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Sensors: Temperature


Typical applications for temperature sensors include:

  • HVAC – room, duct, and refrigerant equipment
  • Motors – overload protection
  • Electronic circuits – semiconductor protection
  • Electronic assemblies – thermal management, temperature compensation
  • Process control – temperature regulation
  • Automotive – air and oil temperature
  • Appliances – heating and cooling temperature


Sensor Types

  1. Thermocouples – Thermocouples are pairs of dissimilar metal alloy wires joined at least at one end, which generate a net thermoelectric voltage between the two ends according to the size of the temperature difference between the ends, the relative Seebeck coefficient of the wire pair and the uniformity of the wire’s relative Seebeck coefficient.
  2. Thermistors – Thermistors (Resistance Thermometers) are instruments used to measure temperature by relating the change in resistance as a function of temperature.
  3. Radiation Pyrometer – A device to measure temperature by sensing the thermal radiation emitted from the object.
  4. Radiation Thermometers (Optical Pyrometers and Infrared Thermometers) – Optical Pyrometers are devices used to measure temperature of an object at high temperatures by sensing the brightness of an objects surface.
  5. Resistance Temperature Detectors (RTDs) – RTD’s (Resistance Temperature Detectors) are precision, wire-wound resistors with a known temperature resistance characteristic. In operation, the RTD is usually wired into a specific type of circuit (wheatstone bridge). They are nearly linear over a wide range of temperatures and can be made small enough to have response times of a fraction of a second. They require an electrical current to produce a voltage drop across the sensor that can be then measured by a calibrated read-out device. The output of this circuit can be used to drive a meter which has been calibrated in temperature, or to operate a relay to sound an alarm or shut down the motor. The Platinum RTD is the most accurate and stable temperature detector from zero to about 500°C. It can measure temperatures up to 800°C. The resistance of the RTD changes as a function of absolute temperature, so it is categorized as one of the absolute temperature devices. (In contrast, the thermocouple cannot measure absolute temperature; it can only measure relative temperature.)
  6. Fiber Optic Temperature Sensors – Optical-based temperature sensors provide accurate and stable remote measurement of on-line temperatures in hazardous environments and in environments having high ambient electromagnetic fields without the need for calibration of individual probes and sensors.

    Optical temperature sensor systems measure temperatures from -200C to 600C safely and accurately even in extremely hazardous, corrosive, and high electro-magnetic field environments. They are ideal for use in these conditions because their glass-based technology is inherently immune to electrical interference and corrosion. Since there is no need to recalibrate individual sensors, operator and technician safety is greatly enhanced as the need for their repeated exposure to field conditions is eliminated.

    Probes are made from largely non-conducting and low thermal conductance material, resulting in high stability and low susceptibility to interference, and in increased operator safety. Optical cables also have a much higher information-carrying capacity and are far less subject to interference than electrical conductors.

  7. Silicon Temperature Sensors – Integrated circuit temperature sensors differ significantly from the other types in a couple of important ways. The first is operating temperature range. A temperature sensor IC can operate over the nominal IC temperature range of -55 C to +150 C. Some devices go beyond this range while others, because of package or cost constraints, operate over a narrower range. The second difference is functionality. A silicon temperature sensor is an integrated circuit, including extensive signal processing circuitry within the same package as the sensor.

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