I wish my pressure gauge didn't lie to me...

[charlesml3]

Hi gang,

I took my reg in for an annual and while we were standing there, I asked the guy to connect it up to a known pressure because I thought my computer was reading low. Before he even connected it, he said "Yep, it's going to read 150 pounds or so low." Turns out, it's reading 220 pounds low at 3000, 2000, 1000 and 700 pounds of tank pressure.

Now before someone jumps all over me, I KNOW. It's a "safety thing" that the manufacturer does on purpose. I just find it a bit annoying. I'd rather have ACCURATE information. I wonder if my depth gauge reads deeper than actual as another safety measure

Besides, now that I know it's reading 220 low, I'm just going to figure that offset into my turn pressure.

-Charles

 

[Puffer Fish]

If you want an accurate gauge.... you need to get equipment that costs a lot more... oh and whatever you do, don't offset anything..

What do you think "accurate" means? And how do you know the the gauge that was different was "accurate".

I work with hundreds and hundreds of gauges, and it is impossible to tell a gauge is correct by comparing it with another gauge.

If, and only if, you have a calibration gauge, recently tested, then I might.

Please keep in mind that there are different definitions for gauge accuracy... a 5% accurate gauge (non-electronic) means the reading is within 5% within the middle third of the gauge. If you have a 4,000 psi gauge, 5% = +/-200 psi, within the middle third.

I doubt there are any gauges being used that are more accurate than that (if you are carrying it around), and most dive shops don't calibrate their gauges annually, so who knows where they are.

Digital gauges do offer the potential for better accuracy, but who knows where they really are and how accurate they are...

If you want to know how accurate a gauge is.. have it really tested.

I doubt there are any

 

[charlesml3]

Ah yes. I forgot one of the cardinal sins of Scubaboard. I failed to mention that the gauge was on the bench at the dive shop and it was certified and tested to be accurate.

This reminds me of that hotel scene in "My Cousin Vinney" by the way when Marissa Tomei was describing the torque wrench used to tighten the spigot...

-Charles

 

[Puffer Fish]

Ah yes. I forgot one of the cardinal sins of Scubaboard. I failed to mention that the gauge was on the bench at the dive shop and it was certified and tested to be accurate.

This reminds me of that hotel scene in "My Cousin Vinney" by the way when Marissa Tomei was describing the torque wrench used to tighten the spigot...

-Charles

Testing and certified does not tell one how accurate a guage is... if the gauge you were looking at was say 12" in diameter, and had a mirror surface, and had a testing tag... then you would have an accurate gauge...but even those are "only" accurate to 1/2% (20 psi on a 4,000 psi scale). And then only in the middle 1/3 of the dial. Standard industrial gauges are usually 2 to 5% accurate... so a calibrated 5% gauge, can have a calibration tag and still be off by 200 psi. Please read the ASTM standards for gauges... and you will get a better understanding of the fact that for all practical purposes, all gauges will be off, including the ones in shops.

 

[Otter]

Its funny how when we see 2378 psi, we assume its (1) more accurate and (2) more precise than an analog gauge. Depending on the tolerances of the pressure transducer, there can be quite a swing of measurements.

 

[Puffer Fish]

Its funny how when we see 2378 psi, we assume its (1) more accurate and (2) more precise than an analog gauge. Depending on the tolerances of the pressure transducer, there can be quite a swing of measurements.

Thank you Otter.. it is somewhat difficult to explain the difference between display resolution and accuracy. There are now some very, very accurate, full scale digital gauges out there, but I have yet to see one at a scuba shop... mostly because of cost...nor do I see any scuba mfg using extremely high cost, high accuracy technology, again because of cost.

 

[Garrobo]

Good rule, no matter what your guage says: If your reg starts sucking hard, go to the surface.

 

[Meng_Tze]

It is not that we are working in a train of scientific testing here. There are so many roundings, errors, tolerances etc, etc.

1BAR is not 1 ATM, 21% compressed air is only 21% on the analyzer that verified/filled it, your depth is only your depth as far as the gauge tells you, and you have the surface moving on top of that..... Hmm am I at 80ft, or exactly at 79.32656837857835875ft?


What gives? Are you sure your tank holds 77.4cft? maybe it holds 77.2cft since the metal weight used was not accurately measured?

:shakehead

 

[Melicertes]

Good rule, no matter what your guage says: If your reg starts sucking hard, go to the surface.

If he's using a balanced first stage it might not be so easy to tell. Better rule: better manage your gas usage/planning so a 220psi reading swing either way will not put you into a situation to worry about.

 

[Temple of Doom]

As for having a margin of error in a gauge (be it analog or digital), it's absolutely important.

There will always be a margin of error in the measurement. There will also always be potential drift in calibration. Ultimately if your SPG can measure 500psi with +/- 10 psi accuracy when it's new, but when it's 10 years old, it's possible some of the units built can then only measure 500psi with +/- 100 accuracy, then you better be damn sure that nothing leaves your factory that can't ever read 100psi when there's 0 in the tank. You have to create a safety margin to prevent the absolute no-no of an air pressure gauge: over-reporting. That means that every unit has to be tuned to under-report pressure when they're built.

Craig