It is up for discussion again although there should be plenty in the
archives.

Any vacuum gauge that costs less than $300 new is not going to cover
the range needed in the player piano accurately. Only a very expensive
hand calibrated gauge will be accurate at a few inches as well as at
full scale.  Of course the error at high levels is less important but
the cheap gauges tend to be worse at the low end. In general a cheap or
medium price gauge is considered to be only reliable at the center of
the range. This is just the nature of gauges. There are three measures
of the quality of a gauge, accuracy, sensitivity and repeatability.
Accuracy requires calibration and sensitivity and repeatability require
good bearings etc. You will not find these in a cheap gauge with one
exception and that is the Magnahelic. It uses a clever design that
overcomes many of the deficiencies of low cost gauges. However you need
several to cover the range accurately. Fortunately they can be placed
in parallel since they are rated at 15 PSI differential max.. so even
a .5 water column inch gauge will not suffer damage if connected to
75" of vacuum.

I also have used the air speed indicators and find that they are very
good too. I have one that I calibrated years ago and it is still good
after much banging around. My favorite, however, is my Wallace And
Tiernan 0 to 120" gauge. It makes two turns for the full scale on a an
eight inch dial. You can read it to .1" and it is very accurate and
sensitive. It is not very portable and is fragile and very expensive.
I believe they are over $1000 new but they can be found surplus.  I
paid $5.00 for mine.

If you want a very cute gauge for very low (0 to 6") ranges you can use
an inclined tube oil manometer of the type that used to be used for
indicating  flow through furnace filters. Even though they use a
special colored low volatility oil, they are calibrated in inches of
water. You can read them to extreme accuracy. They have a built in
float type trap to prevent the oil from getting into the piano but it
is best to provide a second large capacity one too.  They are great
for checking for leaking valves at minimal vacuum level. You can
compare the drop for each valve as the note is played and easily read
the difference.

In case you haven't noticed,  my second hobby is measuring devices.

Spencer Chase
http://nt.mcn.org/spencer/