SAC / RMV Different for Different Size Tanks?

[rob.mwpropane]

I am having a bit of a dilemma. Quick background, I dive pretty much LP85's and HP100's year round. I contracted Covid in Dec of 2021 and watched my SAC go way up and just thought that I had some type of permanent lung damage. Looking back on it it's around the same time that I was primarily using my 85's (easier to just leave them rigged up and fill and go).

Fast forward to this past week and I switch back to the 100's. SAC went back down to what it was pre covid? All my #'s are pulled direct from SubSurface using a transmitter and a SW Perdix, plugging the size tank in. (I believe SubSurface says SAC, but it's actually giving RMV?).

I "thought" that SAC was supposed to be the same between all tanks? When I switch to a 40 it goes back down again.

I'm sure I'm missing something silly or not understanding something that I should have paid more attention too, but what is it? I plug in 170 for my double 85's with a working pressure of 2640, and 200 for my double 100's with a working pressure of 3500?

My SAC / RMV in SubSurface is .55 for the 100's / 40 and .75 for the 85's. In the long run it's kind of irrelevant because I plan with 1.0 anyway, but I'd still like to understand.

Should also note that I fill the 85's to 3400psi and the 100's to 3900psi.. but that shouldn't matter... right?

 

[tursiops]

Tanks are not quite the same size, so slightly different drag.
Tanks are not quite the same weight, so slightly different suit/wing inflation.
Your HP100s are PST? They hold more than 100 cuft.
Because of gas compressibility, 3600 psi is not really 3600/2640 more gas.
All the little things add up.

 

[inquis]

I plug in 170 for my double 85's with a working pressure of 2640, and 200 for my double 100's with a working pressure of 3500

You've overstated the capacity for the 85s and understated it for the 100s. You should enter the ideal volume into Subsurface: 2x82.5 cf (LP85) and 2x106.7 cf (HP100). The latter is for a Faber but will hopefully be closer to truth for your tanks than 100 cf.

Subsurface considers compressibility, and you can hover your mouse over the starting pressure to see the real gas capacity at that pressure and mix.

 

[inquis]

Tanks are not quite the same weight, so slightly different suit/wing inflation.

Ideally the buoyancy difference maps to a difference in lead used and thus similar suit/wing inflation. For 2x Faber's, it's about a 6 lb difference. I doubt any residual difference in buoyancy would noticeably impact RMV.

 

[scubadada]

Hi @rob.mwpropane

The difference in your RMV between the 85s and the 100s seems too large to be explained by cylinder characteristics. The 100s would be more affected by deviation from the ideal gas law at the higher pressure. You are correct, RMV is cylinder independent.

Is it possible that your gas consumption improved over time after recovery from Covid? Seems you just switched back to the 100s from the 85s. Do you have your RMV by month and by cylinder used since you restarted diving after your illness.?

Of course, there would be an easy way to look at this. Do a couple of dives now with your 85s and then switch back and do similar dives with the 100s.

Let us know what you figure out.

 

[inquis]

The 100s would be more affected by deviation from the ideal gas law at the higher pressure.

I don't see that as relevant to RMV. The gas entering the lungs behaves ideally, so an inhale removes the same amount of gas from the cylinder, all else being equal. It's true the PSI drop per breath is greater at 3900 psi, but Subsurface takes that into account.

 

[inquis]

The difference in your RMV between the 85s and the 100s seems too large to be explained by cylinder characteristics.

I'd agree with this. The volume errors only explain about a 10% difference, but your values are around a 35% difference. I wonder if the water temp may be an influence if dives in each type of cylinder were at differing times of the year.

 

[scubadada]

I don't see that as relevant to RMV. The gas entering the lungs behaves ideally, so an inhale removes the same amount of gas from the cylinder, all else being equal. It's true the PSI drop per breath is greater at 3900 psi, but Subsurface takes that into account.

Thanks. Neither Shearwater Cloud nor Oceanic OceanLog take deviations from the ideal gas law into account. I forgot that Subsurface does. As I nearly always dive AL80s, this is less important to me than someone diving at higher fill pressures.

 

[inquis]

When I switch to a 40 it goes back down again.

Is this in the context of using an AL40 during deco? My SAC usually drops between 0.1-0.2 cf/min from the rest of the dive.

Yes, SAC in Subsurface has units of vol/min and is equivalent to RMV. It should be independent of cylinder IF the conversion from pressure is accurate (which does depend on the cylinder's characteristics). You're also correct that *in Subsurface* there should be no impact to SAC/RMV by starting at 3900 psi vs something lower.

 

[rob.mwpropane]

Sorry guys, I should never post on a Friday, my weekends are always so busy.

Tanks are not quite the same size, so slightly different drag.
Tanks are not quite the same weight, so slightly different suit/wing inflation.
Your HP100s are PST? They hold more than 100 cuft.
Because of gas compressibility, 3600 psi is not really 3600/2640 more gas.
All the little things add up.

The 85's are 1/4" longer, they are the same in every other way. I can't tell a difference on my back whatsoever

You've overstated the capacity for the 85s and understated it for the 100s. You should enter the ideal volume into Subsurface: 2x82.5 cf (LP85) and 2x106.7 cf (HP100). The latter is for a Faber but will hopefully be closer to truth for your tanks than 100 cf.

Subsurface considers compressibility, and you can hover your mouse over the starting pressure to see the real gas capacity at that pressure and mix.

That could be part of the problem for sure.

Ideally the buoyancy difference maps to a difference in lead used and thus similar suit/wing inflation. For 2x Faber's, it's about a 6 lb difference. I doubt any residual difference in buoyancy would noticeably impact RMV.

I have PST 100's and Worthington 85's

Hi @rob.mwpropane

The difference in your RMV between the 85s and the 100s seems too large to be explained by cylinder characteristics. The 100s would be more affected by deviation from the ideal gas law at the higher pressure. You are correct, RMV is cylinder independent.

Is it possible that your gas consumption improved over time after recovery from Covid? Seems you just switched back to the 100s from the 85s. Do you have your RMV by month and by cylinder used since you restarted diving after your illness.?

Of course, there would be an easy way to look at this. Do a couple of dives now with your 85s and then switch back and do similar dives with the 100s.

Let us know what you figure out.

I have been using just the 85's for the past 4 months or so, just switched to the 100's for my 1st time last Tues and for the 1st time in a long time SAC came up as .55 or a very benign dive. I have all my SAC for at least the last 18 months.

I don't see that as relevant to RMV. The gas entering the lungs behaves ideally, so an inhale removes the same amount of gas from the cylinder, all else being equal. It's true the PSI drop per breath is greater at 3900 psi, but Subsurface takes that into account.

Agreed.

Is this in the context of using an AL40 during deco? My SAC usually drops between 0.1-0.2 cf/min from the rest of the dive.

Yes, SAC in Subsurface has units of vol/min and is equivalent to RMV. It should be independent of cylinder IF the conversion from pressure is accurate (which does depend on the cylinder's characteristics). You're also correct that *in Subsurface* there should be no impact to SAC/RMV by starting at 3900 psi vs something lower.

Normally I would say yes, but for this particular dive we did a good bit of swimming / looking around at the stop, so it didn't really feel like a normal deco stop. For the dive I entered 40cf @ 3000psi in SubSurface and it says SAC is .59


@inquisit do you know the exact #'s to plug in for PST 100's and Worthington 85's? I "think" that's most likely the reason why my #'s are so different.