Just a couple of additional points to add to this thread.  Magnehelic
gauges are quite accurate for the price and can usually be found on
E-Bay.  A 100 WCI and a 15 WCI or so gauge cover that reproducing piano
vacuum range pretty well with sufficient sensitivity at the low range
and no chance of sucking water into the instrument or needing to have
a water gauge taller than a person.  Of course, you can use a water
trap but the little gauges are much more convenient.

Best regards,
Spencer Chase