Testing an SPG

Soakedlontra · Jun 26, 2012

Is there a way to test a submersible pressure gauge? My regular buddy thinks that his SPG is not giving him a correct reading. During a deep dive it looked like it was off of 300psi and I am wondering how we can test it to make sure that there is nothing wrong with it.

CT Sean · Jun 26, 2012

Compare it with another SPG? As in check tank pressure with questionable SPG (presumably attached to a reg set) , remove reg set, check pressure again with another set

You can probably also check it at your LDS. I'm sure they'd confirm the pressure after filling the tank, then you check it with your reg set and see if it matches - not a bad idea to check tank pressure after you get them filled anyway.

Maybe do both - then you have 3 separate readings.

chrpai · Jun 26, 2012

Reminds me of the SPGs I've seen in Jamaica. I start doubting it when it reads the same amount for awhile. Smack it with the palm of my hand and suddenly they drop 300 - 500 psi. Time for service...

RonFrank · Jun 26, 2012

There is no service on an SPG. It is either working or it is not but it can not be calibrated. Your buddy can dive it to zero knowing that it reads low. If that makes him nervous buy a new SPG.

chrpai · Jun 26, 2012

Service to me is a generic word for repair or replace. I didn't mean to imply that you could repair it.

Scared Silly · Jun 27, 2012

RonFrank:
There is no service on an SPG. It is either working or it is not but it can not be calibrated. Your buddy can dive it to zero knowing that it reads low. If that makes him nervous buy a new SPG.

Any gauge can be calibrated. Calibration is the process of comparing against a known reference to check accuracy. That said a SPG can not be adjusted as it is sealed.

BTW A gauge that is not accurate but is precise can be used. An gauge that is accurate but not precise should not be used.

I.e. if a gauge consistently reads high by the same amount, using it will not cause a problem just subtract the difference from the true value. A gauge that reads a different value each time it is used but on average gives the correct valve should not be used as one will not know whether the reading is high or low.

Accuracy != Precision != Resolution

As for checking the SPG in question. Compare it to other gauges that are known to be accurate and precise. Typically, those will be panel gauges at a fill station which is will have high accuracy and ticks every 50 psi so you should be able to tell within 25psi.

Soakedlontra · Jun 27, 2012

CT Sean:
Compare it with another SPG? As in check tank pressure with questionable SPG (presumably attached to a reg set) , remove reg set, check pressure again with another set

You can probably also check it at your LDS. I'm sure they'd confirm the pressure after filling the tank, then you check it with your reg set and see if it matches - not a bad idea to check tank pressure after you get them filled anyway.

Maybe do both - then you have 3 separate readings.

Thanks!

I mentioned your suggestion to my buddy and he thinks that it would be better to attach the two SPGs to the same regulator to have a more accurate reading. We will try this after coming back from a non diving week long vacation.

---------- Post Merged at 09:49 PM ---------- Previous Post was at 09:45 PM ----------

Scared Silly:
Any gauge can be calibrated. Calibration is the process of comparing against a known reference to check accuracy. That said a SPG can not be adjusted as it is sealed.

BTW A gauge that is not accurate but is precise can be used. An gauge that is accurate but not precise should not be used.

I.e. if a gauge consistently reads high by the same amount, using it will not cause a problem just subtract the difference from the true value. A gauge that reads a different value each time it is used but on average gives the correct valve should not be used as one will not know whether the reading is high or low.

Accuracy != Precision != Resolution

As for checking the SPG in question. Compare it to other gauges that are known to be accurate and precise. Typically, those will be panel gauges at a fill station which is will have high accuracy and ticks every 50 psi so you should be able to tell within 25psi.

Thanks. If it turns out that his SPG is precise it will be up to him to decide whether he want to use it as it is or not. Personally, I would prefer to buy a new one.

Scared Silly · Jun 27, 2012

Soakedlontra:
I mentioned your suggestion to my buddy and he thinks that it would be better to attach the two SPGs to the same regulator to have a more accurate reading.

Segal's law:
"A man with a watch knows what time it is. A man with two watches is never sure."

supergaijin · Jun 28, 2012

Mine reads a couple of hundred lower than 'normal' and has done for years. ie. a full tank would read 2700 or so.
Like people say, there is no way to fix them effectively- it'd cost the same as buying a new one. IME the cheaper SPGs do not last as long as the pricier ones. Some cheapies will only last a few hundred dives. My current one I pulled out of a 'parts' bin in a DC I worked in a couple years ago- aside from the low reading, it works sweet.

Fishpie · Jun 28, 2012

There are some older gauges that can be serviced and re calibrated.
The armature can be disconnected from the spring and the spring coiled more or less and the needle pulled and re zeroed.
.....is it worth the time and effort, probably not, but it can be done.

Testing an SPG

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