The principle of construction of the dial steam gauge is, that the
pressure may be indicated by means of a pointer in a divided dial
similar to a clock face, but marked in division, indicating pounds
pressure per square inch instead of hours and minutes.
Figs. 87 and 88 show the ordinary style of gauge which consists of an
elliptical tube, connected at one end to a steam pipe in communication
with the boiler pressure and at the other end with gearing to a pointer
spindle as shown in cut.
An inverted syphon pipe is usually formed under the gauge, its object
being to contain water and thus prevent the heat of the steam injuring
the machinery of the gauge, or distorting its action by expansion.
[Illustration: Fig. 87.]
[Illustration: Fig. 88.]
[Illustration: Fig. 89.]
A small drain cock should be fitted to the leg of the syphon of a steam
gauge, leading to the boiler, at a level with the highest point the
water can rise in the other leg, otherwise an increased pressure will
be indicated, due to the head of water which would otherwise collect in
the boiler leg of the syphon.
Steam gauges indicate the pressure of steam above the atmosphere only,
the total pressure being measured from a perfect vacuum which will add
14-7/10 lbs. on the average to the pressure shown on the steam gauge.
These gauges are apt to get out of order in consequence of water
lodging in the end of the heat tube and corroding the latter. It may
be easily known when they are out of order by raising the pressure of
the steam in the boiler and watching when it commences to blow off at
the safety valve, and then noting the position of the index finger. The
pressure registered by the finger should, of course, then correspond
with the known blow off pressure of the valves; if it does not, one or
the other or both of these instruments must be out of order; therefore,
when this is the case and a disagreement occurs, the steam gauge may be
presumed to need correction.
It should also be noted that the steam gauge finger points to zero when
steam pressure is cut off. A two-way cock should be used for closing
the connection between the steam gauge and the boiler, and at the same
time to let air into the steam gauge.
The steam should never be allowed to act directly on a steam gauge when
located in cold situations where they are liable to freeze. The valve
on the boiler should be closed and the water allowed to drip out, and,
before the steam is turned on from the boiler, the drip on the gauge
should be closed, in order that sufficient steam may be condensed in
the pipe to furnish the quantity of water necessary to keep the steam
from striking the gauge.
_A ready method for being always able to prove the correctness of your
steam gauge._
When steam is at some point not over half the usual pressure, place the
ball on the safety valve at the point where it commences to blow off
and mark the place. Move the ball twice as far from the fulcrum as this
mark, and it should blow off at twice the pressure as indicated by the
gauge, or it is not right. Any other relative distance may be used to
advantage.