Air integrated vs. SPG, a small study

[Charlie99]

As some of you know, my husband and I have been engaged in an ongoing wrangle about air-integrated gauges versus analog SPGs. ...... His feeling was that the analog gauge would be much better, because you directly read the numbers, instead of having to interpret the position of a needle.

Depends on what you are trying to do with the reading.

Kind of like my automobile gas gauge. If I were trying to accurately measure miles per gallon over a short span, then I'd want a high resolution (not necessarily high accuracy) gauge. OTOH, for just avoiding running out of gas, I'm happy with an occassional glance at the intuitive analog gauge.

Digital gauges tend to have higher resolution, and may or may not have higher accuracy. Analog gauges are better as a quick, intuitive display that can be understood without conscious thought.

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A couple of years ago down in the Keys my instabuddy was a guy from UK that had a horrible time trying to understand what his imperial rental gauge was telling him. What finally go through to him was "by the time it is in the yellow, we should be headed back. By the time it's in the red, we should be on the boat, or at least at safety stop".

 

[Peter Guy]

OK, just to defend myself a tad here. The original problem attempting to be solved was, Why can't my mathematician wife come up with the same answer to a math problem -- the issue over her SAC rate and how much time she'd have in the double 72's. I made the suggestion that she wasn't accurately "recording" her beginning and ending pressures -- and if the underlying numbers are off, the calucations are certainly going to be off (GIGO). I then made the obvious mistake of saying, "That's one of the really nice things about the air integrated computer -- the data is correct." (Geesh, you'd a thunk I'd accused her of something horrible by suggesting, perhaps, just maybe, she'd not remember the right numbers?)

In fact, I was surmising that the temperature factor was the big culprit in her calculations -- but NO, she said, I'm always precise and I know what I'm doing. WELL, I said, if your beginning and ending numbers are correct, then maybe the problem is in how your are reading your gauge? Could there be a problem with that? If so, let's test it -- and we did.

Me, I'm a fan of digital readouts over analog readouts -- MPH, Temp, Heading, whatever. (Maybe it is a rebellion against having to teach kids math using colored sticks so that they learned that red (2) + blue (3) = purple (5) and what they learned was red + blue = purple -- NOT that 2 + 3 =5 -- but that is another story from a period and place long, long ago.)

 

[The Kraken]

Spouse Fight !!!! Spouse Fight !!!!! Spouse Fight !!!!!!



. . . just kidding! You should hear the Yellow Angel Fish and me getting into it over her intergrated alternate air source and my bungied alternate. . .

the K

 

[Walter]

As I was chewing on a gas consumption issue that was coming out differently depending on how I went to solve it, my husband opined that the real problem was that I couldn't accurately read my SPG.

Why would that give you different answers? While a more precise reading will give you a more precise result, a lack of precision in your readings would not cause you to come up with different answers. Multiplying 500 by 2 will result in 1000 every time. While more precise data (502 x 2 = 1004) may or may not be desirable, you shouldn't be getting different answers as long as you are working with the same data.

 

[Scared Silly]

The other question is that though accurate were the readings precise - as in were the measurements repeatable? Which is more imporatant than number of digits used. (Sorry just the scientist in me mucking up the waters).

 

[Walter]

Digital reading are much more precise. There's no guarantee they'll be any more (or even as) accurate, but they are certainly precise. I believe you've confused precision and accuracy.

 

[TSandM]

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.

 

[Walter]

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

 

[Scared Silly]

Digital reading are much more precise. There's no guarantee they'll be any more (or even as) accurate, but they are certainly precise. I believe you've confused precision and accuracy.

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.

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.

Confused? Most people are and I have seem many many discussions on this subject.

 

[TSandM]

Please note that my original post did say that our test gave us no information about which gauge was more accurate. Further, since we did not do repeat evaluations of either gauge, we can say nothing about precision (although I would be shocked if the digital gauge were not more precise). We can only say something about the consistency between the two measurements, which was surprisingly good, and definitely augmented my ammunition