Average Gas Consumption

[TooCold]

I just discovered a big problem in my gas consumption numbers. I have had my logging software set to the wrong size tank this whole time.

Which I think is why Shearwater says they won't add tank volume settings to their DC's. What if your trusted DC was showing you an underestimate of your SAC rate? What if they used that rate for other calculations and showed you an overestimate of GTR, for example?

Still, for myself, I'd really like it if they would let me set the tank volume(s) - of course real volume, not the volume the gas would occupy if it were at 1 bar. They should just contnue to use (and report) pressure change for related calculations, not volume; only use tank volume to report it in the dive log. That way I don't have to add it manually later.

But sorry, that's off topic.

 

[scubadada]

Just discovered this great thread despite years of lurking on SB before joining. Will vote soon - haven't yet because I went cross-eyed looking at numbers in cu ft/min.

I found the discussion around the RMV/temperature interesting (and a little bit entertaining). Eyballing the posted scatterplots, I would be very dubious of interpreting the plotted line as a regression line. In other words, they can be fun to speculate, but a bad idea to think the data supports any prediction about RMV, given the temperature, or vice versa, or worse that it provides information to use for dive planning. This is especially true because a simple scaling of the axes will dramatically change their appearance.

I think this sums up perfectly how to approach the data:

When I am cold, my RMV is elevated, that's just the way it is . I dress warmer than most and have a great boat coat. The older i get, the more intolerant of cold I have become.

 

[TooCold]

When I am cold, my RMV is elevated, that's just the way it is

I think you hit the nail on the head. I would expect RMV to depend (partly) on body temperature, not water temperature. For me, I'm warmer diving in 5C water (cozy and dry) than on the second or third dive of the day in tropical water in a 3mm wetsuit. Lower body temp -> more exertion (goose bumps, shivering) -> more CO2 -> higher RMV. A full 3mm in 28C water just doesn't do it for me any more if I'm multi-diving.

 

[ATJ]

Which I think is why Shearwater says they won't add tank volume settings to their DC's. What if your trusted DC was showing you an underestimate of your SAC rate? What if they used that rate for other calculations and showed you an overestimate of GTR, for example?

SAC by definition is the change in pressure not the gas consumed (which is RMV). Even if you could put your tank volume in, it wouldn't change your SAC. Your SAC will alway be how quickly the pressure in the tank is changing.

Similarly, GTR is calculated on how quickly the pressure in the tank is changing, so, it too, would not change if you put the wrong tank volume in.

Still, for myself, I'd really like it if they would let me set the tank volume(s) - of course real volume, not the volume the gas would occupy if it were at 1 bar.

What is the real tank volume if not the volume of gas at 1 bar (or actually liquid volume which is what is used)?

 

[TooCold]

SAC by definition is the change in pressure not the gas consumed (which is RMV). Even if you could put your tank volume in, it wouldn't change your SAC. Your SAC will alway be how quickly the pressure in the tank is changing.

Gas consumption: the 'C' in SAC. It can be reported as pressure/time, volume/time, mass/time ... the confusion usually starts when the units aren't specified. Here's the first link I found just now:

Know Your Air Consumption: Calculating Your SAC’s - DIVEIN.com

If you’ve go beyond the basics of Scuba diving, and have started planning your own, independent dives, you should know you Surface Air Consumption, or SAC

www.divein.com

My point is NOT that SAC is volume/time, but that different units are used by different people or for varying purposes in different contexts. We can all get along and have intelligent discussions if we know what units we're using. I think dive computers use and report pressure/time, and calculate GTR using that, because they get sensor data - no user error possible.

Volume: the 'V' in RMV. Units are still important since not everyone uses the same ones.

What is the real tank volume if not the volume of gas at 1 bar (or actually liquid volume which is what is used)?

AL80 tanks for example often have volume of 11.1 litres. That's what I'd like to be able to set in my DC, not something related to the 80 in "AL80", which is not the tank volume and wouldn't get the job done.

Just trying to clarify, not get into the SAC/RMV debate here when there are so many of those threads already.

 

[ATJ]

Gas consumption: the 'C' in SAC. It can be reported as pressure/time, volume/time, mass/time ...

You started by talking about Shearwater computers and they use the term SAC correctly: volume by time: bar/min (for the metric people) and psi/min (for the non-metric.

the confusion usually starts when the units aren't specified. Here's the first link I found just now:

Know Your Air Consumption: Calculating Your SAC’s - DIVEIN.com

If you’ve go beyond the basics of Scuba diving, and have started planning your own, independent dives, you should know you Surface Air Consumption, or SAC

www.divein.com

And that site is calculating RMV and calling SAC. Just because someone publishes something n the web doesn't make it right.

I think dive computers use and report pressure/time, and calculate GTR using that, because they get sensor data - no user error possible.

