Michael Taylor asks for a conversion factor between inches of water
and psi.  Please excuse my bluntness for saying that this is a bit
pointless, but the reasons will become evident, since measuring organ
wind pressure in inches of water is wholly sensible.

Pressure is equivalent to head [column height] times density.

Water has a density of one gram per cubic centimeter, which can be
converted into old English units by dividing by the number of grams in
a pound and multiplying by the number of cubic centimeters in a cubic
inch.  This gives a density of 0.036 pounds per cubic inch.

The pressure of one inch of water column is simply this density
multiplied by one inch, which equals 0.036 psi.

Therefore one inch of water is equal to 0.036 psi.

You can see that for typical organ pipe pressures we are always working
well below one psi, hence my opening statement.  To put things into
perspective consider the atmospheric pressure of 14.7 psi.  This
equates to a water column 34 feet high!  Organ pipe pressures ranging
between one and twelve inches water column (for the majority of
mechanical organs) are minute in comparison with this.

Let's keep working in inches of water, and leave psi and bars for
pneumatic and hydraulic machinery.

Nicholas Simons