A temperature measuring instrument is a device used to measure and record the temperature of an object. This paper describes the working principle, classification and application of temperature measuring instrument.
First, the working principle of temperature measuring instrument
1.1 Thermal expansion method
1.1.1 Rotary thermometer: using the nature of material expansion under heat, such as alcohol, mercury, etc. filled in the glass tube, with the temperature change, the liquid volume expansion, so as to read the temperature through the scale.
1.1.2 Bimetallic sheet thermometer: Two metal sheets with different linear expansion coefficients are welded together, bent and deformed when heated, and the temperature is indicated by the transmission mechanism.
1.2 Thermoelectric effect
Thermocouple thermometer: The thermoelectric motive force generated at different temperatures by the two ends of a thermocouple composed of different metals is used to measure temperature.
Thermal resistance thermometer (resistance thermometer) : resistance materials such as platinum resistance, copper resistance, etc., the resistance value changes with the temperature, and the temperature is converted by the resistance value measured.
1.3 Heat conduction/heat radiation
Semiconductor temperature sensor: A sensor designed based on the fact that the resistivity of a semiconductor material changes significantly with temperature.
Infrared thermometer (non-contact) : Use the infrared radiation energy emitted by the object and its temperature relationship to determine the temperature.
1.4 Pressure thermometer
Using the principle that the working medium (liquid or gas) in the closed system changes the saturated vapor pressure with the change of temperature, the temperature is measured by the deformation of the elastic element.
Second, the classification of temperature measuring instruments
2.1 According to the temperature measurement method, it can be divided into contact thermometer and non-contact thermometer
2.1.1 Contact thermometer
Expansion thermometer: such as mercury thermometer, bimetal thermometer, based on the principle of thermal expansion of objects;
Resistance thermometer: including metal resistance thermometer (such as platinum resistance, copper resistance, etc.) and semiconductor resistance thermometer;
Thermocouple thermometer: using thermocouples formed by different metal combinations to generate electromotive force at different ends to measure temperature;
Thermistor thermometer: using the resistivity of some materials with the characteristics of temperature change for temperature measurement;
Liquid-expansion thermometers: thermometers, such as those filled with gas or oil, used for specific temperature ranges.
2.1.2 Non-contact thermometer
Optical pyrometer: such as an optical pyrometer, which determines temperature by measuring the intensity or wavelength of light emitted by an object;
Infrared thermometer (IR thermometer) : by detecting the infrared radiation energy emitted by the object to measure the temperature, without touching the object to be measured;
Far infrared thermometer: the use of advanced infrared sensing technology and signal processing technology, can be used for long-distance temperature measurement;
Radiation thermometer: The working principle is similar to the infrared thermometer, measuring the thermal radiation of the object, suitable for high temperature occasions.
2. 2 Classified by signal output and function:
Digital thermometer: directly display digital reading, some equipped with communication interface, support data transmission and remote monitoring;
Pointer thermometer: indicates the temperature by the position of the pointer on the dial;
Local display instrument: display temperature directly on site without external power supply or controller;
Temperature transmitter: Converts the temperature signal into a standard electrical signal output, such as 4-20mA or 0-10V analog signal;
Intelligent temperature transmitter: In addition to the basic signal conversion, it also has self-diagnosis, fault alarm, remote configuration and other functions;
Temperature recorder and temperature patrol instrument: used to record and monitor temperature changes at multiple points for a long time;
Temperature regulator: not only measures the temperature, but also automatically adjusts the output of heating or cooling equipment according to the set value to achieve temperature control.
2.3 Other Categories:
Gas thermometers, such as hydrogen or helium thermometers, for ultra-low temperature measurement;
High temperature thermometer: such as light pyrometer, colorimetric thermometer, radiation thermometer, for high temperature environment measurement;
Thermometers: Special body temperature measuring tools, including mercury thermometers, electronic thermometers and red ear temperature guns.
Third, the selection of temperature measuring instruments
When choosing a temperature measuring instrument, the following key factors need to be considered:
Measuring range: Select the appropriate instrument type according to the range of temperature to be measured. For example, for low temperature areas (-200℃ to 400℃), thermal resistance (Pt100, Cu50, etc.) can be selected; For high temperature areas (0 ° C to 1600 ° C or even higher), thermocouples (K, S, B, R, etc.) should be selected.
