(Editor: Craig Brougher was kind enough to suggest posting to MMD
 the following info which I sent to him.)  [Thanks to both you and
 Craig, then; it's fascinating!  -- Robbie]

I've been examining my Pistonola action.  Unfortunately it's
disassembled and I'm unable to measure actual vacuum levels, but
physical measurements give the following results:

The reservoir between the exhausters has a diameter of 2.625" (area of
5.412 sq. in.) and a stroke of 3".  Its compression spring has a rate
of about 10 lb/in, thus requiring a force of 30 lb to close completely.
This therefore needs a vacuum of approximately 5.5 psi, or about 150"
of water column or over 1/3 atmosphere or 10" mercury column.

The reservoir has a thick felt pad in the bottom to cushion the
piston at its extreme travel.  The system plays consistently with the
reservoir closed or nearly closed in much the same manner as
conventional player mechanisms and heavy accents are produced at
significantly higher vacuum levels.

The exhausters comprise two cylinders 3" in diameter (7.068 sq. in.)
with a 4.5" stroke max.  At 5.5 psi each would require a force of
nearly 39 lb to move, which is within human capability.  The straps
connecting the pedal and piston are not leather but very strong cotton
webbing, 1" wide x 3/32" thick.

The playing pistons are in fact 17/32" diameter (0.221 sq. in.).
At 5.5 psi they can thus generate a force of 1.2 lb.  They were
connected by strong cotton/silk cord to the whippen ends in the
upright action, which would give them a mechanical advantage of about
two.  Thus a force of about 2.4 lb would be exerted on the jack, which
compares well with the force produced at a capstan/sticker in hand
playing.  The three Pistonolas I have encountered played easily as
loudly as any normal player.

The valves in the Pistonola seem to have been perfectly serviceable
and no more troublesome when new than tyre and other pneumatic valves
working at similar differentials.  The top seats of the secondary
valves are strong thin glazed card or celluloid, probably for
quietness, pressed into their wells and held down by small brass inner
circlips; I suspect they could give trouble after becoming damp or
chemically affected by atmospheric pollution.

From diving and engineering experience I know that producing more
than 20" Hg vacuum is not difficult even with hand pumps.  The time
taken to reach such levels is naturally proportional to the capacity
of the system being evacuated, and estimating that of the Pistonola is
very difficult.  Getting to 27" or 29" Hg of course requires increasing
amounts of energy.

I suspect that the pundits were impressed by this 'high-tech' system
when it was launched and were dazzled by the fact that normal players
will strike notes at about 4" water minimum and the Pistonola amazingly
works at more than 100".

My PPG article will have more tantalizing details of the exquisite
miniature NACA duct shaped tempo control orifice and the ball and
spring tempo and balanced accentor regulators.  There are also lots
of mouthwatering patents of Coldman and Webb to be savoured.

Paddy Handscombe
Wivenhoe, Essex, UK