Bimetal thermometer is a kind of field instrument for measuring medium and low temperature. Bimetal thermometer can directly measure the temperature of liquid, steam and gas medium in the range of -80℃ ~ +500℃ in various production processes.

Bimetal thermometer working principle is the use of two different temperature expansion coefficient of metal, in order to improve the temperature sensitivity, usually made of metal spiral coil shape, when the temperature of the multi-layer metal sheet changes, each layer of metal expansion or contraction amount is different, making the spiral coil rolled up or loosened.

Because one end of the spiral coil is fixed and the other end is connected to a freely rotating pointer, the temperature can be indicated on a circular indexing scale when the bimetallic sheet senses a change in temperature.

Working principle of bimetallic thermometer

Bimetal thermometer is a temperature measuring device. It uses a bimetal to convert the temperature of the medium into mechanical displacement. The bimetal consists of two different metals with different coefficients of thermal expansion. Bimetal thermometers are used in residential equipment such as air conditioners and ovens and industrial equipment such as heaters, hot wires, and oil refineries. They are a simple, durable and cost effective way to measure temperature.

The bimetal thermometer works by using two basic properties of metals.

The thermal expansion properties of metals
At the same temperature, different metals have different coefficients of thermal expansion.

The main component of a bimetallic thermometer is the bimetal. The bimetal strip consists of two thin strips of different metals, each with a different coefficient of thermal expansion. Thermal expansion is the property of a metal to change its shape or volume in response to a change in temperature. The metal strips are joined along their length by fusing or riveting them together. The strips are fixed at one end and free to move at the other.

The two metals usually used are steel and copper, but steel and brass can also be used. Because of their different thermal expansion, the lengths of these metals change at different rates at the same temperature. Due to this property, when the temperature changes, one side of the metal strip expands while the other side does not, thus creating a bending effect.

As the temperature increases, the strip shifts toward the metal with the lower temperature coefficient. When the temperature decreases, the strip bends toward the metal with the higher temperature coefficient. The deflection of the strip indicates the temperature change. This bending motion is connected to a dial on the thermometer that outputs the temperature of the medium. Calibration is an important step to ensure correct temperature readings.

Advantages and disadvantages of bimetal thermometer

Benefits of bimetal thermometers include:

Simple and robust design
Cheaper than other thermometers
They are completely mechanical and do not require any power to operate
Easy to install and maintain
Near linear response to temperature changes
Suitable for a wide temperature range

Some disadvantages of bimetal thermometers are:

They are not recommended for use at very high temperatures
They may require frequent calibration.
May not give accurate low temperature readings.
Rough handling can interfere with calibration

Types of bimetallic thermometers

There are two types of bimetallic thermometers, spiral strip bimetallic thermometers and spiral strip bimetallic thermometers. Spiral and spiral strips are used to keep the thermometer size within manageable limits.

Spiral band bimetal thermometer

As the name suggests, this type of thermometer uses a spiral-shaped bimetal to measure temperature. The pointer is connected by a shaft at the free end of the strip. The strip is spirally wound inside the stem. As the temperature increases, the spiral strip senses the temperature change. The strip metal, which has a high coefficient of thermal expansion, expands and coils up along the stem, which causes the shaft to rotate. This rotation causes the pointer to move its position in the dial indicating the temperature of the medium. As the temperature decreases, the metal with the lower coefficient of thermal expansion contracts and rotates the shaft. The pointer then reads the lower temperature in the dial.

They are primarily used in industrial applications because they can be placed inside thermocouple sleeves to provide operation in high temperature and high pressure environments.

Spiral Strip Bimetal Thermometer

Spiral bars are used to measure the temperature in a bimetallic spiral bar thermometer. As the temperature increases, the two metal strips expand differently. This creates a bending effect and the strip coil proceeds in such a way that the metal with the higher thermal coefficient forms the outer side of the arc. As the temperature decreases, the metal with the lower thermal coefficient forms the inner layer of the arc. A pointer and dial attached to the spiral read this deformation indicating the temperature of the medium.

These are mainly used in thermostats or to measure the ambient temperature because they are sensitive to lower temperature changes.

Selection standards

When selecting a bimetal thermometer for your application, the following selection criteria should be considered.

Temperature range: The bimetal thermometer should be within the upper and lower temperature limits. Due to extreme temperatures, the metal may reach its expansion limit and not bounce back, resulting in permanent damage to the thermometer.
Rod: The length and diameter of the bimetal rod thermometer rod should be determined based on the application requirements. This may require determining the immersion length or depth of the tank in which the thermometer will be used.
Thermocouple Sleeve: A thermocouple sleeve is a cylindrical tube fitting that protects the temperature sensor installed in an industrial application. It acts as a barrier to protect the expensive thermometer from any potential damage from the process fluid. Thermowells must be used where the stem may be exposed to extreme temperatures, pressures, high velocities, or corrosive fluids. With the thermowell installed, the thermometer can be easily removed and replaced without stopping the process. Because the thermowell protects the thermometer, it lasts longer, reducing maintenance and replacement costs.
Thermometer type: Bimetal thermometers can have either spiral strips or spiral strips. Spiral strip thermometers are the preferred choice for industrial applications such as refineries and oil burners. The bimetal strip spirals are wound inside the stem and they can be supported by a thermowell to operate at extreme temperatures and pressures. Spiral strip thermometers are used in thermostats because they are sensitive to low temperature changes.

Bimetal thermometer calibration

The most accurate method of calibrating a bimetal thermometer is the freezing point method. To calibrate a bimetal thermometer using this method, fill the glass completely with ice, add cold water, and let it sit for 4 to 5 minutes. Then, insert the stem of the thermometer into the ice water. Make sure the stem is not touching the bottom or sides of the glass. Let it sit until the dial stops moving. If the thermometer is accurate, it should measure 0°C or 32°F. If not, turn the nut located under the dial so that it reads 0°C. Check periodically to ensure accuracy. Depending on the requirements of your application, a weekly or monthly thermometer calibration process should be implemented.

Applications

Applications in industrial production

Petrochemical industry: Measure the temperature inside the reactor, pipeline, storage tank and other equipments to control the reaction process and ensure the product quality.
Metallurgical industry: Monitor furnace temperature, steel temperature, etc. to control the smelting process.
Machinery Manufacturing: Measure the temperature of machine tools, molds and other equipment to ensure processing accuracy.
Food Processing: Monitor the temperature during food processing, such as baking, sterilization, etc.
Power industry: Monitor the temperature of transformers, motors and other equipment to prevent overheating.

Applications in Daily Life

Household appliances: such as refrigerators, air conditioners, water heaters, etc., used to control the temperature and improve the efficiency of use.
Automotive: Measure engine water temperature, oil temperature, etc., to monitor vehicle operation status.
Construction industry: Measurement of indoor and outdoor temperatures for control of air conditioning systems.

Other applications

Laboratory: Measurement of temperature in experimental devices.
Meteorological observation: Measurement of air temperature.
Agriculture: Measurement of soil temperature, greenhouse temperature.

Thermometer