Analog Pressure Gauge with no EN837 accuracy class information

#1
Hi,
We inherited a set of pressure equipment of which there were some analog pressure gauges (0-30psi) resolution of 1psi. These gauges were sent for calibration and the results showed 0.5psi error in 1 of the nominal input pressure. In our usage, any readings that is above a marking will be round off to the next marking's value. Would this gauge's calibration results be considered acceptable under these conditions? Is there a industrial standard that provide guideline on handling this type of gauge errors?

Besides this, are gauges with no EN837 indication reliable to use?

Appreciate any thoughts to share on this. Thanks/
 
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Jim Wynne

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#2
Whether or not the gages' calibration results are acceptable depends on your requirements for accuracy. It's your decision to make, and it should be based on the attendant risks.
 
#3
Thanks Jim. I am trying to explain to end users that any measurements with pointers in between 2 marking (resolution of 1psi) should be consider as the next higher values. So in the case if the calibration error is -0.5psi, the gauge will still be consider acceptable as it will always be rounded off to the next value. However, if the error is +0.5psi, this will render the gauge unusable as the gauge will always have a reading that is 1psi higher than actual. Is this an acceptable explanation?
By the way, is it a common practice that calibration lab will indicate results as 0.5 if it is in between 2 markings? There seemed to be various explanations within other forum threads found in this site.
 

dwperron

Trusted Information Resource
#4
Hi,
We inherited a set of pressure equipment of which there were some analog pressure gauges (0-30psi) resolution of 1psi. These gauges were sent for calibration and the results showed 0.5psi error in 1 of the nominal input pressure. In our usage, any readings that is above a marking will be round off to the next marking's value. Would this gauge's calibration results be considered acceptable under these conditions? Is there a industrial standard that provide guideline on handling this type of gauge errors?

Besides this, are gauges with no EN837 indication reliable to use?

Appreciate any thoughts to share on this. Thanks/

Pressure gauges are interesting..... The only way to determine with certainty the specifications is when you can identify the manufacturer and model and look up the specifications.
The EN837 gauges are good since they normally have the tolerance printed in the gauge, but most commercial gauges don't do that.
There is an American document ASME B40.1 for pressure gauges that can be helpful in determining "generic" tolerances for unidentified gauges, but that is still not a guarantee of the designed accuracy of the gauge.

Years ago I developed a "cheat sheet" for my people to make an educated guess to the accuracy of the gauge, I have attached a copy.
 

Attachments

#5
Thanks dwperron. The cheat sheet is really helpful as a guide. So the 30psi analog pressure gauge i have will fall under grade 'A'. The accuracy 2-1-2%, does this mean?:

0 - 7.5psi = 2% FS

7.5 - 22.5psi = 1% FS

22.5 - 30psi = 2% FS



I've also attached a snapshot of the calibration report. Noticed the 30psi actual is different for ascending and descending. I'm thinking that they should be the same as when ascending peak is reached, it should continued from descending to check on hysteresis? Please correct me if my understanding is incorrect.
Gage Hysteresis.jpg
 

dwperron

Trusted Information Resource
#6
Thanks dwperron. The cheat sheet is really helpful as a guide. So the 30psi analog pressure gauge i have will fall under grade 'A'. The accuracy 2-1-2%, does this mean?:

0 - 7.5psi = 2% FS

7.5 - 22.5psi = 1% FS

22.5 - 30psi = 2% FS



I've also attached a snapshot of the calibration report. Noticed the 30psi actual is different for ascending and descending. I'm thinking that they should be the same as when ascending peak is reached, it should continued from descending to check on hysteresis? Please correct me if my understanding is incorrect.
View attachment 26214
You are interpreting the specifications correctly.

Yes, the 30 psi should be the same for the descending hysteresis check as in the ascending. That would indicate one of a couple of possible issues:
Your pressure standard is not stable.
More likely, you did not give enough time for your gauge to stabilize on the ascending reading.

Common practice is to give the gauge a light tap with the fingers before taking a reading to overcome any meter friction, that might have helped here.
Also, on analog gauges it is common to adjust the calibrator pressure until to reach one of the test points for the gauge. For instance, if you adjust the calibrator to read exactly 15 psi on the gauge you would report that the Nominal pressure was 15 psi, the reading was 15 psi, and the Source pressure was 14.87 psi or whatever the calibrator was reading. That is a lot easier and more accurate than just setting the calibrator to 15 and interpreting what the gauge is reading between marks on the dial.
 
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