_The Piezometer, or pressure gauge, is an instrument for measuring the
pressure of water in a pipe._
[Illustration: FIG. 107.]
It may be broadly stated that all pressures and weights relating to
water, steam, gases, etc., are now recorded by gauges.
The principle of construction of the dial gauge is that the pressure
may be indicated by means of a spring and pointer upon a divided dial
similar to a clock face, but marked in divisions, indicating pounds,
hundreds, etc., _pressure_ instead of hours and minutes. The more
approved forms of gauges are now constructed upon the principle of the
_Bourdon_ spring or metallic barometer invented in 1849. (See page 113
for illus.)
[Illustration: FIG. 108.]
The essential principle—or discovery—is this: that a metal curved
tube—oval cross section, _under pressure, tends to straighten itself_
according to the force exerted by the pressure inside. Figs. 107 and
108 show the ordinary style of gauge which consists of an elliptical
tube, connected at one end to a pipe in communication with the
pressure, and at the other end with toothed arc and pinion to a pointer
spindle as shown in cuts.
NOTE.—Hydraulic gauges are indispensable as it is often necessary to
stop the pressure at points below that at which the safety valve has
been set.
Within the gauges—or cases, is a small coiled tube closed at one
end, while the other end is attached to the socket through which
the water is admitted; this tube has a tendency to straighten when
under pressure, and thus its free closed end moves, and this motion
is communicated to the pointer; when the pressure is relieved the
tube assumes its original position and the pointer returns to zero.
There are many modifications and special adaptations of the Bourdon
discovery, but the principle remains, and the same useful results are
obtained with both single and double tubes, the latter being the most
resensitive.
Fig. 107 shows the dial of a hydraulic gauge which is graduated to suit
the work to which it is related. These gauges are made for pressures
from 1,000 to 20,000 pounds per square inch. The springs are formed of
heavy solid bar steel turned and bored to size and are of the Bourdon
style. They are in use in large railroad shops, sugar refineries and
cotton-seed oil mills. These gauges are also made with connections
through the back of the case.
The gauge illustrated in Fig. 108 is used in connection with hot
water heaters, denoting the height of column of water in the tank or
reservoir, one hand being painted red and the other black. As it is
necessary to have at all times in the tank or reservoir a certain
height of water, the red hand is set at the point on the dial which
denotes this height. The black hand is connected with the working parts
of the gauge and indicates on the dial the actual height of water in
the tank or reservoir.
The dial of this gauge is graduated in feet, instead of pounds.
A check valve is almost indispensable in using a hydraulic gauge, as
the pressure is often suddenly removed and the momentum of the hand
will throw the pinion out of gear with the toothed arc, and is liable
to break the hair-spring. A check valve prevents any trouble of this
kind and should always be used.