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MMD > Archives > March 2009 > 2009.03.05 > 06Prev  Next

Dial Gage vs. Microscope for Valve Calibration
By John Phillips

Hello MMD.  Just over a year ago I posed a query here about my concern
that using a dial gage to measure valve travel in a player action might
distort the results, on account of the significant force required to
move the dial gage plunger.  I mentioned that last time I set up some
valves, I used a travelling microscope.

I received a couple of replies urging me to forget about the microscope
and to get myself a dial gage, so I went out and bought myself a new
one.  Only recently I started making valve measurements in earnest and
decided to compare the two methods.  I borrowed a low power microscope
from my ever-accommodating former employers, the University of Tasmania,
and started measuring.

At this stage I had, and have, the completely original stack out of
my Aeolian Steck Upright Half Duo-Art.  I believe that it has never
been touched since its manufacture in about 1919.

I made measurements with both gage and microscope, first by blowing
into the pouch nipple and observing what happened, and then by
connecting a vacuum supply to the treble half of the stack and
measuring valve travel under something approaching working conditions.
I chose five valves, spread across the treble side of the stack.  The
results were much the same for each valve; here are those for valve
number 60.

Blowing: Microscope, 0.042"; Gage 0.041"

Under Vacuum: Microscope at 8" Water Manometer, 0.042";
  Dial Gage at 21" Water Manometer,  0.039"

The results from the other four valves revealed a similar small
scatter of results.  It seems that it doesn't matter whether one uses
a microscope or a gage; the results will be much the same.

But why did I use very different vacuum levels for the microscope
compared to the gage?  Because the valves didn't flip from their 'off'
states to their 'on' states until the vacuum level was about 15" of
water.  I used 21" of water so that the excess pressure under the pouch
over what was needed to get the valve moving was roughly the same in
both microscope and gage situations.  (Okay -- it should have been 23"
but I was getting worried about the strain on the pouches.)

I was really surprised at the considerable pressure difference
required across a pouch in order to get the dial gage plunger moving.
I therefore made a rough measurement of the minimum necessary upwards
force to move the plunger.  It was about 77 grams weight.  A 15" water
manometer pressure difference means that the pouch will deliver an
upwards force of 38 grams weight per square centimeter to the foot of
the valve.  So, if the surface area of the foot is 2 square centimeters,
which implies a diameter of 1.6 centimeters, the need for the 15" of
vacuum becomes clear.  Well, it convinces me.

It's a long time since I looked at an Aeolian 88-note valve so I don't
know what the diameter of the foot really is, but 1.6 cm [0.63 inch]
diameter doesn't seem ridiculous.  This back-of-the-envelope calculation
(I really did use an old envelope!) ignores any effects due to the tension
in the pouch leather, but I suspect they will be comparatively small.

I think I'll stop now, before I lose all readers.

John Phillips in Hobart, Tasmania

(Message sent Thu 5 Mar 2009, 10:23:54 GMT, from time zone GMT+1100.)

Key Words in Subject:  Calibration, Dial, Gage, Microscope, Valve, vs

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