Air pressure can be either positive or negative, in which negative is
generally referred to as suction. I have worked with a range of pressure
units, such as pounds per square inch (psi) when I was employed in
power stations. These stations were pneumatically controlled, and the
reference pressure was 30 psi. There is also the bar, which is how
pressure gauges in espresso coffee machines are calibrated, in which
a bar is the pressure of one atmosphere, or about 15 psi.

I have always used inches water gauge (WG) when dealing with pneumatic
pianos, although I recall the power stations used inches of mercury to
measure suction. I use inches WG because that is the unit used by
Ampico in its service manual. My first reproducing piano was an Ampico,
so you could say I was "Ampico trained".

Inches WG suits a reproducing piano well, as a gauge can be made by
anyone. Just stick a glass tube in a pot of water and you have
a manometer that will be quite accurate. I have made many manometers
over the years, both U-tube and straight tube. I have since made
several electronic vacuum gauges, all calibrated in inches WG.

Put simply, the best unit to use for pressure is the unit that suits
the application. For instance, one would never uses inches WG to
express the air pressure in a car tyre. I guess you could use
centimetres WG, but it doesn't roll off the tongue like inches WG.

Peter Phillips
Sydney
https://petersmidi.com/