suitable for steam locomotive use: https://rover.ebay.com/rover/0/0/0?mpre ... 3507711540

Well, without a current qualification sticker on it, it's just another gauge...right?

somebody, sometime, used it as a master gauge. I have two real test gauges but they are out of calibration, however they are marked with the last date they were tested. Picked them up at an antique shop for $20 each so I could do an uncertified test on gauges I own.

True, but you can always get it certified. Even with a brand new one you're going to have to get it certified eventually anyway.

It needs to be tested with a dead weight tester so it will be up to date aka certified or calibrated.

What makes this a master gauge? Is it more accurate across the range? Would the calibration schedule be any different than a gauge used on a locomotive?

Why would one use this versus a dead weight tester?

Many large industrial plants with large numbers of gauges would have test connections for such a master gauge so if there was any question about the accuracy of the gauge in service the master gauge could be added to confirm or condemn the function of the service gauge.

Master gauges were also manufactured to must stricter tolerances than most gauges. so they are 27 jewel Hamilton pocket watch of the bourdon tube world.

Thanks.

Also some places will create a `master calibrated gauge' to calibrate other general purpose gauges at a lower tolerance, instead of having to pull out the dead weight calibrator which takes more time to use correctly. The checked gauges wont get a calibrated tag, but the company will know they are reading correctly. The `masters' will rotate through the calibration room to get updated and verified.

How they are handled all depends on the tolerance the field gauge needs to maintain. It is similar to machine shops where they have certified measurement standard blocks that never leave the inspection room and regular standards that the machinists have access to to maintain their own measuring equipment.

Rich C.