[rotorwrench]

I have an old hand hydraulic pump that is connected to a T-manifold with a test gauge on one side. I have all sorts of reducer bushings to fit the gauges to be tested and a cap plug to keep the test set clean when I'm not using it. I install the gauge to be tested on the other side of the T-manifold but leave it loose. I then bleed the air out & tighten the gauge to the manifold. I pump the handle to get pressure on the test gauge and compare to the tested unit through several stages. This way I only have to get one gauge calibrated per year and I can test the rest of them for calibration in house.

I use the same set up to test electric gauge indicating systems as well. It passes FAA requirements. I use motor oil for the test set since most of my stuff is for oil pressure. I just have to clean the fuel pressure gauges after testing to prevent any fuel system contamination.

Air or nitrogen gas pressure gauges are tested with compressed air and a regulator with a similar T-manifold. It's easier to test them. Gasses compress to a certain degree so I don't mix gaseous pressure gauges with liquid pressure gauges when testing them. For a basic test of any pressure indicator, compressed air works fine but the difference lies in accuracy and concerns for contamination.