Average Gas Consumption

[Divin'Papaw]

Hey @dirkhh

Your graph was a great idea. I have known that my RMV was adversely affected by colder water for a long time, but I have never taken the time to prove it. My graph is not as pretty as yours, did it with Excel. The scattergram represents my last 533 dives from the beginning of 2018 until the present. The line is the linear trend. Clearly, as the temperature goes down, my RMV goes up. My RMV for these dives was 0.35 +/- 0.03 (mean +/- std dev). This needs to be taken with a grain of salt as many of my colder dives were also more strenuous. Revillagigedos and Galapagos were more challenging dives than those in Bonaire or Grand Cayman. Florida represents nearly the entire temperature range.

View attachment 658273

Mine is highly influenced by cold as well plus anything causing me to either work harder or be uneasy or uncomfortable. I'll have to work on doing a scatterplot like this for mine too.

 

[dirkhh]

Hey @dirkhh

Your graph was a great idea. I have known that my RMV was adversely affected by colder water for a long time, but I have never taken the time to prove it. My graph is not as pretty as yours, did it with Excel. The scattergram represents my last 533 dives from the beginning of 2018 until the present. The line is the linear trend. Clearly, as the temperature goes down, my RMV goes up. My RMV for these dives was 0.35 +/- 0.03 (mean +/- std dev). This needs to be taken with a grain of salt as many of my colder dives were also more strenuous. Revillagigedos and Galapagos were more challenging dives than those in Bonaire or Grand Cayman. Florida represents nearly the entire temperature range.

View attachment 658273

My graph is straight from the Subsurface 5 statistics module. And I am plotting RMV/SAC over temperature, while you are oddly seem to be plotting it the other way around -- which makes your trendline's meaning, err... confusing... you are tracking the temperature for a given RMV value... which really isn't how diving works, I think.

Also, I repeat my caveat. While a couple commercial dive log programs have finally started following our lead, most RMV/SAC data that I see still assumes linear compressibility of gases and is therefore factually wrong. And depending exactly which pressure range we are talking about, the error is often 10% and more.

 

[dirkhh]

Mine is highly influenced by cold as well plus anything causing me to either work harder or be uneasy or uncomfortable. I'll have to work on doing a scatterplot like this for mine too.

Simply import your dive data into Subsurface 5 and it will create this plot for you.
(open source, zero cost, no hidden anything - just a tool you can use and then delete and go back to however you track your dives today)

 

[rob.mwpropane]

I know someone who makes such a claim, and I believe him. I have dived with him, and his air consumption is extraordinary. (Well, it was. He is pretty much a rebreather diver now.)

I hesitate to say here how he does it, because people will scream, but I will reveal it. He skip breathes. Seriously. He would practice it on the surface, sometimes on long car trips.

Notice I am not advocating it; I am just a reporter.

Run @boulderjohn run!!

 

[scubadada]

My graph is straight from the Subsurface 5 statistics module. And I am plotting RMV/SAC over temperature, while you are oddly seem to be plotting it the other way around -- which makes your trendline's meaning, err... confusing... you are tracking the temperature for a given RMV value... which really isn't how diving works, I think.

Also, I repeat my caveat. While a couple commercial dive log programs have finally started following our lead, most RMV/SAC data that I see still assumes linear compressibility of gases and is therefore factually wrong. And depending exactly which pressure range we are talking about, the error is often 10% and more.

I assume my graph came out that way because of how I constructed the spreadsheet, RMV in column A and Water Temp in column B. it is still RMV vs. water temp and not difficult to interpret. This is simply two variables, plotted against one another, with linear correlation. As water temp goes down, RMV goes up, or, as water temp goes up, RMV goes down. Perhaps I'll take you up on your suggestion and import my data into Subsurface, would be a lot easier than constructing a spreadsheet.

Nearly all of my dives have been using AL80s. At a fill pressure around 3000 psi, I believe the error is a little over 3%, if you use the ideal gas capacity.

 

[dirkhh]

Nearly all of my dives have been using AL80s. At a fill pressure around 3000 psi, I believe the error is a little over 3%, if you use the ideal gas capacity.

Age old discussion. Math is really annoying. At 3000psi the error is around 3.5% one way, at 1000psi the error is around 1.5% the other way. And because math sucks, combining these two errors means you are actually almost 10% off, depending on exactly which gas you are breathing (because those errors are different depending on the O2 content), and depending exactly where your exit pressure is.
And of course, the moment you use HP cylinders, things get far worse. When playing with this a while ago I find a reasonably realistic scenario where the error was around 15% (but a quick search through my email didn't find that example)... and then you look at those alleged 400bar cylinders that I hear people talking about... oh my... Z=1.2 for air at 400 bar (so you only get ~333l of air per liter of wet volume @400bar)

 

[scubadada]

I know someone who makes such a claim, and I believe him. I have dived with him, and his air consumption is extraordinary. (Well, it was. He is pretty much a rebreather diver now.)

I hesitate to say here how he does it, because people will scream, but I will reveal it. He skip breathes. Seriously. He would practice it on the surface, sometimes on long car trips.

Notice I am not advocating it; I am just a reporter.

@rsingler has previously told us what the lower limit of RMV is, but I cannot find it. I believe this was from ventilating patients under anesthesia. Perhaps he can explain that to us again.

 

[rsingler]

The thread you were referring to is here:
RMV Spinoff from Accident & Incident Discussion - Northernone - aka Cameron Donaldson
Yes, commonly 0.2-0.25 cfm under anesthesia (which does not reduce metabolism unless the patient is cooled).

This thread shows my resting RMV at 5.1 lpm (0.18 cfm) sitting quietly in a chair with stable end-tidal CO2.
Overshooting NDL and mandatory deco stops

 

[Divin'Papaw]

The thread you were referring to is here:
RMV Spinoff from Accident & Incident Discussion - Northernone - aka Cameron Donaldson
Yes, commonly 0.2-0.25 cfm under anesthesia (which does not reduce metabolism unless the patient is cooled).

This thread shows my resting RMV at 5.1 lpm (0.18 cfm) sitting quietly in a chair with stable end-tidal CO2.
Overshooting NDL and mandatory deco stops

Science is kool!

 

[scubadada]

The thread you were referring to is here:
RMV Spinoff from Accident & Incident Discussion - Northernone - aka Cameron Donaldson
Yes, commonly 0.2-0.25 cfm under anesthesia (which does not reduce metabolism unless the patient is cooled).

This thread shows my resting RMV at 5.1 lpm (0.18 cfm) sitting quietly in a chair with stable end-tidal CO2.
Overshooting NDL and mandatory deco stops

Thanks Rob, I couldn't find your posts. So just under 0.27 for a dive over an hour is not too shabby.