Quantcast Sensors: For Controls Without Digital Displays

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Next, turn the dial slowly downward to lower temperature differential settings. This
increases the resistance shown to the collector sensor terminal, and at the "off"
differential, the control should shut off.
Many testers also include a high limit test function. Generally, the tester shorts out
the collector sensor's terminals, to ensure the control is trying to run the system.
Then, the resistance supplied to the storage sensor terminals is reduced, simulating
a rising storage temperature. When the high limit is reached, the control should
shut off.
The dials on most testers are not very accurate. If the control has a digital display,
use that instead of the numbers on the tester dial. In descending order of accuracy,
the dials and readouts encountered during control and sensor testing are:
Control digital display
Most accurate:
Digital ohmmeter
Analog ohmmeter
Resistor color code
Control tester dial
Least accurate:
Control adjustment pot dials
Sensors: For Controls Without Digital Displays
To check suspicious sensors, disconnect the wires from their terminals at the
control. With all sensors removed, the control should be off when it is in the "auto"
position. Leave the connections intact at the sensor for the time being. Using an
ohmmeter and the appropriate temperature vs. resistance chart (Table 3-3 or 3-4),
determine if the resistance of the sensor is appropriate for the temperature it should
be measuring. If using an analog meter with several resistance scales, the 100 x R
scale is normally the most useful.
Confirm also that the sensor resistance changes as the sensor temperature
changes. This may require warming a cool sensor in your hand, or moving a sensor
off a warm collector or tank into the cooler air.
Do not immerse sensors in warm or cold water. Most sensors
are not waterproof and can be damaged with water. Resistance
readings of an immersed sensor will not be correct. If
temperature extremes are needed to check the sensor, wrap it
in a plastic bag before immersing it in water.


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