monometer selection?

[emree]

Hii...during hydrostatic testing of piping (B31.3 or API 1104) what is the criteria for pressure gage selection...?
Eg. desing pressure is 10bar , so test pressure is 1.5 times design pressure so 15bar...which one of the pressure gage should be used...?(a 25bar gage or 40bar or 60bar)
thanks...

 

[BigInch]

25

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[JStephen]

You generally would want a pressure gauge to be reading in the higher part of it's range for best accuracy- the tick marks are a smaller percentage of the reading.

 

[JR97]

My experience is that analog pressure gages should be used at approximately mid-scale. They tend to get non-linear at either end of scale. ASME B&PV Code stipulates analog gage range be 1.5 to 4 times test pressure which puts the test pressure between 25% and 67% of gage range.

Digital gages are entirely different. Digital gages should be used closer to their upper limit. Manufacturer certs for digital gages always state accuracy as % of full scale reading. Again using ASME B&PV Code as standard, acuracy of digital gage must be 1% of reading. If your gage is certed at .25% accuracy (standard gage), you need to be above 25% of full scale to achieve 1% of reading accuracy.

JR97

 

[vpl]

JStephen and JR97 have both given the reason behind Big Inch's "25"



Patricia Lougheed

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

 

[JLSeagull]

I mostly agree. For process gauges in operation we like the normal operating pressure to be within the middle third of the scale - and the maximum pressure well on the scale.

I worked in my home-town refinery carrying tools as an apprentice in the instrument craft after I graduated - before starting my engineering career. I rebuilt and calibrated pressure gauges and did not find pressure gauges to be non-linear at the high end. However, the bottom 5 % is just slop. We liked to select the test gauges used for gauge calibration to match the selected gauge. We did calibration mostly to the top of the scale.

If the test gauge was non linear near the top, it matched the process gauge well. We checked the test gauges with dead-weight.

 

[hacksaw]

for what it is worth, the normal pressure for bourdon tube gages is not supposed to exceed 80% of its maximum range.