Average Gas Consumption

[scubadada]

All of the data needed to calculate SAC and RMV is included in the bottom half of the Oceanic logging app; start pressure, end pressure, dive time, avg depth, cylinder gas capacity and working pressure. Sorry for the imperial example, it's what I do

 

[inquis]

SAC is pressure/time/atm. It is cylinder dependent.

That is the convention, but the tangent started because the logical concept of "consumption" doesn't mandate units (nor does everyone follow that convention). It's also perfectly valid to state:
Consumption in units of pressure/time/atm is cylinder dependent
Consumption in units of volume/time/atm is cylinder independent

I think it would demystify the concept for new divers to realize it's simply two ways of looking at the same thing (i.e., consumption). Further tangent: auto fuel consumption is no different: miles per gallon, km per liter, or even liters per 100 km are all in use today, and AFAIK, "consumption" conveys the point.

 

[Divin'Papaw]

All of the data needed to calculate SAC and RMV is included in the bottom half of the Oceanic logging app; start pressure, end pressure, dive time, avg depth, cylinder gas capacity and working pressure. Sorry for the imperial example, it's what I do

Cylinder Gas Capacity is the one element Shearwater computers don’t capture, as has been mentioned. Therefore, those of us with Shearwaters need to convert SAC to RMV either using other software or manually. Personally, I use a spreadsheet. I added a formula to calculate RMV from SAC using the cylinder volume. It’s easy math. That works for me because that spreadsheet is my log book. It has every dive I’ve ever done. Date, location, site, charter if applicable, equipment, buddies, max depth, avg depth, time, temp, etc.

I’d imagine SW hasn’t bothered with Tank volume because it is so variable and setting it is very easily forgotten.

 

[scubadada]

Hi @inquisite,

This topic has been discussed numerous times on SB. Standard or conventional definitions would help considerably in accurate communication.

I rarely talk about SAC, when i do, I give the units, psi/min, generally leaving out the per atm. With SAC, it is explicitly at the surface, just the pressure units are not clear.

When I talk about RMV, I also mention the units, cu ft/min, again leaving out the per atm. With RMV, the surface conditions are not explicitly stated, but the volume is. I've thought it should be SRMV, but we already have enough miscommunication as it is

 

[scubadada]

...I’d imagine SW hasn’t bothered with Tank volume because it is so variable and setting it is very easily forgotten.

I'd be happy if Shearwater would simply allow you to enter the cylinder information to calculate RMV in the Cloud log. I don't care if my dive computer calculates it.

My 11 1/2 year old Oceanic VT3 gives none of the required data in the on-board log except for dive time, all of the information is displayed in the downloaded log, as I posted above Average Gas Consumption

My VT3 does give me an ATR, including safety or deco stops, which has provern quite accurate. So, it does know my SAC.

 

[Divin'Papaw]

I'd be happy if Shearwater would simply allow you to enter the cylinder information to calculate RMV in the Cloud log. I don't care if my dive computer calculates it.

I think that would be a nice compromise. I'd sure like that feature and would certainly use it. You should submit it as a recommendation to SW.

 

[scubadada]

I think that would be a nice compromise. I'd sure like that feature and would certainly use it. You should submit it as a recommendation to SW.

I have, along with a couple of others, easy stuff

 

[Joneill]

All of the data needed to calculate SAC and RMV is included in the bottom half of the Oceanic logging app; start pressure, end pressure, dive time, avg depth, cylinder gas capacity and working pressure. Sorry for the imperial example, it's what I do

View attachment 701541

View attachment 701544

0.26? That's a crazy low SAC (actually meant RMV) - you must have gills ! The best I've achieved to date was on my November Aruba trip where I was consistently below 0.4 and got as low as 0.35.

 

[tursiops]

Hi @inquisite,

This topic has been discussed numerous times on SB. Standard or conventional definitions would help considerably in accurate communication.

I rarely talk about SAC, when i do, I give the units, psi/min, generally leaving out the per atm. With SAC, it is explicitly at the surface, just the pressure units are not clear.

When I talk about RMV, I also mention the units, cu ft/min, again leaving out the per atm. With RMV, the surface conditions are not explicitly stated, but the volume is. I've thought it should be SRMV, but we already have enough miscommunication as it is

I have understood that RMV (as a correctly define technical term) is independent of depth; it is a physiological concept that describes the uptake in the lungs, at the surface, for example 0.7 cuft/min or 20 l/min. If I increase my ambient pressure to (say) 3ATM, it isn't RMV that changes, it is just the amount of gas I breathe in 1 minute. So I can't say my RMV has tripled...it stays the same. It is my gas need that triples.

In a perverse sort of way, SAC also doesn't need to be stated as at 1 ATM; the S in SAC does that. Similarly to RMV, if I go to 3 ATM, my SAC doesn't change, only my gas needs.

Perhaps all would be clearer if Shearwater -- and others -- did not display SAC (or RMV, for that matter) during he dive but rather displayed psi/min or cuft/min AT DEPTH, so you can more easily see that you are using gas up faster if you are deep. If I glance down and see that SAC stays at 31 psi/min while I am diving, perhaps I might misinterpret that.

 

[inquis]

but rather displayed psi/min or cuft/min AT DEPTH

I would not like that. I use memorized values to gauge stress level during the dive.