Air pressure gauges and hydraulic pressure gauges operate on the same principle, correct? bourdon tube expands proportional to pressure, turns a little gear attached to a needle? The question arose when I couldn't find a hydraulic gauge to troubleshoot a hydraulic system so I got the idea to use an air gauge. My boss said it wouldn't work. I tried it anyway. It did work. Afterwards, I went and looked at all the gauges we have in stock. I couldn't find on any of them where it specified air or hydraulic. I think they might be one in the same. We refer to the liquid filled ones as hydraulic gauges and the non liquid filled ones as air gauges; but I seem to remember seeing a liquid filled gauge on an air system before... somewhere....