Blue Chips
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- 622
- Location
- Maine, USA
I think it's probably feet of water, 35 PSI equates to 80 feet.
35 PSI = about 71 inches of mercury or roughly 81 feet of water. Even though the 64 reading on the gauge that I got from applying 35 PSI is closer to 71 than 80, the gauge could be, and probably IS, way off.
It appears that @Maker is correct. I just found this listing in the 1920 National Boilers, Radiators and Specialties Catalog. The gauge is virtually the same as mine, and it says that it measures the height of the column of water in the system, in which case it would have to be feet of water. Apparently, my gauge is badly in need of calibration.
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