and if the gauge reading is equal to the pressure
represented by the total weight added, the gauge is
accurate. If the gauge is not accurate, it must be
adjusted to read correctly.
When a Bourdon-tube pressure gauge is in-
accurate, the following adjustments should be made:
1. If the pointer travels too far or not far enough as
each weight is applied, change the ratio of movement
between the Bourdon tube and the pointer. The
movement of the sector gear meshes with a pinion on
the pointer spindle. Lengthening the distance between
the spindle and the link connection to the sector gear
reduces the amount of travel to the pointer. Shortening
this distance increases the amount of travel.
2. If the amount of travel is correct as each weight
is added but the total reading is wrong, the pointer must
be reset. Gauges of recent design have a countersunk
split-head screw in the dial for setting the pointer. On
some older types of gauges, the pointer must be pulled
and reset. Pointer pullers are supplied with the gauge-
3. If the gauge cannot be made to read correctly
over the entire scale, it should be adjusted so the reading
is correct at the working pressure. A table or curve
should then be made that shows the corrections required
for other readings.
From time to time, you may be required to adjust
the diaphragm type of air pressure gauge, as shown in
figure 5-56. The zero adjustment on these gauges
should be checked frequently. Each gauge of this type
has a three-way cock that can be turned to shut off the
gauge without disconnecting the gauge piping. When
the handle of the three-way cock is at right angles to the
valve body, the gauge unit is open to the outside air
pressure, and the reading on the scale should be zero.
When the handle of the cock is parallel to the valve
body, the gauge is open to the pressure in the line. A
zero adjusting screw is provided either below the
gauge or on one side of the gauge. Turn this screw in or
out to bring the pointer to zero while the handle of the
three-way cock is at right angles to the valve body.
After making this zero adjustment, restore the gauge to
service by turning the cock handle so it is again parallel
with the valve body.
Figure 5-56.Diaphragm type of pressure gauge.
If you are required to replace any part of a pressure
gauge, handle the mechanism carefully so none of the
elements are bent or distorted.
You may occasionally have to replace the
diaphragm in a diaphragm type of air pressure gauge
(fig. 5-56). First, disconnect the pressure line below
the unit. Remove the outside zero adjustment screw
and the three-way cock (with its coupling), and remove
the unit from its case.
Disassemble the unit by compressing the small
spring on top of the calibrating spring to loosen the
retaining pin. Remove the stem that holds the
calibrating spring, and remove the screws (usually 10)
around the edge of the diaphragm housing. The oil
diaphragm can then be lifted out.
Clean both surfaces of the housing. Apply a small
amount of gasket cement to the edge of the lower
housing, and immediately place the new diaphragm-
gasket assembly over the edge of the lower housing.
Replace the top housing. Tighten the screws, being
careful to draw them up uniformly. Replace the
calibrating spring stem, compress the spring, and
insert the retaining pin in the stem. Then replace the
unit in the case. The gauge is ready for service.
Q27. On a deadweight tester, what is used to apply
pressure to the gauge for testing?
Q28. What type of gauge has a zero adjusting screw?