Recalibrating Pressure Gauge

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  #1  
Old 05-11-10, 04:42 PM
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Recalibrating Pressure Gauge

I have a beautiful, 60-year-old pressure gauge that I've cleaned up and use as coffee-table novelty. The thing that annoyed me was the needle didn't return quite to zero - so I took it apart.

The only way to calibrate the gauge is to physically bend the bourdon tube a little. So now the gauge reads correctly, at least with zero pressure applied to it. But, by bending the tube, you can calibrate for zero or for span, but not both.

Anyway, this exercise got me interested in the technology of pressure gauges.

I think that over time, pressure gauges tend to read high because the bourdon tube deforms (straightens out) slightly. It's probably a good idea to replace a boiler gauge - maybe every ten years or so?

I'm aware of digital pressure gauges that proclaim 0.25% or even 0.05% (full-scale) accuracy. For example: Pressure Gauges: Digital Pressure Gauges

The biggest problem is that they start at $250. And, they depend on batteries. I'd like to have one - I'd install it in addition to my conventional analog boiler gauge.

The digital gauges can be calibrated for zero and span with two little potentiometers. You can send it back to the factory, and they will recalibrate it to NIST-traceable standards

If I were to get a digital gauge, I'd probably select this one: http://www.omega.com/Manuals/manualpdf/M3359.pdf

I'd go with the 0-30 psig model for best precision (30 x 0.25% = +/- 0.06 psi), which more than satisfies my requirements! (The overpressure rating is 2x full scale, or 60 psig.)
 
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Old 05-11-10, 08:58 PM
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Pressure gauges are recalibrated by removing the pointer and then pressing it back on to read properly. Ideally you would use a dead weight calibrator and calibrate to the pressure that is most important. There most likely will be linearity errors at the ends of the scale. Some pointers have a rack and pinion for adjustment.
 
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Old 05-12-10, 08:02 AM
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Originally Posted by furd View Post
Pressure gauges are recalibrated by removing the pointer and then pressing it back on to read properly.
OK, thanks!..............
 
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Old 05-13-10, 01:42 PM
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Gauge Calibration

I was a mechanical calibration tech for many,many years. Zero is not a calibration point, thats why a lot of gauges use pins as pointer stops or sometimes the zero is a square on the dial anywhere in the box is OK. To adjust a bourdon tube gauge, you need to change the distance between the fulcrum and the end of the bourdon tube. Higher quality guages have a link that can be made longer and shorter by loosening the set screws and sliding the link in or out. You move it in if it's slow and out if fast. On cheaper guages there is a U shaped link that is bent in or out to adjust. Most are calibrated at least 3 point lower 1/3, middle 1/3 and upper 1/3. Most cal facilities use guages of higher accuracy to check lower accurate gauges. What we used were test guages that were calibrated to + or - 1% of indicated value and they were calibrated by gages that were accurate to .1% and those were calibrated by dead weight tester that was accurate to .001% which was calibrated by a lab that was 2 steps from NBS (National Bureau of Standards) I hope this helps and is not to confusing.
 
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Old 05-13-10, 01:48 PM
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Right on Dunoon

Reminds me of the HVAC tech that thinks turning his AC gage set to 0 is calibration, but only could make it worse.
 
 

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