possible to get wider variation in differential
settings.
Adjustments
To set the operating range of the switch, turn
the differential adjustment screw (fig. 5-11)
counterclockwise against the stop for minimum
differential. Bring the temperature to the value
at which the circuit is to be closed. If the switch
contacts are open at this temperature, turn the
range screw slowly clockwise until the contacts
close. If the contacts are closed when the desired
temperature is reached, turn the range screw
counterclockwise until the contacts open; then
turn the screw slowly clockwise until the contacts
close. These adjustments set the closing
temperature.
The temperature is now raised to the point
where the circuit is to be opened. Since the
differential adjustment is now set at minimum,
the circuit will probably open at a lower
temperature than desired; therefore, turn the
Figure 5-10.--Temperature switch with two types of
differential adjustment screw clockwise to widen
sensing elements. A . Bulb unit. B. Helix unit.
the differential until the desired opening
temperature is obtained. For further instructions
on how to adjust these switches, refer to the
control circuits. For the thermal unit to be most
manufacturers' technical manuals.
effective, it must be located at a point of
unrestricted circulation. This is so it can "feel"
the average temperature of the substance you wish
PRESSURE MEASURING
to control.
INSTRUMENTS
Temperature changes cause a change in the
Pressure, like temperature, is one of the basic
volume of the sealed-in gas, which causes move-
engineering measurements and one that must be
m e n t of a diaphragm. The movement is
transmitted by a plunger to the switch arm. The
moving contact is on the arm. A fixed contact may
be arranged so the switch will open or close on
a temperature rise. This allows the switch contacts
to be arranged to close when the temperature
drops to a predetermined value and to open when
the temperature rises to the desired value. The
reverse action can be obtained by a change in the
contact positions.
The difference in temperature for the contact
opening set point and closing set point is the
differential. The switch mechanism has a built-in
differential adjustment so the differential can
be varied over a small range. Once set, the
differential remains essentially constant at all
temperature settings.
Some switches are stamped WIDE DIF-
FERENTIAL. These switches are adjusted in the
same manner described for the regular controls.
However, because of slight design changes, it is
Figure 5-11.--Temperature switch.
5-9
