Maker
Most folk just call me; Orange Joe
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I think it's probably feet of water, 35 PSI equates to 80 feet.Thanks @Onoff. That looks like a virtually identical gauge, other than the company. Since it was manufactured or sold to a boiler company, it seems logical that it could have been used on a boiler or on some kind of steam equipment.
I just tried applying some air pressure to the gauge. The mechanism seems to be working smoothly, but I have NO idea if it's even reasonably accurate anymore. When I applied exactly 35 PSI of air to it, the gauge read 64. The closest pressure unit equivalent to 35 PSI I've found was 71 inches of mercury, which isn't terribly far off, considering that the gauge is probably way out of calibration. Inches of mercury is/was commonly used to measure atmospheric pressure, so that would be a possible unit for an 'altitude' gauge. Maybe one of our steam afficionados would know more about it.