[ Peter Phillips wrote in 231009 MMDigest:

> 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.

Inches of water is also used in pipe organ work. Church organs are
usually at 4", theatre organs at 8" to 10", and also some ranks are
at 15, 20 and 25" (usually the louder sounds and 16-foot ranks --
and I may be losing some of you with all this terminology). 

Now, if one converts that to inches of mercury (usually referred to
in the USA as pounds per square inch, or PSI), 10" of water pressure
is about 0.38 PSI, which is one reason to use inches of water as it
gives you a greater measuring range to work with at such a low
pressure, or vacuum.

When we were planning on re-installing a pipe organ in the Oroville,
California, city's theatre -- which involved running a wind line of
14" diameter from the basement up to the pipe chambers about 10 feet
off the auditorium floor and then over the stage's proscenium arch to
the chamber on the other side -- the city engineer was concerned with
how we would "vent all that pressure." I had to explain to him that 
there wasn't much pressure by converting it to PSI, which he was more
familiar with, and that it was more volume of air than pressure of air. 

Satisfying the structural engineers for that project was quite an
undertaking -- but it's now all done and working! And, because we
are using a computerized control system, it is also a "self-playing"
instrument that can record a player's actions at the console and then
play it back at any time, just like a live performance (complete with
any errors the organist made).

David Dewey
Oroville, California

 [ Read about the rescue of Oroville's State Theatre organ at
 [ https://www.mmdigest.com/Archives/Digests/201702/2017.02.15.03.html 
 [ https://www.mmdigest.com/Archives/Digests/201702/2017.02.16.02.html 
 [ -- Robbie