Analog SPG inaccuracy

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tceylan

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Location
san diego, ca, usa
# of dives
100 - 199
Just wondering if this is common. I have a pretty new expensive steel analog SPG which seems to show 200 psi higher than the pressure (3900psi) compared to 2 other (way cheaper)
SPG's... (3700psi)

Halcyon to be specific. One of my lds told me that this is common, where the other told me that
this rarely happens especially with Halcyon.

Not a big deal, since I'm asssuming this accuracy will get smaller as the tank pressure drops?
Also I know that the last 200-300 psi is considered unreliable in SPG's anyways...

Any comments?
 
One of my dive buddies had the same problem with his Halcyon spg. Oh, wait. That was you!

Does anyone know if it is possible to re-calibrate the spg? If so, how much does this service cost? Or would it be better to wait for the dive fairy to leave a new spg under your pillow?
 
You'll never know if it's inaccurate unless you have it calibrated. All of the gauges we use in my work have to be calibrated straight out of the box some are good, some aren't. Usually most gauges are considered unreliable at the lower 10% and upper 10% of the guage. Strangely enough on a 5000 psi guage the lower 10% is 500 psi.:coffee:
 
You'll never know if it's inaccurate unless you have it calibrated.
Actually, you don't need to calibrate the spg to determine inaccuracy.

What is the acceptable tolerance for a new analog spg out-of-the-box? +/- 100psi?
 
Actually, you don't need to calibrate the spg to determine inaccuracy.

What is the acceptable tolerance for a new analog spg out-of-the-box? +/- 100psi?

So how do you propose to know it's accurate? It may be close to another gauge that's currently being used, but that may be off too. Just because you can duplicate another reading doesn't mean it's accurate.

As to your second question, that's up to the user and the risk your willing to take.
 
I don't know who makes Halcyon's SPGs, but I suspect they're made in Italy like most others. Probably all in the same factory. I'm afraid that like so much dive gear these days the quality is generally poor. In my dive center I have to throw away two or three SPGs a year because they've become too inaccurate. Even so, I rarely throw away an SPG with the original hose on it - I find most hoses don't last more than a few months. When I started diving I NEVER had to replace a hose, yet on my personal gear I'm just about to replace the third hose this year.

A common fault with an SPG is that it reads too high, and still reads positive even when not on a tank. I don't think there's much doubt that that's wrong.
 
So how do you propose to know it's accurate? It may be close to another gauge that's currently being used, but that may be off too. Just because you can duplicate another reading doesn't mean it's accurate.

I think you have to think outside the reg repair box. :)

I don't mean to be glib, but let's say that you happen to be diving with a bunch of other people. Every diver has an analog spg on his reg setup -- all various brands. You attach each of their regs to the same tank. All of the spgs share an identical reading. Then you hook up your new Halcyon spg and it reads 200-300psi lower. I guess you could make the case that all of the other spgs were wrong...but the odds of that being the case are quite low (at least lower than one spg being inaccurate). I'll concede that if you only have one other spg to compare it to, no conclusion can be made. But what happens if there are 2 spgs...or 3...or 4...or 5 to compare to the Halcyon spg? By attaching multiple spgs to the same tank and consistently getting a different pressure, it seems to me that you could make a pretty strong case that the Halcyon spg is inaccurate.

I will also concede that another troubleshooting technique would be to detach the Halcyon spg from the OP's first stage...and then reinstall it on another first stage known to be in good working condition (preferably one used in the previous set of tests). This would address the possibility that there's a problem with the HP port of the OP's first stage, although I'm not sure if this is a common occurrence.

One could also bleed the tank down until the gauge read zero. If any air remained, then the troublesome gauge is giving inaccurate readings.

As to your second question, that's up to the user and the risk your willing to take.

I must not have stated my question clearly enough. My question was not referring to how much "risk tolerance" is involved in using faulty/inaccurate equipment. I was referring to allowable "specs" for a measuring device...you know, the +/- number. This is commonly referred to as the tolerance of the device. I apologize if I wasn't clear about that. The reason I asked is that if the spg is not "within spec" then it's possible that Halcyon might replace it as a defective item.
 
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I left the whole reg to my lds today and they will measure the accuracy of the SPG, but they said there's nothing else they can do.

The SPG reads zero when no tank is connected...

Is there a way to calibrate a halcyon analog spg? Neither of the lds's mentioned this to me today...

Should I get it replaced?
 
I have seen the Halycon spg and it looks identical to the one I have - same SPG made by the same company with different graphics. There is no magic status bestowed by the H.

Generally speaking mechanical bourdon tube SPG's are calibrated in the middle of the range. Those that show errors greater than zero when not connected ever make it out the door (although more than one brand uses a zero line that is more like a large black zero range, so that is not really the case as you can't really determine where zero is).

Gauges that may read 200-300 psi low at the upper end of the range are fairly common nad since the error is on the conservative side I don't get concerned. Oddly, people will complain less about a gauge that reads high - I suspect because it makes them think they have a great fill even if they don't and it makes it easier to accept the error - even if it causes gas planning problems for them.

To be fair to the SPG cmpare it to the other 2 at pressures in the 1200-1700 psi range as well as in the 500-700 psi range. What is really important is the middle of the range as that is where you determine when to turn the dive.
 
Seems everyone praises the reliability of B&G spg over computers but no one discusses the fact that accuracy of B&G varies greatly and can change seemingly without reason.
It's nice to see a thread discussing the calibration and scale inaccuracy.

One thing I'd like to add for AI computers users strictly from a electronics perspective.
Electronic pressure transducers have far less accuracy problems they tend to stay in calibration once they are accurately set. When they fail is is more often a catastrophic failure that is obvious. Most modern dive computers will only have problems due to battery issues. (low battery or water seeping into the battery compartment after a battery change)
If we were to compare computer failures against B&G gauges counting inaccuracies of more than 100 psi as failures I think the failure rate of the B&G gauge would well outweigh failures on current model AI computers.

Sorry for the hijack
 
https://www.shearwater.com/products/swift/

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