They do.

AL80 tanks for example often have volume of 11.1 litres. That's what I'd like to be able to set in my DC, not something related to the 80 in "AL80", which is not the tank volume and wouldn't get the job done.

OK, that's the opposite of what you said before and was why I was confused:

Still, for myself, I'd really like it if they would let me set the tank volume(s) - of course real volume, not the volume the gas would occupy if it were at 1 bar.

The real volume IS the volume the gas would occupy at 1 bar. i.e. you can put exactly 11.1L of air at 1 bar into an "AL80" tank.

 

[Bob DBF]

And that site is calculating RMV and calling SAC. Just because someone publishes something n the web doesn't make it right.

The SCUBA industry is pretty fast and loose with the terms, and will use SAC to reference them both.

 

[ATJ]

Which I think is why Shearwater says they won't add tank volume settings to their DC's. What if your trusted DC was showing you an underestimate of your SAC rate? What if they used that rate for other calculations and showed you an overestimate of GTR, for example?

Thinking about this further, I don't believe it would make any difference to GTR calculations if they used RMV instead of SAC and you had entered the tank size wrong.

Let me use an example to illustrate. My RMV ranges from around 10 to 14L/min (using ideal gas calculations) but let's assume 12L/min for the calculations.

If I was diving with a 12L tank, my SAC would be around 1.00bar/min. If Shearwater also showed RMV and I correctly entered my tank size as 12L, it would show my RMV as 12.

Let's say I had incorrectly entered the tank size as 10L but was using a 12L tank. My SAC would still be 1 but RMV would show as only 10L/min. It would look like I was using less gas than normal. However, the GTR calculation would still turn out the same as it would see me using less air but from a smaller tank. In fact, it would most likely calculate the GTR in the same way: how long will the remaining gas last based on the current rate.

Let me add the workings.

I have 100bar left in my tank and GTR is going to calculate to 40bar. So the calculation will be based on 60bar.

I am at 10m and my SAC is currently 1.00.

Using SAC to calculate GTR gives me 60/2 = 30 minutes.

Using RMV to calculate GTR:

First with a 12L tank and size set correctly to 12L. RMV is 12L/min and I have 720L remaining before I get to 40bar. GTR will be 720/12/2 = 30 minutes.

Second with a 12L tank but size set to 10L. RMV is 10L/min and I have 600L remaining before I get too 40bar. GTR will be 600/12/2 = 30 minutes.

* Note I am assuming air behaves as an ideal gas.

 

[Dan]

Thinking about this further, I don't believe it would make any difference to GTR calculations if they used RMV instead of SAC and you had entered the tank size wrong.

Let me use an example to illustrate. My RMV ranges from around 10 to 14L/min (using ideal gas calculations) but let's assume 12L/min for the calculations.

If I was diving with a 12L tank, my SAC would be around 1.00bar/min. If Shearwater also showed RMV and I correctly entered my tank size as 12L, it would show my RMV as 12.

Let's say I had incorrectly entered the tank size as 10L but was using a 12L tank. My SAC would still be 1 but RMV would show as only 10L/min. It would look like I was using less gas than normal. However, the GTR calculation would still turn out the same as it would see me using less air but from a smaller tank. In fact, it would most likely calculate the GTR in the same way: how long will the remaining gas last based on the current rate.

Let me add the workings.

I have 100bar left in my tank and GTR is going to calculate to 40bar. So the calculation will be based on 60bar.

I am at 10m and my SAC is currently 1.00.

Using SAC to calculate GTR gives me 60/2 = 30 minutes.

Using RMV to calculate GTR:

First with a 12L tank and size set correctly to 12L. RMV is 12L/min and I have 720L remaining before I get to 40bar. GTR will be 720/12/2 = 30 minutes.

Second with a 12L tank but size set to 10L. RMV is 10L/min and I have 600L remaining before I get too 40bar. GTR will be 60/12/2 = 30 minutes.

* Note I am assuming air behaves as an ideal gas.

I don’t think so. Tank size matter. 12L tank at the 200 bar = 2400 L at 1 bar. 10L tank at 200 bar = 2000 L at 1 bar. So 12L tank would have 20% more air to breathe than 10L tank.

SAC unit is in bar/min. To covert it to L/min for RMV (V stands for VOLUME) correctly, you need to use the correct tank volume.

If you use the same tank to see how your SAC improving over the years, then your tank size doesn’t matter.

 

[ATJ]

I don’t think so. Tank size matter. 12L tank at the 200 bar = 2400 L at 1 bar. 10L tank at 200 bar = 2000 L at 1 bar. So 12L tank would have 20% more air to breathe than 10L tank.

Please reread my post. I don't think you have followed what I was saying.

I certainly wasn't saying that a 12L tank holds the same air as a 10L tank. I was talking about GTR calculations.