question on RMV

[Tobagoman]

Not sure if this is the right forum for this question, but if anyone knows the answer, you guys will.
Why is the RMV greater for a higher capacity cylinder. My SAC rounds up to 21 psig, and the RMV for a 3442 psig /100 cf cylinder is .61 cf/min, and for a 3000 psig /45 cf deco cylinder it is 0.32 cf/min?
Logic tells me that the smaller cylinder should breath down at a faster rate than a larger capacity cylinder. What am I not seeing here?
Thanks!

 

[simbrooks]

Are you working whilst using your deco bottle (ie not decoing?). Your SAC/RMV will be much lower on deco than during the working portion of your dive, you are usually fairly still, more work = higher SAC rate. Those rates sound about right for my cave/working and stop rates, my drift diving and non-overhead working rates are a little lower - still working on them

As for larger cylinders, a little extra to push through the water, particularly doubles, but not a huge difference.

 

[Tobagoman]

Are you working whilst using your deco bottle (ie not decoing?). Your SAC/RMV will be much lower on deco than during the working portion of your dive, you are usually fairly still, more work = higher SAC rate. Those rates sound about right for my cave/working and stop rates, my drift diving and non-overhead working rates are a little lower - still working on them

As for larger cylinders, a little extra to push through the water, particularly doubles, but not a huge difference.

My SAC rate was calculated on a single tank at around 2 ATA for a 60 min dive. I then took the SAC and converted it into the RMV (surface) for the 100 cf cyclinder and the 45 cf cylinder. Since there is "more cf" available in the larger cylinder at only 442 more psi then the 45, I would think that for each breath you take from the larger cylinder, less cf / breath would be consumed compared to the smaller cylinder (this is the calculated RMV for the surface). I'm missing something here.
I calculated RMV by taking the rated cylinder working pressure divided by the tated cylinder volume and dividing that into my SAC.

 

[mempilot]

SAC is Surface Air Consumption, not 20' Air Consumption

SAC remains constant

You adjust SAC by a factor of pressure(ATA) and exertion.

Calculate your resting SAC by breathing off a tank at sea level for several minutes and averaging the cf/min.

Multiply that by ATA's to get a depth compensated rate.

Then adjust for workload, light current, moderate current/tasking, heavy, etc...

I use the following EF multipliers:

Single tank recreational rig:
No exertion: 1
Light: 1.5
Moderate: 2.0
Heavy: 2.5

Double tanks tech gear:
No exertion: 1
Light: 2.0
Moderate: 3.0
Heavy: 4.0


My standing SAC is .23
Let's plan a 99' dive with doubles in a moderate current or work load

.23sac x 4ata x 3ef = 2.76rmv at depth

20 minutes at that depth

20m x 2.76rmv = 55.2 cf required for bottom time + decent/ascent and reserve

I record my actuals and the conditions to verify the EF values. It's dead nuts on for me right now.

 

[simbrooks]

I hadnt split mine up by exertion factors, might have to go in to my logs and think about it and if there is a particular method to my workload in terms of a factor.

Tobagoman - you are correct in a sense, the guage reading will go down faster (not noticable unless over a long enough time period) for a smaller tank than a larger one all other things being equal. This does not mean you are breathing faster from one than the other, you would still be breathing the same cuft/min rate (its just the pressure difference per cuft is higher on a smaller tank). But as i said, and Mempilot pointed out, work rate really affects your SAC rate and you have to account for your depth in absolute atmospheres (ATA). I dont know what my no work rate on the surface is, but its in the 0.3's somewhere if not less, like i said i dont have those factors worked out, but it makes sense. For a drift i will be down in the 0.4's, moderate work 0.5's, heavy work or cave stresses 0.6's+ (still working on that).

 

[mempilot]

Make sure you are calculating cf properly.

Take the volume of the tank divided by the rated pressure. That will give you cf/psi. Then you can always go back and forth between cf and pressure.

Example:

98cf OMS tank rated at 2640

98cf / 2640psi = .037 cf/psi

When you check your SAC, you can use this number to convert psi breathed in so many minutes to a cf breathed in so many minutes.

Also, at the end of the dive, if you started with 2600psi and you finished with 500psi, then the total breathed was 2100psi.

2100psi x .037cf/psi = 77.7 cf used

Divide 77.7 by minutes and you can rough check your RSV for the depth you were at.

 

[Tobagoman]

I see where I'm looking at this wrong. The RMV is Minutes/cf not cf/min. So it is telling me that it takes me .61 minutes to breath 1 cf from the larger tank compared to .32 minutes to breath 1 cf from the deco bottle.
Now it makes perfect sense.
Whew.
Paul

 

[simbrooks]

I see where I'm looking at this wrong. The RMV is Minutes/cf not cf/min. So it is telling me that it takes me .61 minutes to breath 1 cf from the larger tank compared to .32 minutes to breath 1 cf from the deco bottle.
Now it makes perfect sense.
Whew.
Paul

Cant recall quite what RMV was a calc for (Reduced Mean Volume), but most people refer to SAC (Surface Air Consumption) as cuft/min and use that to work out their gas requirements. The psig/cuft rating is used to gas match on dissimilar tanks, particularly if you are diving thirds.

So in your examples i quoted you would breathe 0.61 cuft/min from your larger single (100cuft) during the working part of your dive and 0.32cuft/min from your deco bottle during deco. These numbers sound reasonable for SAC's. Multiplying these by your average depth in ATA will tell you how much you need for the dive to complete the profile you wish to dive - 2 ATA 1.22 cuft/min, 3 ATA 1.83cuft/min etc - so for an 100 you could expect to stay at max depth (3ATA) for about 45mins (if you plan a rock bottom of 1cuft/min for a 2 min ascent (33fpm) and 3 min safety stop).

 

[mempilot]

RMV - respiratory minute volume

Basically, the volume of gas breathed in a minute for a particular pressure

 

[simbrooks]

RMV - respiratory minute volume

Ok, thanks for correction - i think RMV is what is actually termed in some instruction texts as cuft/min and they call SAC for psig/min (which of course means next to nothing for dive planning), but most people use SAC when they are talking of cuft/min - hence forgetting quite what RMV was (including the acronym). So Tobagoman, is this clearer now, what you want is cuft/min at the surface and work out your gas requirements from there - as shown above, call it whichever you prefer RMV or SAC. Was it this phrasing and two different ways of quoting such things in psig or cuft per minute that was confusing the issue?