Inherent Gauge Error in Analog Gauges?

[talonraid]

I have noticed that my gauges always show about 200 lbs less psi than the pressure gauge at the dive shop. It's not due to a hot tank fill that later cools because I've cross checked it with tanks that have cooled for a few days and with recently filled tanks. The error is the same in both instances.

I assume, but I don't know that this is some sort of safety feature that's just built in to analog gauges, but perhpas it's something that occurs with use and time and can be adjusted or fixed.

If it's a safety feature installed by the manufacturer, I wonder if it can be corrected. I would much rather have my guages read the correct pressure and I will apply the margin for safety, than have the manufacturer pad my safety that I already take into consideration and then the dive boats also pad.

Is this pretty typical of analog gauges or am I off the mark?

[Crimediver]

Your guage is probably off a bit. I have a guage like that which is about 200 psi off as well. My other guages and a tank checker guage are all consistent with each other with the exception of this one USD guage. It shows 200 psi less than what is in the tank so it is a bit conservative. I have no problem using it as I am aware of how it reads. I would toss it if it said I had 200 psi more than what I actually had.

I would check your guage against others and see how it calibrates. I am not aware of any repair that can be made to your SPG so if it troubles you get a new one. I don't see a safety issue in diving it as it gives you a reserve, so to speak.

There is no deliberate error built into the guage by the manufacturer but sometimes guages don't calibrate accurately. Looks like you and I got stuck with one of them.

[ScubaToys Larry]

I'm sure what you'll find is there is a percentage off... not a pressure amount off. For example, if you have 100 psi in the tank, the gauge will not show -100. So if you are 200 off at full, you will probably be about 100 off at 1500, 50 off at 750, 25 off at 375. So really, it becomes relatively insignificant since zero is still zero. But a fluctuation of 100 psi or so at full is not uncommon.

Of course if it bothers you... Aeris Atmos AI Nitrox Computer, Computers: Nitrox, Aeris, Aeris Atmos AI Nitrox Computer

[awap]

What makes you think the shop's gauge is correct?

[cummings66]

That's the real question, is it your gage or the shops gage? I have 3 gages that measure all pretty close, one shop I use is off about 250 psi, consistently, so I never get a full fill there. When 3 of mine say it's 3200 psi and they say it's 3500 psi I will believe 3 vs 1.

[CompuDude]

Yup. Entirely possible for any given gauge to consistently read +/- 250 psi. Seen several threads about this.

Chances are, one gauge or the other is off by a good amount. Some manufacturers may let you replace it, others will tell you it's within the margin of error. (if you think of 4000 psi as the top end, a 5% margin of error means +/- 200 psi.)

[talonraid]

Originally Posted by ScubaToys Larry

I'm sure what you'll find is there is a percentage off... not a pressure amount off. For example, if you have 100 psi in the tank, the gauge will not show -100. So if you are 200 off at full, you will probably be about 100 off at 1500, 50 off at 750, 25 off at 375. So really, it becomes relatively insignificant since zero is still zero. But a fluctuation of 100 psi or so at full is not uncommon.

Of course if it bothers you... Aeris Atmos AI Nitrox Computer, Computers: Nitrox, Aeris, Aeris Atmos AI Nitrox Computer

Interesting. I'll have to check my gauge against a few other gauges with tanks at various pressures. I hadn't really thought about it being a percentage off, but had been thinking it was a constant 200 lbs off. On a side note, and to probably spark some follow on debate....I just can't bring myself to go with an integrated air computer and NOT back it up with analog. I have the Aeris Atmos II wrist computer (from ST) and I back that up with Tusa analog gauges (from ST). So, Larry I saw the thread, but I'm still pretty happy with my Atmos II. I think when I need to replace the computer, I'll get a hoseless wrist mount.....and keep the analog on the HP hose. Call me overly conservative and reluctant if you wish, but I like the redundency.

I have only really paid much attention when I first turn on my tank to dive. The gauge reads about 200 psi less than the shop's analog gauge that is on the refill system and the stand alone tank pressure checker.

I'll also grab a few rental regs next time and cross check them all just to see where I stand.

By the way when are Larry and Joe going to put that new Scuba Shop in San Antonio?

Thanks everyone for the input so far.

[Crimediver]

Originally Posted by cummings66

That's the real question, is it your gage or the shops gage? I have 3 gages that measure all pretty close, one shop I use is off about 250 psi, consistently, so I never get a full fill there. When 3 of mine say it's 3200 psi and they say it's 3500 psi I will believe 3 vs 1.

Kinda like the crooked butcher shop with the rigged scales. Many states have a Bureau of Weights and Measures to make sure consumers aren't getting short-changed at the gas pumps and scales. Might be pretty funny to sic them on the shop and see what happens.

[ChrisA]

There is an old saying:

"A man with a watch knows what time it is. A man with two watches is never sure of the time."

The other week end three of us got out of the water al at the same time and as we were waling up the beach we compared tempatures. One had 49F another had 50F and mine read 51F. Which was right? We all did the same dive.

In order to resolve this you would need a guage that s known to be in calibration. There are labs that can do this or at least can give you a correction sheet that tells you the error at each 100 PSI or so. But do you want to pay for that?

Most scuba gauges are designed such that they are most accurate below 1000 PSI and at the high end they don't worry so much about. So expect a larger, even a larger percentage of error at 3,000 than at 800.

If you see a 200 PSI difference at 700 PSI then you need to worry and figure out which gauge to chuck in the trash or send back for warenty work

[mixahl]

When it comes down to it, would u trust an analog gauge or a digital computer gauge? I have a wrist computer (non-air integration) and dont know whether to get another digital computer (hose) with air integration or just an analog one...