Tangley calliopes usually bear a nameplate on the side of the case
stating that instrument was originally voiced at the factory to
play at operating pressure, e.g., 3/4" or 1", meaning 3/4 or 1 psi.
Below is a short table of calliope air pressure, relating water
column inches (in-H20) and pounds-force per square inch (psi):

  psi  in-H20

  1/8   3.46
  1/4   6.92
  3/8   10.38
  1/2   13.84
  5/8   17.30
  3/4   20.76
  7/8   24.22
1       27.68
1-1/4   34.60
1-1/2   41.52

A gage suitable for checking the air pressure supply, and also the
vacuum supply for the player system, would be a differential (dual
port) gage of 40 inches water column, such as made by Dwyer
Instruments and sold at eBay auction, http://www.eBay.com/  Search
for Dwyer Magnehelic differential pressure gauge.

Dwyer Magnehelic model 2040 reads 40" water column full scale, or
about 1-1/2 psi.  Dwyer says it's accurate to 2 percent full scale,
or +/- 0.8 inches water column.  The price from Dwyer Instruments
is $63.50 new; available used at eBay for around $30.

References:

  http://en.wikipedia.org/wiki/Atmosphere_(unit) 
  http://www.mmdigest.com/Archives/KWIC/C/calliope.html 
  http://www.mmdigest.com/Archives/Digests/199906/1999.06.16.17.html 
  http://www.dwyer-inst.com/Product/Pressure/DifferentialPressure/Gages/Series2000 

Robbie Rhodes
Etiwanda, California