All bourdon or diaphragm-capsule gages with a rack-and-pinion dial
linkage are flawed with non linearity, inaccuracy, and uncertain zero
at the low end of the scale, and can suffer zero-shift or irreversible
calibration error if over-ranged accidentally.

Many people would wish to use the water manometer for any precision
shop work, but there are cases where portability or fast response in
dynamic situations might dictate the use of a gauge instead.

May I suggest the Dwyer 'Magnehelic' low pressure gage.  Dwyer is the
industry leader in all kinds of low pressure measuring technology, and
the 'Magnehelic' is the standard for reasonably priced vacuum or low
pressure dial measurement.  They have the following advantages:

  Their simple large cloth diaphragm and disk, opposed by a flat spring
(whose length is adjustable for calibration with a water gage) makes the
action linear all along the scale.

  They have a true floating zero.  When the needle is zeroed with a screw
on the front, accessible without disassembly, you know the reading is
truly zero, and the lowest readings will be accurate.

  They can be over-ranged up to 15 psi in either direction without damage.
In such instances, the diaphragm simply bottoms in the 'pouch well',
and the magnetic linkage to the pointer movement lets go, preventing
any bending or distortion of the works.

  Both the upper and lower halves of the case are sealed, and fitted with
nipples, so the gage can work for pressure, vacuum, or differential
pressure, equally well, depending how you tube it up.

I would suggest the model 2040 (40" wc) would be most useful, altho 25,
30, 50, 60, and many others are available.  Price $49.00 (last time I
looked in the catalog) from: Dwyer Instruments, Inc.  PO Box 60725
Houston, TX 77205 713-446-1146; Fax 713-446-0696

Another approach would be to get a model 2030 (30" wc) gage, and make a
'Hickman pressure divider' for higher readings.  Solder a bleed in both
run ends of a 3/16 tee; tube one run to the measurement and tube the
branch to the gage.

With the other end plugged with a knotted tube, the gage will read the
measurement; when unplugged, it will read about half, because the
pressure drop in the air flowing through the two bleeds will be about
equal.  Put this on a known supply, say 27", whiff out the outer bleed
with a small wire-sized drill used as a 'scraper', (only a tiny amount
of size increase is needed) till the gage reads 9", and you have a 3/1
scale multiplier.  Typically measurements on high vacuums, such as
setting spill valves, will tolerate a little flow without serious loss
of significance.

Richard Vance