Air integrated vs. SPG, a small study

[cerich]

TSandM,

If you are trying to work out your SAC your results gain in resolution as you:

Extend the period of the test i.e. note the beginning and ending pressures over a 30 period vs a 10 minute period

Increase the WP of the tank, that increases the PSI / cuft

Decrease the volume of the test tank. That also increases the PSI / cuft.

My personal favorite for SAC rate tests is a slung AL 30. With a fill pressure of 3000 psi, and a capcity of 30 cuft, you end up with 100 psi / cuft. Pretty handy.

Swim that 30 for 10 minutes at 33 ft and you can calc your SAC rate in your head.

Example start pressure = 3000, end pressure = 1800 psi. PSI comsumed = 1200

1200 psi / 100 psi per cuft = 12 cuft. 12 / 2 x 10 = .6

With an analog guage the minimum resolution should be 1/2 division or 50 psi. If you look at the impacts of the maximum resolution error induced, i.e. 1300 psi on the high end and 1100 psi on the low the results vary from .65 Sac to .55

If you repeat this 10 minute test using double LP 120's you get 11 psi / cuft.
Now the impacts of max error is about nine times greater, and the observed change in pressure approaches an order of magnitude less than the minimum resolution.

In my view a small HP tank trumps a digital guage.



Tobin

Tobin, you have just found me a use for my 30 alum that hasn't been used in 6 years because I went standard on everything else. It shall now accompany me when I teach and my students shall get to use it!

Best,

Chris

 

[cool_hardware52]

Unfortunately you are confusing arithmetic precision (the number of digits used to represent a value) with measurement precision (how well the measurements can be repeated). This is a common mistake. It is also a common mistake to use accuracy and precision interchangably when speaking of a measurement.

When making measurement there are two important factors:
Accuracy - meaning the readings are correct.
Precision - meaning the readings are repeatable.

When making a measurement you it want it to be both accuracy (correct) and precise (repeatable). And have an instrument that has the arithmetic precision to give it to you.

For the most part I agree, but I'd also say I'll take good repeatability over absolute accuracy every time. Why? I can adjust for the lack of accuracy, but poor repeatability cannot be compensated for. An example: Lets say I have an SPG that always reads 100 psi high at the high end of the range (actual 3000 psi, reads 3100) and always reads 50 psi low at the low end (actual 550, reads 500) I can adjust for this. If on the other hand my SPG sometimes reads 3100 when it's 3100, and some times reads 2900 when it's 3000 I need a new SPG.

Here is a pretty decent explanation - http://en.wikipedia.org/wiki/Accuracy

Now back to arithmetic precision. A digital guage is more precise measurement wise but not arithmetic wise. Here is why:

For a digital guage in PSI 4 digits are typically used to represent the pressure. However the last digit is garbage. This is, because the transducer used has a precision of +/-0.25% or +/- 12.5psi over a range of 0-5000psi means the best you can is get three significant digits. Now for a 5000 psi analog guage with a precision of +/-1% or +/-50psi there is no need to show the divisions on the guage at more than 100psi (remmber you can read in between the divisions to get 50psi). Which means you can also get three significant digits. Now the accuracy of the last digit for the digital reading is more accurate than the analog but the arithmetic precision is no better.

I agree here, significant digits are very poorly understood. My favorite is unit converstions used by the media. A foreign source estimates the weight of something (sadly these days it's usually a bomb) at say 1000 kg. This is nothing but a guess, based on the size of the crater, and has 1 significant digit. The US press picks this story up, and converts kilos to lbs. 1 kg = ~2.2 lbs. Now they report that the bomb was 2200 lbs., gaining a significant digit via unit conversion!

One minor point regarding digital readouts; they eliminate parallax and interpolation errors.

Tobin

 

[Diver0001]

Stop calculating on average depth and do some tests at a single depth for various levels of activity. Average depth is a WAG anyway.

WAG?

R..

 

[Diver Dennis]

Wild A s s Guess

 

[Peter Guy]

Just to throw in some more confusion -- do any of you know HOW the Suunto engineers attempt to determine the SAC rate? It is clear from TSandM's calculations that the Cobra does NOT calculate the SAC rate by using the stated average depth, gas used and time -- but, in fact, must be doing something else. Is the "average depth" a weighted average (time AND depth) or merely the mean depth regardless of time (for example).

TSandM, being a scientits and all, needs to know these things -- me, being a non-precise type of person just takes what the 'puter gives me and then compares that to my rough guestimate of the world.

 

[TheRedHead]

Just to throw in some more confusion -- do any of you know HOW the Suunto engineers attempt to determine the SAC rate? It is clear from TSandM's calculations that the Cobra does NOT calculate the SAC rate by using the stated average depth, gas used and time -- but, in fact, must be doing something else. Is the "average depth" a weighted average (time AND depth) or merely the mean depth regardless of time (for example).

It's not really SAC because the computer doesn't know the size of the tank either. I don't find it very accurate. The manual lists the assumptions the computer makes in real time. The Dive Manager does calculate SAC. I've hacked inside the Access database and all I can say is those wacky Finns have a sense of humor.

 

[Thalassamania]

Not to complicate further, and knowing nothing about the Cobra, I've seen manufacturer's do wierd things with air integrated computers, weighted averaging being one, but the strangest was having a non-zero intercept so that the computer (or should it be confuser) reported you as out of gas when it thought you should be back on the boat (e.g., 500 psi).

 

[lamont]

Walter, you are, of course, quite right that the same data should give the same answers.

It was interesting, back when I was using the Cobra, to see that the SAC rate I came up with using the average depth was always HIGHER than what the Cobra was calculating -- by quite a bit; I was getting numbers between .4 and .45, the Cobra was often coming up with values below .4. If I use my numbers, I end up with the paradox that I'm using quite a bit less gas than my SAC rate and depth say I should be. If I use the Cobra's numbers, I come up much closer to what is actually happening.

It isn't critical, but it tells me that, if I estimate my gas requirement based on the SAC rate I'm calculating, there will be some generous padding over what's actually required. And for the diving I'm currently doing, that's just fine.

Does the Cobra know how big your tank is? If it thinks you've got an Al80 and you've got a larger water volume tank, it'll read smaller psi drops and think you've got a better SAC...

 

[Peter Guy]

Lamont, what I (and Lynne) wrote is incorrect regarding "the Cobra" calculating the SAC rate. The computed SAC rate comes from the Dive Manager program which combines the Cobra's saved data with the information manually inputted (is that really a word?) regarding the tank size/working pressure. When we wrote about "the Cobra" we were really referring to the Dive Manager report.

 

[TSandM]

It's the logging software that does the calculation, and you enter the tank size and working pressure so it can do it.