Accuracy requirements: If the accuracy requirements are high, such as chemical, pharmaceutical or laboratory applications, you can choose a liquid thermometer, precision thermal resistance or thermocouple; In the case of low precision requirements, bimetallic thermometers or ordinary thermocouples can meet the requirements.
Response speed: Fast response applications, such as the monitoring of transient temperature changes, need to choose a fast response sensor, such as a thin film thermal resistor or thermocouple.
Contact or non-contact measurement: contact measurement is suitable for direct contact with the measured medium, such as thermocouples, thermal resistance, etc. Non-contact measurement is suitable for applications where direct contact is not possible, such as infrared thermometers.
Working environment: Considering the environmental conditions, such as whether there is corrosive media, vibration, impact, high pressure, etc., it is necessary to select the corresponding protection level and material of the thermometer and protective sleeve.
Installation conditions: The installation position should ensure that the temperature measurement point is representative, and the layout of the junction box and protective sleeve should take into account waterproof, dustproof, shock-proof and other factors.
Signal transmission: If you need to remotely transmit signals or integrate them into the control system, you should choose a temperature meter with a transmission function, such as a thermal resistance temperature transmitter or thermocouple temperature transmitter, and consider using a two-wire, three-wire or four-wire connection.
Safety and reliability: For applications with high safety requirements, especially those involving SIS (Safety Instrumentation system), a temperature meter with a high reliability and redundancy design should be selected, and sometimes it is necessary to use two temperature measuring elements.
Economy: On the basis of meeting the measurement requirements, but also consider the acquisition cost, maintenance cost and service life, choose cost-effective instruments.
In short, the selection of temperature measuring instruments is a comprehensive consideration process, which needs to be combined with specific process conditions, equipment requirements, budget constraints and other factors to determine the most appropriate instrument type and model.
Fourth, the installation of temperature measuring elements
The temperature measured by the contact thermometer is determined by the temperature measuring (temperature sensing) element. After the correct selection of the temperature measuring element and the secondary instrument, if the correct installation of the temperature measuring element is not paid attention to, the measurement accuracy is still not guaranteed. In industry, it is generally installed according to the following requirements.
4.1 Installation requirements of temperature measuring elements
When measuring the temperature of the pipeline, it should be ensured that the temperature measuring element is in full contact with the fluid to reduce the measurement error. Therefore, it is required that the temperature measuring element should be inserted against the flow direction of the measured medium (countercurrent) during installation, and must be at least orthogonal to the measured medium (90°), and must not form a cocurrent with the measured medium.
The temperature sensing point of the temperature measuring element should be at the maximum flow rate in the pipeline. In general, the end of the thermocouple, platinum resistance, copper resistance protection sleeve should cross the beam center line 5~10mm, 50~70mm, 25~30mm.
The temperature measuring element should have sufficient insertion depth to reduce the measurement error. For this reason, the temperature measuring element should be installed at the Angle or at the elbow.
If the process pipe is too small (diameter less than 80mm), the expansion pipe should be installed where the temperature measuring element is installed
The junction box cover of the thermocouple and thermal resistance should be upward to avoid rain or other liquids or dirt entering the junction box and affecting the measurement.
In order to prevent heat loss, the temperature measuring element should be inserted in the pipe or equipment with a thermal insulation layer.
When the temperature measuring element is installed in the negative pressure pipeline, its tightness must be ensured to prevent the outside cold air from entering and reduce the reading.
4.2 Cabling Requirements
Use the compensation wire of the thermocouple according to the specified model, pay attention to the positive and negative poles of the thermocouple and the positive and negative poles of the compensation wire, do not connect wrong.
The line resistance of the thermal resistance must meet the requirements of the secondary instrument.
In order to protect the connecting wire and compensation wire from external mechanical damage, the connecting wire or compensation wire should be inserted into the steel pipe or trough plate.
Wires should be as far as possible to avoid connections. It should have good insulation, and it is forbidden to share a threading pipe with AC power lines, so as not to cause induction.
The wire should avoid AC power wires as far as possible.
The compensation wire should not have an intermediate connector, otherwise a junction box should be installed. In addition, it is best to lay separately from other wires.
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