Air integrated vs. SPG, a small study

[TSandM]

As some of you know, my husband and I have been engaged in an ongoing wrangle about air-integrated gauges versus analog SPGs. 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. He felt I was probably several hundred psi off in my measurement of starting and ending pressures. 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.

So we decided to check it out. We measured the pressure in seven different cylinders, ranging from 600 to 3500 psi, with each of the gauges. I read the SPG the way I would normally do during a dive (rounding to the closest 100 psi) and also as accurately as I could do it, with no rounding. The air-integrated unit was my husband's Suunto Cobra.

The results surprised both of us. The furthest apart the two measurements were was 70 psi, and that was on one of my rounding readings (on that tank, the "best" reading was only 19 psi off the Cobra). The closest I got was 6 psi.

Although this tells us nothing about which gauge is more ACCURATE, it does tell us that the results of using those gauges are quite consistent, and the issue I was having with the original problem is most likely NOT due to my inability to read my gauge accurately.

Thought this was interesting enough to share.

 

[Peter Guy]

So since the gauges are both accurate (at least compared to each other) that just leaves the error factor to be in her head!

 

[Diver0001]

Why the need for all the accuracy?

R..
P.S. BTW, you said "His feeling was that the analog gauge would be much better" but I think you meant to say that Peter thinks digital is better.

 

[Packhorse]

19psi diffrence is nothing. at 3000psi it is 0.63 %. While at 500psi its 3.8%

 

[cool_hardware52]

As some of you know, my husband and I have been engaged in an ongoing wrangle about air-integrated gauges versus analog SPGs. 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. He felt I was probably several hundred psi off in my measurement of starting and ending pressures. 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.

So we decided to check it out. We measured the pressure in seven different cylinders, ranging from 600 to 3500 psi, with each of the gauges. I read the SPG the way I would normally do during a dive (rounding to the closest 100 psi) and also as accurately as I could do it, with no rounding. The air-integrated unit was my husband's Suunto Cobra.

The results surprised both of us. The furthest apart the two measurements were was 70 psi, and that was on one of my rounding readings (on that tank, the "best" reading was only 19 psi off the Cobra). The closest I got was 6 psi.

Although this tells us nothing about which gauge is more ACCURATE, it does tell us that the results of using those gauges are quite consistent, and the issue I was having with the original problem is most likely NOT due to my inability to read my gauge accurately.

Thought this was interesting enough to share.

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

 

[The Kraken]

There's probably not any great degree of need for accuracy, but it does provide one with some psychological reassurance.

Knowing that one is interpreting a gauge reading to an acceptable resolution is, in and of itself, a factor in one making a more relaxed and competent dive.

Nice test, TS&M.

the K

 

[TSandM]

Yes, Rob, you're right, I misspoke. Peter thought the DIGITAL gauge would be better.

There is no need to be that accurate, and that's been one of my arguments all along.

What I have been doing is trying to figure out whether I can reliably get two dives out of the doubled 72s I'm diving, keeping an appropriate reserve. If I look at my usual gas usage out of my 95's, I come up with about 47 cu ft (rounding everything up). That should easily give me two dives out of the 72s. But when I look at my usual SAC rate and average depth on my usual dives, I don't come up with 47 cu ft out of the 95s. So something's out of whack, but I haven't figured out which of the numbers I'm using (none of which is very accurate) is the source of the error.

The SPG accuracy was one possible culprit, hence the test.

 

[1_T_Submariner]

I'll give some background from the Industry that I work in.

Gauge accuracy is usually expressed in % of full scale.

For the price of an SPG they are most likely 1% gauges so a 5000 PSIG spg would be designed to be accurate to 50 psi. (My guess and your experiment would indicate they are better than this, however over time the Bourdon tube bends a little etc...)

The lowest accuarcy that an electronic pressure transducer comes in is (For industry NOT Scuba) is .25% of FS. So a 5000 psi transducer should be accurate to 12.5 psi.

Anyway I'll quit rambling. Even though the electronic pressure transducers are probably more accurate there are more things to go wrong so i use an SPG.

 

[bperrybap]

I would think tank-air temperature change could be a much bigger
factor than gauge accuracy for your SAC calculations,
especially for you folks that dive the northwest where there
can be a large difference between air and water temperature.

Using charles law and solving down a bit:

p2 = p1 x T2/T1

And since temperatures are in Rankin (F + 460)
That means that each degree F alters your gauage pressure
reading by 1/460 or .217% or around 6 or more PSI for
a tank with 3000 PSI.

And since it looks like you guys up northwest can have 30-40
degrees more between air temperature (initial tank-air temp) and
water temperature in the summer,
it would seem that this becomes a significant factor.

With a 40 temp change, the 3000 PSI you started with is
around 8% lower or only 2760 or that 95 you started with
was really only an 87.

--- bill

 

[cool_hardware52]

I'll give some background from the Industry that I work in.

Gauge accuracy is usually expressed in % of full scale.

For the price of an SPG they are most likely 1% gauges so a 5000 PSIG spg would be designed to be accurate to 50 psi. (My guess and your experiment would indicate they are better than this, however over time the Bourdon tube bends a little etc...)

The lowest accuarcy that an electronic pressure transducer comes in is (For industry NOT Scuba) is .25% of FS. So a 5000 psi transducer should be accurate to 12.5 psi.

Anyway I'll quit rambling. Even though the electronic pressure transducers are probably more accurate there are more things to go wrong so i use an SPG.

1_T,

Cool info. The .25% FS for transducers, is that just the strain guage, and does it include any correction for temp of other effects? (I'm assuming a whetstone bridge SG, but maybe that's old news)

The digital fill whip guages (at least some of the one's I've used) seem to be a little flaky with a much less repeatability.

Tobin