Temperature sensors measure temperature by detecting a change in a physical property. This change then translates into a readable signal. The specific property used varies depending on the type of sensor.
Temperature sensors exploit different physical phenomena to determine temperature. Here's a breakdown of common methods:
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Thermocouples: These sensors rely on the Seebeck effect, which generates a voltage when two different metals are joined together and that junction is heated. The voltage is directly related to the temperature difference between the junction and a reference point.
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Resistance Temperature Detectors (RTDs): RTDs utilize the principle that the electrical resistance of a metal changes predictably with temperature. Typically, platinum is used because of its stable and linear resistance-temperature relationship over a wide range. As temperature increases, resistance increases.
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Thermistors: Similar to RTDs, thermistors measure temperature based on resistance changes. However, thermistors are made of semiconductor materials and exhibit a much larger change in resistance per degree Celsius than RTDs. They can be either Negative Temperature Coefficient (NTC) – resistance decreases with increasing temperature – or Positive Temperature Coefficient (PTC) – resistance increases with increasing temperature.
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Semiconductor-Based Sensors (e.g., Diodes and Integrated Circuits): These sensors often exploit the temperature dependence of the voltage across a diode junction. As temperature increases, the voltage drop across a forward-biased diode decreases. Integrated circuit (IC) temperature sensors can contain sophisticated circuitry to amplify, linearize, and convert this voltage change into a digital or analog output signal. According to the reference, a voltage increase at the diode terminals indicates a temperature rise.
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Infrared (IR) Sensors: These sensors measure temperature by detecting the infrared radiation emitted by an object. All objects above absolute zero emit infrared radiation, and the amount of radiation increases with temperature. These sensors are often used for non-contact temperature measurements.
Summary of Temperature Sensor Types:
| Sensor Type | Sensing Principle | Advantages | Disadvantages |
|---|---|---|---|
| Thermocouple | Seebeck Effect (voltage generation due to temperature difference) | Wide temperature range, robust, inexpensive | Low accuracy, requires cold-junction compensation |
| RTD | Resistance change with temperature | High accuracy, stable, linear response | Slow response time, relatively expensive, self-heating effects |
| Thermistor | Resistance change with temperature | High sensitivity, inexpensive | Non-linear response, limited temperature range |
| Semiconductor Sensors | Voltage/current change in semiconductor junctions | Small size, low cost, integrated signal conditioning | Limited temperature range, less robust |
| Infrared Sensors | Detection of infrared radiation | Non-contact measurement, fast response time | Affected by emissivity of the object, sensitive to environmental factors |
The choice of temperature sensor depends on factors such as the required accuracy, temperature range, response time, cost, and application environment. The sensor output (voltage, resistance, current) is then processed by electronic circuitry to display or transmit the temperature reading.
