be relatively constant regardless of cylinder.
- To convert from SAC to RMV you need to know the "baseline" of the cylinder you are breathing from. Baseline is the volume of the cylinder divided by the working pressure (e.g. 95/2640 for a Faber LP95 ... = 0.036 CF/psi). You convert SAC to RMV by multiplying your SAC by the baseline of your cylinder (e.g. 16 psi/minute x 0.036 CF/psi = 0.576 CF/minute).
- You should calculate your SAC for at least a resting (normal) and working (exertion) rates. This will give you a "range" which is useful when planning how much gas you will need for the conditions you plan to dive in.
- Both SAC and RMV are useful to know for dive planning. You use SAC when determing turn pressure and rock bottom. You use RMV for planning how much gas you will need for a specific dive plan.
That's just a nutshell overview ... but these are concepts any diver should be clear on before doing deep dives, and certainly before getting into tech diving. I teach it in AOW.
... Bob (Grateful Diver)
- SAC is the amount of gas you breathe at the surface in pressure (psi or bar) per minute. It is specific to the cylinder you are breathing from.
- You can calculate your SAC rate from any dive if you know the average depth by using the formula (depth/33+1) to calculate the pressure in ATA at that depth, dividing the total gas consumed (in psi) by the ATA, and dividing by the total minutes you consumed the gas.
- RMV is the amount of gas you breathe at the surface in volume (cubic feet or liters) per minute. It is independent of the cylinder you are breathing from.
- SAC will vary, depending on the cylinder you are using. Within certain limits (imposed by the amount of exertion you are doing on a specific dive) your RMV will be relatively constant regardless of cylinder.
- To convert from SAC to RMV you need to know the "baseline" of the cylinder you are breathing from. Baseline is the volume of the cylinder divided by the working pressure (e.g. 95/2640 for a Faber LP95 ... = 0.036 CF/psi). You convert SAC to RMV by multiplying your SAC by the baseline of your cylinder (e.g. 16 psi/minute x 0.036 CF/psi = 0.576 CF/minute).
- You should calculate your SAC for at least a resting (normal) and working (exertion) rates. This will give you a "range" which is useful when planning how much gas you will need for the conditions you plan to dive in.
- Both SAC and RMV are useful to know for dive planning. You use SAC when determing turn pressure and rock bottom. You use RMV for planning how much gas you will need for a specific dive plan.
That's just a nutshell overview ... but these are concepts any diver should be clear on before doing deep dives, and certainly before getting into tech diving. I teach it in AOW.
... Bob (Grateful Diver)
Split hairs if you must!! Most on this side of the pond, hey its what i was taught and what 90% of all SB members would be dealing in unit-wisegrazie42:I´d point out that MOST people propably quote RMV & SAC in something quite different than cft/min but that propably wouldn´t help tobagoman so I won´t
I know you point, what do you guys use, litres/min? But the point was that even though RMV and SAC are supposed to be in different units - one in volume change the other in pressure change over a unit time, the terms are often changed back and forth and most people call it SAC as that acronym sounds more like the result we are calculating than RMV - which is just a mouthful
My GF's little brother was looking for a science project that involves graphs and such (he wants to dive soon too), so i suggested that he take my SAC rate data and correlate them to my estimated working/stress on each dive to work out exertion factors like yours - he didnt buy, little punk!mempilot:It's specific to you, so if you are accurate and verify it, call it what you will!
simbrooks:My GF's little brother was looking for a science project that involves graphs and such (he wants to dive soon too), so i suggested that he take my SAC rate data and correlate them to my estimated working/stress on each dive to work out exertion factors like yours - he didnt buy, little punk!
I've built an Excel spreadsheet with graphs plotting my actuals against my EF lines by depth. It's kinda cool.
Yeh but using this kid brother would have been a bunch easiermempilot:
