Question What is your tolerance for error in your SPG?

[MacDuyver]

I suspect my SPG is bad. I got a tank this morning and the tech showed me the 3k reading on her tank gauge at the shop. When I set up, my SPG was showing about 2500. This has been pretty consistent with what I thought were just light fills from this shop.

So I’m taking it into another place that has a full on calibration setup to get it checked out. Expecting to be replacing it here by the end of the week if he confirms that’s it’s 500psi off.

This raises the question, what’s an acceptable margin for error with an SPG?

 

[davehicks]

Probably less that 50 psi. Anymore than that and you really don't know how far from empty you really are.

 

[CptTightPants21]

She showed you 3K on a digit gauge just after she filled the tank or hours after it was filled and as you were picking the tank up from the shop?

Get a 2nd regulator from a friend or dive buddy and see what their spg reads.

The tolerance from manufacturer is 10%. I would question anything above 200 psi when analyzed at the same time and know that one of those gauges is off.

When your regulator is depresurized, is the needle at 0 or somewhere above/below?

 

[BoundForElsewhere]

I would say about 300psi. or whatever your comfortable surfacing with.

 

[phoque]

Well, I had a gauge which gave me under readings of about 150-200 psi on full tanks, and eventually replaced it, but more because it annoyed me a bit rather than presenting an actual risk of any sort. As outlined above, the potentially serious issue it may bring would be over reading at low pressures, however in your case the gauge reads lower, so would be annoying in the sense that it may lead you to end dives unnecessarily early, but otherwise does not present any risk of causing you to unknowingly breathe your tank empty. Also, I suspect as was the case for me that this may be a stiffness defect in the mechanism, leading to a percentage error rather than a fixed gap. My gauge was accurate at low pressures, with a 5-6% under reading across the whole range. In your case, the gap is quite large at~17% at 3000 psi, but I suspect it would read ~830psi on a 1000psi tank rather than 500psi.
Personally, I chose to replace it just because I'm anal, but I didn't really need to. I always knew how much gas I had left. In any case, these things aren't really precise, especially the smaller stage-size gauges (1.5" diameter). A 5% error is pretty much within the acceptable range.

 

[scubadada]

My SPG is 75-100 psi higher than the reading on my Oceanic VT3 and Shearwater Teric, which are the same. I have always thought this is fine.

 

[Lorenzoid]

When doing gas calculations based on an SPG reading, we generally round in the conservative direction to the nearest 100 psi, the reasoning being that we should not expect a mechanical SPG to be more accurate than about 100 psi. I have heard others say that electronic transducers maintain better accuracy than mechanical SPGs. Still, for consistency, I would answer 100 psi to your question.

And that is my 2 psi on the subject.

 

[arew+4]

A couple hundred psi is fine as long as it's stable, preferably just a bias (offset), or close to linear.

 

[Wibble]

SPGs do fail.

But the temperature of the tank will massively affect the pressure. If it was filled to 250 Bar and left to cool it'll be maybe 220 bar, or less if it was really hot.

Get yourself a standalone gauge to keep at home. Then you've one you know works.

 

[Bigbella]

With the increasing dominance of electronics, we've become far too "granular" with our conventional gauges, where it's no longer a satisfactory 600 psi but must register 617 psi.

For decades, that had seldom been an issue -- and my former business partner dove without SPGs for years, when he was a kid. They were then expensive, a real rarity. and his regulator model, at the time, couldn't even accommodate one.

My SPGs all jibe with my electronics, so far as I can gather; but the resolution on even my finest gauges cannot parse out a 617 psi reading -- not to mention the button gauges on ponies, which only register pressure through a series of 25 bar (366 psi) hashmarks, from 0 to 360 bar (5200 psi and change).

I felt much the same way about my IP gauges, since I had always used analogue gauges on the bench, when working on regulators. Some newer digital counterparts allow for greater granularity, I suppose; but should I be just as satisfied with an 8.55 or even 8.6 bar, when I was aiming for 8.5?

What is your tolerance for error in your SPG?

That the conventional equipment at least jibes with others, say, another SPG or a standalone pressure gauge -- analogue or digital, especially at the lower end . . .