Pressure gauge testing and calibration

[somersetsimplex]

A recent discussion at my club was around locomotive pressure gauge checking and raised the question as to what is acceptable accuracy for pressure gauges.  Section 6.10 of the boiler test code  states "the boiler's pressure gauge shall be checked against the calibrated pressure test gauge..."  but nowhere does the code appear to state what the acceptable range of accuracy is.

Logic states it should be +0% /-10%, ie a gauge marked at 100psi when checked against the calibrated gauge at 100psi should read between 90 and 100 psi, but I've been unable to find anything  stating this in the code  - does anyone know where acceptable accuracy is stated? 

Apologies if this has been asked before, but a search of the forum posts didn't find an answer.

[runner42]

The Australian boiler code is similarly prescriptive in specifying that the gauge be checked against a guage of known accuracy. In practice this is checked at the boiler operating pressure, eg 100psi and again at +10% above operating pressure. At these pressures the guage under test readings are noted. The red line marking of the guage under test at the boiler operating pressure will be the indicator at which the safety valve(s) should blow and during the steam test where the blower is operating fully, to produce maximum heat, the gauge pressure should not increase by 10%, the second noted reading. This reading is to ensure that the safety valve(s) have sufficient capacity to exhaust the steam. So the gauge is more of an indicator than one of a specified accuracy. An aside the gauge should have a full scale reading of at least 20% or preferably 50% above the boiler operating pressure.

Brian

[tenor]

There is an explanation of accuracy ranges here:
www.brannan.co.uk/knowledge_base/pressure-gauges-accuracy-explained/
Realistically for the miniature gauges used on a 5" gauge loco for example, I reckon 10% of FSD accuracy would be a very good result.
Back in the day, I oversaw the calibration of company test gauges to 0.25% class. Over many years and many gauges we only ever needed to adjust the needle on it's spindle. So for model gauges, make it agree with the master gauge at working pressure and all will be good.
Martin