17 - Temperature Sensors * Flashcards
What is a temperature sensor
A temperature sensor is a device that provides temperature measurement in a readable form through an electrical signal.
What are some different kinds of temperature sensors?
Thermocouple: A device made of a pair of two dissimilar wires joined at one end that can generate a voltage based on the amount of heat applied to it. Each wire is made of a different metal. The end of the wires that are welded together is called the hot junction, the other end of the wires is connected into circuity. When the joint is heated, a small voltage develops between the two metals. This voltage can be measured at the other ends of the two wires with a sensitive voltmeter. Multiple thermocouples can be wired together in series to create a device called a thermopile.
Thermistor: A solid-state device that changes its resistance as its temperature changes.
Thermometer: An instrument used for measuring temperatures
What is a primary sensing element, how to check it
Primary sensing element is the material used to produce the electric reading. (Ex: thermocouple, thermistor)
They can be checked using the following methods:
- Visual inspection for damage, exposed wire, cracks
- Resistance reading (Thermistor)
- Millivolt reading (Thermocouple)
Where would you normally find them
Thermocouples are often seen used in pilot light assemblies, pipes, and controlled rooms where access is difficult. Thermocouples are also often used in temperature clamps and sensing devices technicians use to check superheat and sub-cool.
They are commonly used in low voltage electronic applications to function as a circuit protection device in circuits and motors. They can also be used in buildings as remote temperature sensing devices, and permit a technician to monitor the temperature in a remote location from an operating console.
How to determine one has failed
A faulty sensing element can produce the following errors:
- an OL or excessively high/low reading
- Erratic, Inconsistent, or inaccurate readings
- Static reading in the presence of temperature changes
How to replace one
Determine the part needed for replacement. Remove old part and replace with new one. Test for accurate reading.
How to calibrate one, how to determine the desired calibration
How to check and verify polarity, and determine what the desired configuration is
How to check controller parameters
How to determine what the correct type of sensor is
What is a well
Clean exhaust port (temperature flapper)
How to replace a thermostat, or determine one has failed
Thermocouple strength is measured in?
Millivolts
