Question: Why can't you calibrate SPG?

[cocoajoe]

OK, so I take my reg in to get serviced today. This isn't the shop who I've used in the past...they're slammed & I didn't want to wait for 2 weeks. But I've been in there and bought stuff ( new mask, wetsiut top, spool, yadda yadda) before. And they've always come across as professional.

Today when I walked in the door, there's this kid who I'd never seen before and clearly new. So, I ask who's gonna be the one to service my stuff? About that time the owner comes out of the back and says I am. I'm relieved.

Making small talk while the owner is helping this kid with paperwork, I say I've always heard you can't calibrate a SPG, why? He says, yeah you can..buy a new SPG. I go huh? What makes you think a new SPG is correct? The following is why I should have not said anything...LOL

He says, if you come up with 500# you won't have to worry about it. I'm already regretting bringing it up. But it's too late. I go, are they still teaching that in class? Big mistake, the guy's clearly not happy with me challenging the old 500# rule. He says, get this...I suspend guys for the next dive who come up with less than 500#

I go on to say, I go by dive conditions and depth. At the bridge (BHB W Palm where it's 20' max and usually 8-10' depth) I've gone down to 200# several times. 35-40' reef dives, I'll start up with 5-600#. 80' I'll sart up with 8-900#. 100' dive, my rule is 1000# no matter what. (please no responses about S.A.C. & dive plans.....I know how how to do it and will use it at times)

He agrees about the 1000# at 100' and I just want to get outta there by then. But here's the question: Why is it not possible to calibrate a SPG? OK, I understand you might not be able to calibrate guages as they are constructed now. But why couldn't a SPG be designed so that it could be calibrated? My work history has included testing pipe & boilers. Hyro test reports will not be signed off if guage calibration dates are not correct. I have witnessed techs calibrating guages in the past. So, why can't this be done for SPGs?

 

[oly5050user]

they usually not designed to be opened..Not a good idea to plan on coming back to beach or boat with 200psi..lets say the gauge is off by something close to 200 psi..you may read it as "hey i still have 250psi when in reality you are on fumes at 50psi..analog gauges are not so accurate and as they age they can get less and less accurate..

 

[JohnN]

Accuracy of the gauge aside, the arbitrary rule of 500 psi seems pretty silly to me.

For example on an AL80:
500/3000*77 cu ft = 12.8 cu ft of gas remaining

I dive steel HP120s:
500/3442*120 = 17.4 cu ft of gas

If you were diving double HP100's:
500/3442*200 = 29 cu ft of gas

 

[lowviz]

1) Do you want "calibrated" or do you want to just believe what your SPG is telling you?

I care about my IP (cold water diver) so I paid for an Ashcroft gauge. See pic, note where zero is.
I am conservative with gas so I just want to know if my SPG is pretty much working. Bought a cheap welding gauge and a really useful cylinder pressure checker with pressure release. If my SPG and the welding gauge agree, chances are that neither is "bad". I'm willing to take that risk. (Note that my cylinder is DIN, they come in yoke also. Useful tool.)

So: consider spending the yearly calibration expense on a pair of really useful gauges...

 

[spectrum]

Accuracy of the gauge aside, the arbitrary rule of 500 psi seems pretty silly to me.

For example on an AL80:
500/3000*77 cu ft = 12.8 cu ft of gas remaining

I dive steel HP120s:
500/3442*120 = 17.4 cu ft of gas

If you were diving double HP100's:
500/3442*200 = 29 cu ft of gas

Nobody with an appreciation of gas planning will argue with you. It's a simple no brainer way to manage resort charters when the playing field has been leveled with AL80 cylinders. After that is begns to fall apart.

However... Since calibration or even verification againinst a true standard is a rarity it would be folly for most to plan on ending with 250 PSI because they have twin 80's as an example. Yes, in theory they have the same volume but you can't breathe theory.

Pete

 

[Bob DBF]

It's cheaper to replace the pressure gauge than to build one that calibrates and the pay someone to calibrate it and hope it dosen't flood afterwords.

As one who has been diving from before they were available, all the ones I have had worked well. Just watch for sticking and that the low end is close. I have a surface pressure gauge that can use to check pressures against the SPG's.



Bob
--------------------
I may be old, but I'm not dead yet.

 

[FM1520]

SPGs can be calibrated, any gauge can be calibrated. Depending on the gauge if it fails it may be disposed of instead of repaired. In the Navy, on ships down in the plant, all the gauges have calibration stickers and get calibrated on some scheduled. However, in Naval Aviation the gauges in the aircraft are not calibrated but the support equipment that is used to maintain the aircraft are calibrated. The application and the user determines whether calibration is required.

 

[flots am]

Why is it not possible to calibrate a SPG? OK, I understand you might not be able to calibrate guages as they are constructed now. But why couldn't a SPG be designed so that it could be calibrated? My work history has included testing pipe & boilers. Hyro test reports will not be signed off if guage calibration dates are not correct. I have witnessed techs calibrating guages in the past. So, why can't this be done for SPGs?

Because most new SPGs are throw-away items and have no calibration screw.

If you have a gauge that can be calibrated, then it can be. It depends on what you own.

OTOH, if it needs calibration, it's probably actually worn or damaged and should be tossed anyway.

flots.

 

[dfx]

they usually not designed to be opened..Not a good idea to plan on coming back to beach or boat with 200psi..lets say the gauge is off by something close to 200 psi..you may read it as "hey i still have 250psi when in reality you are on fumes at 50psi..analog gauges are not so accurate and as they age they can get less and less accurate..

I don't buy that. If the pressure is zero, then the SPG will read zero. Very easy to verify. If it doesn't read zero with the tank turned off, then you know it's bad. Ok. So with the SPG reading zero at zero pressure, why would it jump up to 250 with an only 50 psi increase in pressure? While still reading 3000 at 3000 psi? That would be seriously off. I'm not saying that it's technically impossible, but the realistically plausible chance of this happening gotta be close to zero. It's much more likely that the inaccuracy goes the other way, i.e. the SPG reading zero when there's still some pressure left.

 

[lowviz]

Doesn't work that way. Zero is a mechanical stop on cheap gauges. See second gauge in post #4.