What is the Difference Between a Bimetal Thermometer and a Pressure Thermometer

What is the Difference Between a Bimetal Thermometer and a Pressure Thermometer

1. Bimetallic Thermometer

WSS series bimetallic thermometer measures low and medium temperature field detection instruments. The bimetal thermometer can be -80 ° C – +500 ° C range of liquid, steam, and gas media to measure the temperature of various production processes directly.

Bimetallic Thermometer

1.1 Main features

  • On-site temperature display
  • Intuitive and convenient
  • Safe and reliable
  • Long service life
  • A variety of structural forms
  • Can meet different requirements.

1.2 Classify

Ordinary bimetallic thermometer, shock-resistant bimetallic thermometer, electric node bimetallic thermometer. According to the bimetallic thermometer pointer disc, the protection of the direction of the tube connection can be divided into bimetallic thermometer axial type, radial type, 135 ° to the kind, and universal type four.

  • Axial type bimetallic thermometer: pointer disc and the protection of vertical connection.
  • Radial type bimetallic thermometer: pointer disc and the protection of the parallel connection.
  • 135 ° to the type of bimetallic thermometer: pointer disc and the protection of the tube into 135 ° connection.
  • Universal type bimetal thermometer: pointer disc and protection tube connection angle can be adjusted.

1.3 Selection and use

In selecting bimetallic thermometers, fully consider the actual application environment and requirements, such as dial diameter, accuracy level, installation and fixation, the type of medium to be measured, and environmental hazards.

Besides, it also pays attention to cost-effectiveness, maintenance workload, and other factors. Additionally, the bimetallic thermometer in the use of the process should pay attention to the following points:

  • Bimetallic thermometer protection tube immersed in the measured medium length must be greater than the length of the temperature-sensitive components. It is generally immersed in the length of more than 100mm, 0-50 ℃ range of immersed in the length of more than 150mm, to ensure the accuracy of the measurement.
  • Various types of bimetallic thermometers should not be used to measure the temperature of a medium in an open container. And electric contact thermometers should not be used in environments with high vibrations within the control loop.
  • Bimetallic thermometer in storage, use installation and transport, should avoid collision with the protective tube, do not make the protective tube bending deformation, and use the table as a spanner too.
  • Thermometer in normal use should be regularly inspected. Generally, every six months is appropriate. An electric contact thermometer is not allowed to work under strong vibration to not affect the contact’s reliability.
  • The instrument’s temperature often works best in the scale range of 1 / 3 ~ 2 / 3.

2. Pressure Thermometer

The principle of pressure thermometer is based on the closed temperature measurement system within the saturated vapor pressure of the evaporating liquid and the temperature of the relationship between the changes in temperature measurement. When the temperature packet feels the temperature change, the saturated vapor within the closed system produces the corresponding pressure, causing changes in the curvature of the elastic element so that the free end of the displacement, and then by the gear amplification mechanism to the displacement into the indication value.

2.1 Composition and Classification

The sensitive element temperature package, pressure transfer capillary tube, and spring tube pressure gauge composition use a pressure thermometer.

  • If the system is filled with gas, such as nitrogen, known as an inflatable pressure thermometer, the upper limit of temperature measurement is up to 500 ℃, the relationship between pressure and temperature is close to linear, but the temperature packet volume is large, large thermal inertia.
  • If filled with liquid, such as xylene, methanol, etc., the temperature package is smaller, with the temperature measurement range of -40 ℃ ~ 200 ℃ and -40 ℃ ~ 170 ℃, respectively.”
  • If it is filled with liquid with a low boiling point, its saturated vapor pressure should be changed with the measured temperature, such as acetone, used for 50℃~200℃. However, the thermometer scale is non-uniform because the saturated vapor pressure and temperature are non-linear.

2.2 Features:

  • The temperature package must be fully immersed in the measured medium.
  • The maximum length of the capillary tube does not exceed 60m.
  • The instrument’s accuracy is low, but easy to use and vibration-resistant.

3. Differences Between Bimetallic Thermometers and Pressure Thermometers

Pressure thermometers and bimetallic thermometers are expected temperature-measuring devices.

Bimetallic thermometer with thermowell

3.1 Definition and principle of operation

3.1.1 Pressure thermometer

Pressure thermometers use the change in temperature against pressure to achieve temperature measurement. It usually consists of a closed vessel and a sensor that measures the pressure of the measured medium. As the temperature of the measured medium changes, the pressure inside the ship changes accordingly. It also uses a sensor that can convert the pressure into an electrical signal, which is then converted into a temperature value by a conversion circuit, thus achieving temperature measurement.

3.1.2 Bimetallic Thermometer

Bimetallic thermometers are based on the thermal expansion characteristics of bimetallic materials to achieve temperature measurement. A bimetallic thermometer consists of two metal strips. The coefficient of thermal expansion of the two metals is different; when the temperature changes, the length of the two metals will also have different degrees of change, resulting in the bending deformation of the entire bimetallic material. This deformation can be used by measuring the temperature, usually installed in the bimetallic strip resistor, through the measurement of the resistance value to calculate the temperature value.

3.2 Comparison of advantages and disadvantages

From the temperature range, accuracy, response speed, and the use of the scene to compare, pressure thermometers and bimetallic thermometers have advantages and disadvantages.

3.2.1 Temperature Range

Pressure thermometers are generally suitable for low-temperature range (-200 ℃ ~ 600 ℃), while the measurement range of bimetal thermometers is generally -70 ℃ ~ 250 ℃.

3.2.2 Accuracy

Bimetallic thermometers have higher measurement accuracy in the same temperature range than pressure thermometers.

3.2.3 Response speed

The pressure thermometer response speed is faster and can measure the temperature value in a shorter time, while the bimetal thermometer response speed is slower.

3.2.4 Usage Scenarios

Pressure thermometers are suitable for measuring the temperature of pipelines with smaller diameters. They can measure the temperature of gases, liquids, and even vapors. However, bimetal thermometers are suitable for applications that require accurate measurement of temperatures as low as -70℃, such as in the manufacture of electronic devices, food processing, biopharmaceuticals, and other fields of application.

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