Sensors Designed Based on the Principle of Metal Expansion
Metals expand in response to changes in ambient temperature, and sensors can convert this response into signals in various ways.
Bimetallic Strip Sensors: Bimetallic strips consist of two metals with different coefficients of expansion bonded together. With temperature changes, material A expands more than the other metal, causing the strip to bend. The curvature of this bend can be converted into an output signal.
Bimetallic Rod and Tube Sensors: As temperature increases, the length of the metal tube (material A) increases, while the length of the non-expanding steel rod (metal B) does not. This linear expansion of the metal tube due to the change in position can be transmitted. Conversely, this linear expansion can be converted into an output signal.
Sensors Designed Based on the Deformation Curves of Liquids and Gases: Liquids and gases also undergo volume changes with temperature.
Various structures can convert this expansion change into a position change, thus generating a position change output (potentiometers, sensing bias, baffles, etc.).
Resistance Sensing: The resistance of a metal changes with temperature.
For different metals, the change in resistance varies with each degree Celsius change in temperature, and this resistance value can be directly used as the output signal.
There are two types of resistance changes:
Positive Temperature Coefficient: Temperature increase = Resistance increase
Temperature decrease = Resistance decrease
Negative Temperature Coefficient: Temperature increase = Resistance decrease
Temperature decrease = Resistance increase
Thermocouple Sensing
A thermocouple consists of two metal wires of different materials welded together at their ends. By measuring the ambient temperature of the unheated area, the temperature of the heated point can be accurately determined. Because it requires two conductors of different materials, it is called a thermocouple. Thermocouples made of different materials are used in different temperature ranges, and their sensitivities also vary. Thermocouple sensitivity refers to the change in output potential difference when the temperature of the heated point changes by 1°C. For most thermocouples supported by metal materials, this value is approximately between 5 and 40 microvolts/°C.
Because the sensitivity of a thermocouple temperature sensor is independent of the thickness of the material, temperature sensors can be made using very thin materials. Because the metal materials used to make thermocouples have excellent ductility, these tiny temperature sensing elements have extremely high response speeds and can measure rapidly changing processes.