Overpressure requested from manufacturers for pressure gauges is 120-130% of scale as normal practice. What about a gauge with 0-6 barg scale but 18 barg as design pressure of relevant process line?

 

I think what you are suggesting is the design spec for the line is much higher than the normal operating pressure. In this case it's common to use a lower range of gauge but the proof pressure should be 18 bar i.e. the gauge may be wrecked by overpressure but it won't burst letting the process fluid out.

I like to pick a gauge that's approx 50% scale at normal pressure.

 

Install a valve to isolate the gauge for pressure tests.

Regards,
cww

 

If I read pages 50-56 (pdf) of Wika Handbook

http://www.wika.com/wikaweb/support/pdf/Wika_Handbook_2008_LR.pdf

properly, it says that the maximum overpressure a bourdon tube gauge can tolerate before deforming is 30% of the maximum scale value.

Page 56 (pdf, pg 39 paper), has a graph where the "permanent deformation (boundary) A" is 130% of B, the full scale range value of the gauge.

Various gauge vendors (from some years past) used values between 10% and 50% of a gauge's maximum scale for the limit before overpressure would alter the indicated value.

Our rule of thumb is the same as Roy's, normal operating pressure mid-scale on a mechanical gauge.