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mksmith713
02-25-2009, 09:42
Well? Do you?
I was reading responses on a thread I started awhile ago and SPG accuracy came up in discussion.
That got me to wondering.... so I had to check mine out....just for curiosity sake.
I have 5 set of regs that my family uses.
I hooked them up to a tank and here's what I got.

Newer ScubaPro
(not new, not vintage) 2850 psi

Old brass & glass ScubaPro- 2500 psi

Old plastic Dacor- 2500 psi

Mares (black & Yellow face)- 3000 psi

Two Old brass & glass Cressi's- 3000 & 2950 psi


My question is......which one is closet to actually pressure?
Hell, lives depend on these guages.

bassplayer
02-25-2009, 09:48
wow, I would not have expected a 500 psi difference

robjoubert
02-25-2009, 09:57
When I took my first & second stages & octo in for their annual service, I left the SPG attached to the first stage. When I got it back, included was a comparison report of the pressure the SPG indicated and what the actual pressure was. As I had only bought my regs etc a year ago, this is the first service interval & I thought that the SPG comparison was something that is usually done by the service tech. Am I wrong?

mksmith713
02-25-2009, 10:10
Also, is there a way to calibrate SPG's to make them accurate?

Straegen
02-25-2009, 10:19
I would guess the higher numbers more accurate. The "good" news is that it is probably a linear display so while the actual number may be off the progression is probably very close. So most likely the 3000 will read 1500 at half full and the 2500 will 1250. At 1/3 you would have one reading 1000 and the other reading 830. Not great news, but still means the gauges are probably safe to use. All that said, I would want my gauge bench tested and I would buy a new one if it was lying to me by 500.

bubbletrubble
02-25-2009, 10:27
My question is......which one is closet to actually pressure?
Hell, lives depend on these guages.

I'm not surprised by the divergent SPG readings. I would not even hazard a guess as to which SPG is giving an accurate reading. Anyone who would tell you otherwise...well, I would probably discount anything he/she had to say. :D

If you would like to know the accuracy of the readings, you'll need to calibrate your SPGs. Not just one...but all of them.

Some points to ponder:
The accuracy of an SPG may vary throughout the measurement range, e.g., more accurate in the middle of range than at either extreme.
The tolerance of SPGs can be as high as +/- 10% of the range. If the range is 0 - 5000 psi, 10% is 500 psi. :-)
Accuracy and precision are distinct characteristics. A good instrument is both accurate and precise.
Many SPGs originate from one (or a couple) factories in Europe. They are simply "re-badged" with different company logos.I would consider the above points before ditching my current gear and buying new SPGs from another company.

Even with an accurately calibrated instrument, it's good practice to regard SPG readings with a certain degree of "suspicion." :smiley2: Personally, with my typical conservative diving, I'd be happy to use any SPG that: (1) reads zero when unpressurized and (2) gives a consistent reading in the middle of the measurement range. That's just me.

Check your SPG before, during, and after a dive. Ensure that the gauge reads zero when the reg setup is unpressurized. Pay particular attention to it during the dive. Have fun and be safe.

acamato
02-25-2009, 10:42
Newer ScubaPro
(not new, not vintage) 2850 psi

Old brass & glass ScubaPro- 2500 psi

Old plastic Dacor- 2500 psi

Mares (black & Yellow face)- 3000 psi

Two Old brass & glass Cressi's- 3000 & 2950 psi

4 gauges (ScubaPro, Mares, (2) Cressi's) are within 150 lbs of each other. I would look at the problem being with the Old ScubaPro and the Dacor.


Accuracy is defined as the conformity of an indication to its true value. Accuracy is a percentage of the full range. For example, a gauge that has a scale of 0-3000 psi with an accuracy of 1% would mean that the gauge is accurate to within (plus or minus) 30 psi.

Even cheap commercial grade gauges have a 3% of span accuracy (ASME B40.1 Grade B).

ianr33
02-25-2009, 11:34
I would expect to get a range of readings. Its only potentially dangerous if a gauge reads say 500 when its really at zero. Thats easy to check by looking at the reading when not on a tank.

mksmith713
02-25-2009, 13:56
When not on the tank, all of the guages read "ZERO".

CompuDude
02-25-2009, 14:20
When not on the tank, all of the guages read "ZERO".

Is that because the needle is "pinned" or because that's where it rests, though?

Best is to actually take a tank to zero and be sure. Watch the needle closely for the last 800 psi or so and make sure it progresses at the rate expected, and stops where expected. As as long this works, you should be good.

bubbletrubble
02-25-2009, 15:02
Also, is there a way to calibrate SPG's to make them accurate?
Your LDS repair shop will not be able to do this calibration, although they may be able to state how accurate and precise your SPG is. Technically, the factory that manufactured the SPG could carry out this calibration. Considering the cost of labor, the factory would just junk your SPG and give you a new one.
Practically speaking, if you're uncomfortable with the accuracy of your SPG, you should just buy another one.

bubbletrubble
02-25-2009, 15:10
Accuracy is defined as the conformity of an indication to its true value. Accuracy is a percentage of the full range. For example, a gauge that has a scale of 0-3000 psi with an accuracy of 1% would mean that the gauge is accurate to within (plus or minus) 30 psi.


@acamato: Using your example with gauge accuracy stated as +/- 30 psi, the accuracy of extremely low pressure readings will be limited by the resolution of the "needle" scale being used. Many brass-and-glass SPGs only have four hash marks below 500 psi. My Suunto-brand SPG doesn't have any hash marks below 500 psi.

BouzoukiJoe A.K.A. wrecker130 AKA Chuck Norris AKA joeforbroke (banned)
02-25-2009, 15:34
Your life does *not* depend on the SPG reading, it depends on having breathing gas while under water.

It's documented that people have gon OOA with their SPG reading just fine and it's well known that people dove and some still dive without SPGs

You might consider learning how long you can dive on a full tank at different depths and treat your SPG with skepticism if you've been down a long time and it's still reading high.

Here's what I want in an SPG: Show when I have a full tank and when I have an empty tank and be reasonably linear (i.e. proportional) in between. If it does that I don't care whether it reads pressure in psi, bars, smoots per acre, or ears per square nose.