How to correctly measure the accuracy of bimetallic thermometer

Bimetallic thermometers have the advantages of being able to display temperature on site, intuitive and convenient, safe and reliable, low cost and long service life. They are widely used in the fields of petroleum, chemical industry, metallurgy and textile. Bimetallic thermometers generally use pointers to indicate the on-site temperature value. The measurement process is manual direct reading, so the accuracy of the dial directly determines the temperature measurement accuracy.
Bimetallic thermometer is a field detection instrument suitable for measuring medium and low temperatures. It can directly measure the temperature of liquid, steam and gas medium from -80 to 500℃ in the production process. The main temperature sensing element of the bimetallic thermometer is a multi-layer metal sheet stacked together with two or more metal sheets. In order to improve the temperature measurement sensitivity, the metal sheet is usually made into a spiral roll shape. When the temperature of the metal sheet changes, each layer of metal expands or contracts, causing the spiral roll to roll up or loosen. Since one end of the spiral roll is fixed and the other end is connected to a freely rotatable pointer, when the temperature rises, the metal sheet with a larger expansion coefficient elongates more, causing the entire bimetallic sheet to bend toward the side of the metal sheet with a smaller expansion coefficient. The higher the temperature, the greater the bending degree of the spiral roll, which in turn drives the pointer to indicate the temperature value on the circular graduation scale. The nominal diameters of the dials of common industrial bimetallic thermometers are mainly 60, 100, 150mm, etc.

1. First, install it on a straight pipe section; secondly, make sure that the end of the sheath of the thermal resistor and bimetallic thermometer is in the center of the pipe. For flange connection, be careful not to make a mistake with the connector.

2. The connector depth of the thermal resistor and thermocouple should be greater than 40mm, 43mm, and the sealing surface should be outside.

3. The connector depth of the bimetallic thermometer should be less than 20mm, 16mm, and the sealing surface should be inside. Weld the connector to the DN25D flange.

4. The pressure nozzle of the thermal couple or bimetallic thermometer is at a 45° angle to the process pipeline; the insertion depth is 1/2 of the process pipeline; if the pipeline is less than 80, expand the pipe to 80. In the past, it was important to counterflow 45 degrees, but now it is basically installed vertically. Try to choose a protective sleeve connected with a flange. Most factories now use flange connections.

5. Especially platinum thermal resistors. When the detection temperature is relatively high (generally above 200 degrees), it must not be installed too tightly, and the thermometer sleeve must not be "airtight". Otherwise, the temperature measurement will be inaccurate. The difference is more than 2 degrees.
6. For ordinary assembled thermal resistors, those less than DN80 need to be installed with expansion tubes. The armored thermal resistors should be able to lower the standard and be constructed according to the "Automation Instrument Engineering Construction and Acceptance Specifications" GB50093-2002.
7. If the design institute has issued an installation drawing, just install it according to the installation drawing. Check whether it is flange connection or welding. If it is welding, ensure that the end is 1/3 in the middle of the pipeline. If the process pipe diameter is small, the pipe should be expanded to at least DN80.
8. The installation of thermocouples and thermal resistors, the specification says to insert between 1/3 and 2/3 of the pipeline, not necessarily the center position. At the same time, the specification also clearly states that in order to make thermocouples and thermal resistors interchangeable in the factory, they can be reduced to 2~3 specifications.
9. It doesn't matter whether it is installed in a straight pipe section or at a bend. The main thing is to pay attention to the insertion depth if it is installed in a straight pipe section, and to pay attention to the flow direction of the medium if it is installed at a bend. The connectors of the thermal resistor and bimetallic thermometer should be installed against the flow direction of the medium.