Analog SPG inaccuracy

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i have delt with this in the past..not a halcyon...i tossed it and bought a new one...it was 2 years old with maybe 100 dives on it. i didnt think i could get it fixed...dang!
 
I just had another thought. Considering that B&G spg inaccuracies do occur with a certain frequency, should such "possible" inaccuracies be addressed in Rock Bottom calculations for DIR diving? (Perhaps this is the wrong forum to discuss such an issue.)

Along the same lines, has anyone out there had a B&G spg that read 200psi or higher throughout the entire lower range of the gauge? Obviously, this could be a safety issue.
 
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.
But that doesn't mean the others are accurate, just that they read the same. Calibration does not mean adjustment; it just means applying a known pressure and noting what the gauge reads when that pressure is applied. The resolution on most of the SPGs we use are to the nearest 100 or 250 psi and the user is left to interpolate any space in between. So when you say identical reading I doubt it, they may be close, but not identical. When gauges are calibrated they usually come back with a card so you know when the needle is pointing at a value on the face you know what value it truly represents.

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.

Then you are in that lower 10% where gauges are at their worst accuracy anyway. Despite the appearance the needle does not move linearly the accuracy gets worse the farther you move from the center value of the gauge.

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.
Well I guess that depends on the spec of the guage if it's only spec'd for 5% accuracy it could be as much as 250 psi off and still be good, in the manufactures eyes. Most guages are spec'd to be 10% accurate, but they usually only run a few bucks.
 
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I think it's a good idea to run your tank to zero pressure sometimes (on land, after a dive) to see if the needle hits zero before it's empty and by how much, approximately.
 
But that doesn't mean the others are accurate, just that they read the same. Calibration does not mean adjustment; it just means applying a known pressure and noting what the gauge reads when that pressure is applied. The resolution on most of the SPGs we use are to the nearest 100 or 250 psi and the user is left to interpolate any space in between. So when you say identical reading I doubt it, they may be close, but not identical. When gauges are calibrated they usually come back with a card so you know when the needle is pointing at a value on the face you know what value it truly represents.
You're right. I "misspoke." Only calibration can determine accuracy. I meant to say that, for all of those divers out there who don't have access to proper calibration tools, a quick-and-dirty test to suspect spg inaccuracy would be to compare the
readings of different spgs on the same tank.

Then you are in that lower 10% where gauges are at their worst accuracy anyway. Despite the appearance the needle does not move linearly the accuracy gets worse the farther you move from the center value of the gauge.
From what I know about bourdon tubes, you're correct, the needle doesn't move linearly and accuracy is lowest at both the high and low extremes of measure. However, if a significant amount of air remained after the Halcyon spg went to zero (for argument's sake, let's say 500psi as measured by the other spgs -- still technically within the 10% accuracy that you cite), then I would probably go to the trouble and expense of having the gauge calibrated. Or...heck...I would look into getting another, more accurate spg.

Well I guess that depends on the spec of the guage if it's only spec'd for 5% accuracy it could be as much as 250 psi off and still be good, in the manufactures eyes. Most guages are spec'd to be 10% accurate, but they usually only run a few bucks.
I should have been more specific with my question: What are the accuracy specs for the Halcyon spg in question?

Onfloat, you've piqued my curiosity. What brand analog spgs do you sell at your scuba shop? What manufacturer produces that brand? And what are the accuracy specs on those gauges as reported by the manufacturer? I'm making the distinction between brand and manufacturer with the assumption that the two entities will most likely not be one and the same. Thanks in advance for the info.
 
Electronic pressure transducers have far less accuracy problems they tend to stay in calibration once they are accurately set.

I don't think so... Not only accuracy, but they also have reliability problems. I've seen dives aborted due to electronic gauges in my short diving past.

(For example, we tried several 3 SPG's to compare to my problematic SPG that day and actually of the SPG's was electronic and it was significantly off that the rest. I didn't even mention this in my original posting not to complicate the matters. The owner of the electronic SPG/computer had an additional analog SPG in the same reg setup. He uses his analog SPG since his electronic gauge is off.)
 
I don't think so... Not only accuracy, but they also have reliability problems. I've seen dives aborted due to electronic gauges in my short diving past.

(For example, we tried several 3 SPG's to compare to my problematic SPG that day and actually of the SPG's was electronic and it was significantly off that the rest. I didn't even mention this in my original posting not to complicate the matters. The owner of the electronic SPG/computer had an additional analog SPG in the same reg setup. He uses his analog SPG since his electronic gauge is off.)

You didn't mention if or when it was calibrated or how long between calibration cycles.
Also because of the way they work they can be designed to be linearly accurate across their measuring range.
As with anything quality varies and I'd stay away from cheap junk and a single failure doesn't indicate a general problem. Even with the best of design, quality and testing a individual item that fails may slip through.
 
@Onfloat:
You appear to have some experience with analog spgs. What brand analog spgs do you sell at your scuba shop? What manufacturer produces that brand? And what are the accuracy specs on those gauges as reported by the manufacturer? For the sake of the OP, it might be useful comparison info. Thanks in advance for the info.
 
As usual people do not use the correct terms when discussing instruments:

Accuracy - is the measurement true (checked by calibration)?
Precision - can the measurement be repeated with the same result (checked by calibration)?
Resolution - how fine can the measurement be read (aka arithmetic accuracy)?
Reliability - does the instrument hold up to usage?

All of the these terms are independent of each other.

Example 1) truth is 1000psi, 5 measurements are taken:

1200psi, 1100psi, 1000psi, 900psi, 800psi. The gauge is accurate as the mean value is 1000psi. But poor precision, +/-200psi.

Example 2) truth is 1000psi, 5 measurements are taken:
1075psi 1075psi, 1100psi, 1125psi, 1125psi. The gauge is precise +/-25psi but not accurate as the mean is 1100psi.

Most analog instruments have a precision of +/-0.5% whereas digital have a precision of +/-0.25%. So what does this mean? For a 5000psi gauge this means the reading can be off by 25 psi and 12.5 psi for analog and digital gauges, respectively.

Now the best analog gauge has hash marks every 100psi so the resolution is 50psi. I.e. you can disambiguate whether the needle is on or off the hash mark. So the best you know is 50psi which is more than the precision. For a digital gauge, the opposite is true. You can get as many digits. But the best measurement is based on the precision or 12.5psi.

Now lets talk reliability. I have seen more issues with pressure transducers than with analog devices. However, recently I saw a B&G get banged and end up on the other side of the peg when at zero.

At the end of the day a precise but inaccurate gauge is better because it can be calibrated and the inaccuracy can be known and then adjusted for.
 
You missed one - consistency.

Another issue is whether at a given absolute pressure the gauge will read the same on the way down as on the way up.
 

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