Average Gas Consumption

[scubadada]

There are LOTS of reasons why fingertip capillary saturation can be inaccurate.
But if 88% is real on room air, that should be of concern to you. The "shoulder" of the oxyhemoglobin dissociation curve starts around 90%, so any further drop in your carried oxygen may result in a markedly decreased saturation, which is not good for your downstream tissues (e.g., heart and brain).
If you think that number is real, please consider visiting your doctor.

Diving Doc

Thank you, from another diving doc Average Gas Consumption

Unlike for your RMV, it's not good to have a lower O2 saturation

 

[scubadada]

This illustrates @rsingler point

 

[rx7diver]

I think ...

@scubadada,

I sent the following in a PM to another SB member:

"This is how I think of things: The question is, can a line be used to describe the relationship between ambient water temp and SAC rate? Okay, let's compute the "best" line using data pairs from a bunch of dives.

"Is this "best" line any good? Well, this question has a two-part answer. (1) Is the associated R^2 appreciable? (2) And if it it is, then can we do the formal stuff, like determining whether the line is statistically significant, computing confidence intervals, computing prediction intervals, etc.?

"Actually, things are not quite this linear (pun intended). For example, inspection of the residuals might suggest a "fix" (e.g., add a square term to the regression function, or transform the data, etc.) so that if a new regression line/function is estimated, then the new R^2 might now be appreciable, and/or the new residuals might now suggest that the underlying regression assumptions are satisfied."

The takeaways:

1. R^2 makes intuitive sense, and it always can be computed and interpreted without requiring any additional "underlying assumptions."

2. However, determination of "statistical significance", and/or computation of confidence/prediction intervals/bands, etc. require some pretty specific "underlying assumptions" being satisfied. (These assumptions pertain to independent random errors, normal distributions, constant variances, etc.--all of which can be "tested for", formally or informally.) UPSHOT: If you cannot be reasonably sure that these underlying assumptions are satisfied, then you shouldn't trust the CI's/PI's that a given software product generates for you.

rx7diver

 

[Ulfhedinn]

So Shearwater shows me at 22psi a min on my steel 72... so.. that would be... what?

 

[Mike1967]

So Shearwater shows me at 22psi a min on my steel 72... so.. that would be... what?

That would 22 l/min not psi

<0.3 cu ft/min = <8.5 l/min
0.3-0.39 cu ft/min = 8.5-11.0 l/min
0.4-0.49 cu ft/min = 11.3-13.9 l/min
0.5-0.59 cu ft/min = 14.2-16.7 l/min
0.6-0.69 cu ft/min = 17.0-19.5 l/min
0.7-0.79 cu ft/min = 19.8-22.4 l/min
0.8-0.89 cu ft/min = 22.6-25.2 l/min
0.9-0.99 cu ft/min = 25.5-28.0 l/min
>1.0 cu ft/min = >28.3 l/min

 

[rsingler]

That would 22 l/min not psi

Uhhhh, no. Shearwater tracks and displays psi/min. Check the manual, pg 18.
Perdix AI Manual
"7.1. SAC calculations Surface Air Consumption (SAC) is the rate of change of tank pressure, normalized as if at 1 atmosphere of pressure. The units are either PSI/minute or Bar/minute. The Perdix AI calculates SAC averaged over the last two minutes. The data from the first 30 seconds of a dive are discarded to ignore the extra gas that is typically used during this time (inflating BCD, wing, or dry suit).
SAC vs RMV Since SAC is simply based on rate of tank pressure change, the calculations do not need to know the tank size. However, this means that the SAC is NOT transferable to tanks of a different size. Contrast this to respiratory minute volume (RMV) which is the volume of gas your lungs experience per minute, measured in Cuft/min or L/min. The RMV describes your personal breathing rate, and is therefore independent of tank size.
Why SAC instead of RMV? Since RMV has the desirable property of being transferable between tanks of different sizes, it seems to be the better choice on which to base GTR calculations. However, the main drawback of using RMV is that it requires setting up tank size correctly for each tank. Such setup is easy to forget and is also easy to setup incorrectly."
Shearwater regularly has to defend this choice, and their answer is always the same: calculating this way eliminates one human factor in an error chain. You can convert to RMV yourself.
As seen above, they phrase it more diplomatically.

Thus,
2475 psi divided by 72 cu ft = 34.375 psi/cu ft (Google: tank factor)
so 22 psi/min is about 0.6 cu ft/min

And it's 2475 because the 72 is probably an old 2200 psi tank, and I believe (but could be wrong) that capacity is based upon the originally accepted 10% overpressure for steel tanks. If I'm wrong there, somebody chime in.

 

[Mike1967]

Uhhhh, no.

Oh yeah my bad, I was thinking about after I've downloaded it to Divelog

 

[scubadada]

Today is this thread's 5th anniversary. There have been another 15 votes in the poll since the last update. The discussion has been very good, quite educational.

For the 2016 poll, the median, mode, and weighted average are all 0.5-0.59 cu ft/min (14.2-16.7 l/min)

 

[Ulfhedinn]

Uhhhh, no. Shearwater tracks and displays psi/min. Check the manual, pg 18.
Perdix AI Manual
"7.1. SAC calculations Surface Air Consumption (SAC) is the rate of change of tank pressure, normalized as if at 1 atmosphere of pressure. The units are either PSI/minute or Bar/minute. The Perdix AI calculates SAC averaged over the last two minutes. The data from the first 30 seconds of a dive are discarded to ignore the extra gas that is typically used during this time (inflating BCD, wing, or dry suit).
SAC vs RMV Since SAC is simply based on rate of tank pressure change, the calculations do not need to know the tank size. However, this means that the SAC is NOT transferable to tanks of a different size. Contrast this to respiratory minute volume (RMV) which is the volume of gas your lungs experience per minute, measured in Cuft/min or L/min. The RMV describes your personal breathing rate, and is therefore independent of tank size.
Why SAC instead of RMV? Since RMV has the desirable property of being transferable between tanks of different sizes, it seems to be the better choice on which to base GTR calculations. However, the main drawback of using RMV is that it requires setting up tank size correctly for each tank. Such setup is easy to forget and is also easy to setup incorrectly."
Shearwater regularly has to defend this choice, and their answer is always the same: calculating this way eliminates one human factor in an error chain. You can convert to RMV yourself.
As seen above, they phrase it more diplomatically.

Thus,
2475 psi divided by 72 cu ft = 34.375 psi/cu ft (Google: tank factor)
so 22 psi/min is about 0.6 cu ft/min

And it's 2475 because the 72 is probably an old 2200 psi tank, and I believe (but could be wrong) that capacity is based upon the originally accepted 10% overpressure for steel tanks. If I'm wrong there, somebody chime in.

thanks.. my math skills last light were lacking. Appreciate it!

 

[scubadada]

There appears to be an uptick in the number of views for this thread due to the @rsingler great thread on Rock Bottom calculations Why Surface with 500 PSI? Good reason to start calculating your RMV, if you do not already do so. Ten more votes in the poll since its birthday, 437 now