2. Have the gauges well lighted to make the correct
reading easier to take.
3. Keep the covers tight, and replace broken glass
4. Protect the gauges as far as possible from
vibration, excessive temperatures, corrosive liquids,
and rapid changes in pressure.
Whenever you believe that a gauge is not accurate,
test it with a deadweight tester. Figure 5-55 shows such
a tester. In this testing device, the gauge under test is
subjected to pressure by applying weights to a plunger.
The plunger is accurately fitted into a vertical cylinder
that contains a water-base hydraulic fluid.
Mineral oil or other petroleum products must
NEVER be used in the deadweight pressure
Weights are applied to the plunger, and the
pressure is transmitted to the fluid and then to the
gauge by way of transmission piping and a control
valve. The plunger itself exerts a known pressure of
approximately 5 psi. Additional weights are provided
in sizes that exert pressures equal to 5, 10, and 20 psi.
In figure 5-55, notice that there are two horizontal
cylinders and a main vertical cylinder. The plunger in
cylinder A pumps hydraulic fluid into the instrument
when it is first filled. The plunger in cylinder B exerts
enough force on the fluid so the testing plunger
maintains the weight platform in position about 2
inches above the top of the vertical cylinder. At the
beginning of the test, the plunger in cylinder B should
be screwed out as far as it will go, so cylinder B (as well
as the main cylinder) fills with fluid. If the weighted
test plunger is pushed too far down at anytime during
the test, the plunger in cylinder B should be screwed in
as far as necessary to force the test plunger up so it will
have freedom of movement.
To test a pressure gauge in the deadweight tester,
connect the gauge to the apparatus and fill the tester
with the proper water-base hydraulic fluid, if
necessary. (Some testers of this type are designed to be
kept full of fluid at all times, but others require filling
before each use.) Then level the tester. Apply weights
to the testing plunger, as required, and check the
pressure gauge readings for accuracy. The plunger
should be gently rotated, as each weight is added, to
ensure its freedom of movement. If the gauge reading
increases by the proper amount as each weight is added
Figure 5-55.Deadweight pressure gauge testing apparatus.