1. Basic principles
The bimetallic thermometer is based on two metal sheets with different coefficients of thermal expansion bonded together. When the temperature changes, bimetallic sheets will undergo bending deformation due to the different degrees of expansion or contraction of the two metals. This deformation is transmitted to the pointer through a mechanical transmission mechanism, indicating the corresponding temperature value on the dial. For example, a common combination is brass and Invar, with brass having a higher coefficient of thermal expansion and Invar having a lower coefficient of thermal expansion. As the temperature increases, the bimetallic strip bends towards the Invar side.
2. Structural composition
Temperature sensing element: made of bimetallic sheets, it is the core component of the thermometer, responsible for sensing temperature changes and generating deformation.
Transmission mechanism: amplifies and transmits the slight deformation of the bimetallic sheet, driving the pointer to rotate. Generally includes components such as connecting rods and gears.
Pointer and dial: The pointer indicates the temperature reading, and the dial is marked with a temperature scale, usually in degrees Celsius (℃) or Fahrenheit (℉).
Protective tube: Wrap the temperature sensing element to provide protection from external environmental factors such as corrosion and mechanical damage. There are various materials for protective tubes, such as stainless steel, copper alloy, etc., which can be selected according to different usage environments.
3. Characteristics
advantage:
Intuitive reading: No additional auxiliary equipment is required, and the temperature can be directly read through the position of the pointer on the dial, which is simple and convenient.
Simple structure: Compared to some electronic thermometers, its structure is not complex, high reliability, and low maintenance costs.
Good seismic performance: The mechanical structure enables it to work stably even in environments with vibration, making it less susceptible to vibration interference and affecting measurement accuracy.
No need for external energy: It works by relying on the deformation of bimetallic sheets caused by temperature, without the need for an external power source or battery, and is suitable for situations where energy is not easily accessible.
Disadvantages:
Limited accuracy: The general accuracy is within ± 1% - ± 2% FS (full range), which is slightly lower than high-precision electronic thermometers and is not suitable for situations that require extremely high temperature accuracy.
Slow response speed: The deformation of bimetallic sheets requires a certain amount of time to respond to temperature changes, especially in the case of rapid temperature changes, where the update speed of their readings is relatively slow.
4. Classification
Classified by appearance:
Axial type: The pointer is parallel to the axial direction of the protective tube, and this type is suitable for installation on the side of pipelines or equipment, making it easy to observe temperature from the side.
Radial type: The pointer is radially perpendicular to the protective tube and is commonly installed at the top or bottom of the pipeline. It is convenient to read the temperature from the front.
Universal type: The header and protective tube are connected by a universal joint, which can flexibly adjust the angle of the header to meet different observation position requirements.
Classified by usage environment:
Ordinary type: suitable for general industrial and civilian environments, with no strict requirements for environmental conditions such as temperature and humidity.
Anti corrosion type: Special anti-corrosion materials are used to make protective pipes and related components, which can be used in environments with corrosive gases or liquids, such as measuring some corrosive media in chemical production.
Explosion proof type: Complies with relevant explosion-proof standards, has explosion-proof structure and electrical performance, and can be used in hazardous areas with flammable and explosive gases or dust, such as hazardous areas in industries such as petrochemicals and coal mines.
5. Application Fields
Industrial production: Used for temperature measurement in various process industries such as chemical, refining, power, steel, etc., such as temperature monitoring of equipment such as reaction vessels, pipelines, heat exchangers, etc., to ensure that the production process is carried out within the appropriate temperature range. For example, in chemical reaction vessels, bimetallic thermometers monitor the reaction temperature in real time to prevent excessive or insufficient temperature from affecting the reaction effect and even causing safety accidents.
HVAC: In the heating, ventilation, and air conditioning systems of buildings, it is used to measure the temperature of media such as air and water, help regulate system operation, and achieve a comfortable indoor environment. For example, installing a bimetallic thermometer in the air duct of an air conditioning system can adjust the cooling or heating power of the air conditioner according to the temperature.
Agriculture: Monitoring indoor temperature in greenhouses to provide a suitable temperature environment for crop growth. Staff can adjust ventilation, heating, or cooling equipment in a timely manner based on the readings of the bimetallic thermometer.
Daily life: Some household appliances such as ovens and water heaters also use bimetallic thermometers to control temperature, ensuring the normal use and safety of the products. For example, a bimetallic thermometer in an oven can control the operation of heating elements to maintain the temperature inside the oven within a set range, ensuring the effectiveness of food baking.
