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THE MUSIC TRADE
DECEMBER 31, 1921
REVIEW
A Further Discussion of the Use of Pressure Gauges in the Pla> er Industry,
With Special Reference to the So-called"Ounce Scale"—Laws of Physics Upon
Which Depend the Stability and Accuracy of Pressure-Indicating Devices
A recent correspondent, referring to the article
in the November issue of the Player Section on
the uses of pressure gauges, which appeared in
the Pneumatics Department, writes to the fol-
lowing effect:
"The writer is very much interested in the
item under the heading 'Ounce Scale.' We have
often wanted to compute our pressure and
vacuum in just the terms you have outlined. . . .
In your article you state that if the water column
be of one square inch cross-sectional area the
difference in the level of water will be the inches
pressure or vacuum. Would it make any differ-
ence whether the cross-sectional area is a square
inch or could it not be either greater or
smaller? Would not the result be the same in
either case? In other words, would not a given
strength of vacuum raise the column of water a
certain height, no matter what the area of the
cross section might be?"
Upon consideration it has seemed that others
of our readers might desire to have more light
on these qtiestions. The answers are therefore
given here in detail, for the benefit of all.
Physical Facts
In the first place, the weight of a column of
water in a tube which has a cross-sectional area
of one inch and a height of 408 inches is equal
to the weight of a column of air of the same cross-
sectional area and of a height equal to the height
of the atmosphere. Consequently, if the tube
which is used for the movement of the water
column be less, equal to, or greater, than one
square inch in cross-sectional area the measure-
ments will be equally true. For if the tube be
of smaller area the resistance of the water will
be the resistance of a smaller weight measured
with an equal volume of air. If the tube is larger
then it is simply measuring its larger volume of
water with a larger volume of air, the magni-
tude being measured in all cases by the cross-
sectional area, whatever that may be. There-
fore, the size of the tube can make no difference,
for we are always reckoning our units as taken
from the one-square-inch standard.
The Ounces Measurement
The same, of course, is true as of the measure-
ment by ounces. If the water in a tube of one
square inch of sectional area moves any num-
ber of inches, under a given pressure, then a
tube of either larger or smaller area filled with
water and subjected to the same influence will
move in precisely the same way, neither more
nor less. The pressure is upon the cross-sec-
tional area of the head of the column and, there-
fore, is proportionately the same whatever the
area may be. A smaller area of water column-,
head is pressed upon by a proportionately
smaller body of air, and so on.
Therefore, of course, since a movement of the
water column indicates a difference between the
weight of the air at one end of the column and
the weight at the end, which is expressible
in units of weight, a movement of water meas-
ures the effective working pressure, which is
always equal to that difference. Reduce, by
vacuum'machinery, the normal atmospheric pres-
sure previously existing at one end of a U-shaped
water column and you have an effective pres-
sure at the open end from the normal atmos-
phere. This effective pressure for doing the work
of the machine, which is under test by the pres-
sure gauge is a difference between 14.75 pounds
per square inch and some smaller weight-figure
which represents the reduced pressure in the
machine. This difference is expressible in units
of weight, preferably in ounces. The size of the
water column does not matter.
Relation of the Scales
If 408 inches of water balance 236 ounces of
weight of air, then one inch of water equals
236/408 ounces of effective pressure. Upon the
formula it is easy to make a scale up one leg of
the U-tube by pasting a strip of paper along the
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tube and marking the point to which the water
column rises. Then from this point make a
series of gradations, treating each half-inch as
one whole inch for convenience (as otherwise it
is necessary to have a scale against each leg of
the U-tube and make the reading by noting both
the rise on one leg and the fall in the other).
The scale is made to read, we say, one inch for
each half-inch of actual measurement. Then
each such division, marked as one inch, corre-
sponds' to 236/408 ounces, or about .58 inch. Or,
in other words, each ounce in weight is equiva.-
lont to about 1.72-inch,rise of water column. By
taking a scale marked to equal 102 inches (51
inches actual length of the water column's rise
along one leg) and placing opposite it a scale of
exactly equal length divided into 59 equal parts,
we shall get the nearest practical approximation,
which is 102 inches equal 59 ounces. By so
marking the scale the pressure per square inch
in ounces can easily be read off.
Superiority of the Ounces Scale
For various reasons it seems not to be fash-
ionable to use the ounce scale. Yet we believe
it to be a good deal more scientific than the
ether. For reasons of their own physicists find
it useful, in measuring very high vacua, to em-
ploy the mercury scale, in which the weight of
the atmosphere is balanced by a column of mer-
cury about ^^> inches, not 34 feet, in height. The
measurements are read off on the jnercury tube
in millimeters, and so the custom of using linear
units for measurement of pressure has spread to
the far different cases of the musical pneumatic
mechanisms, with their low pressure and large
quanta of displaced air. Yet, there is no good
reason save the reason of custom for the prac-
tice.
For one special reason the practice of meas-
uring pressure in ounces is to be commended.
In dealing with the piano action one has to deal
entirely with factors expressible in units of
weight. The;value of the touch of a finger upon
a key is expressed in ounces and, in fact, all
work upon the piano action must be expressed
(Continued on page 10)
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