This article mainly explains eight commonly used temperature instruments, from their working principles and installation requirements to the issues that should be noted during product selection and use, as well as the composition of the instruments. It elaborates on the eight common temperature instruments, providing theoretical and experiential assistance to instrument professionals in their later work!
bimetallic thermometer
Working principle:
The working principle of a bimetallic thermometer is based on two metals with different coefficients of thermal expansion. To improve temperature measurement sensitivity, the metal strips are usually made into a spiral shape. When the temperature of the multi-layered metal strips changes, the expansion or contraction of each layer of metal is different, causing the spiral to roll up or unroll.
Since one end of the spiral is fixed and the other end is connected to a freely rotating pointer, the pointer can indicate the temperature on a circular scale when the bimetallic strip senses a temperature change.
The temperature measurement range of this instrument is generally between -80℃ and +500℃, and the allowable error is about 1.5% of the scale range.
Classification:
Ordinary bimetallic thermometers, shock-resistant bimetallic thermometers, and electric node bimetallic thermometers.
According to the connection direction between the pointer and the protective tube, bimetallic thermometers can be divided into four types: axial type, radial type, 135° type, and universal type.
① Axial type bimetallic thermometer: The pointer dial is vertically connected to the protective tube.
②Radial bimetallic thermometer: The pointer dial is connected in parallel with the protective tube.
③ 135° directional bimetallic thermometer: The pointer dial and the protective tube are connected at a 135° angle.
④ Universal bimetallic thermometer: The angle between the pointer and the protective tube can be adjusted arbitrarily.
Selection and Use:
When selecting a bimetallic thermometer, it is essential to fully consider the actual application environment and requirements, such as dial diameter, accuracy class, mounting method, type of measured medium, and environmental hazards. In addition, factors such as cost-effectiveness and maintenance workload should also be taken into account.
In addition, the following points should be noted when using bimetallic thermometers:
A. The length of the protective tube of the bimetallic thermometer immersed in the measured medium must be greater than the length of the temperature sensing element. Generally, the immersion length is greater than 100mm, and the immersion length for the 0-50℃ range is greater than 150mm, in order to ensure the accuracy of the measurement.
B. Bimetallic thermometers are not suitable for measuring the temperature of media in open containers, and thermometers with live contacts are not suitable for use in control circuits in environments with significant vibration.
C. During storage, use, installation and transportation of bimetallic thermometers, avoid collisions with the protective tube, and never bend or deform the protective tube or use the meter as a wrench.
D. Thermometers should be inspected periodically under normal use, generally every six months. Electrical contact thermometers should not be operated under strong vibrations to avoid affecting the reliability of the contacts.
E. The temperature at which the instrument operates most frequently is best between 1/3 and 2/3 of the scale range.
Pressure thermometer
Working principle:
The principle of a pressure thermometer is based on the relationship between the saturated vapor pressure and temperature of the evaporating liquid within a closed temperature measuring system. When the bulb senses a temperature change, the saturated vapor in the closed system generates a corresponding pressure, causing a change in the curvature of the elastic element, resulting in displacement at its free end. This displacement is then converted into an indicated value by a gear amplification mechanism.
Composition and classification:
A pressure thermometer consists of a sensing element (temperature bulb), a pressure-transmitting capillary tube, and a Bourdon tube pressure gauge.
If the system is filled with gas, such as nitrogen, it is called a gas-filled pressure thermometer. The upper limit of temperature measurement can reach 500℃, and the relationship between pressure and temperature is close to linear. However, the temperature bulb has a large volume and high thermal inertia.
If filled with liquid, such as xylene or methanol, the temperature bulb will be smaller, and the temperature measurement ranges will be -40℃ to 200℃ and -40℃ to 170℃, respectively.
If filled with a low-boiling-point liquid, its saturated vapor pressure should vary with the temperature being measured, such as acetone, used for temperatures ranging from 50℃ to 200℃. However, because the relationship between saturated vapor pressure and saturated vapor temperature is non-linear, the thermometer scale will not be uniform.
Features:
The temperature bulb must be completely immersed in the medium being measured; the capillary tube should not exceed 60m in length; the instrument has low accuracy but is easy to use and is vibration resistant.
resistance thermometer
Working principle:
The temperature measurement principle of a resistance temperature detector (RTD) is based on the characteristic that the resistance of a conductor or semiconductor changes with temperature, in order to measure temperature or temperature-related parameters.
The resistance of most metals changes with temperature; the higher the temperature, the greater the resistance, meaning they have a positive temperature coefficient of resistance. In contrast, most semiconductor materials have a negative temperature coefficient of resistance, meaning their resistance decreases as the temperature increases.
Composition material requirements
1. Stable chemical and physical properties within the temperature measurement range;
2. Good reproducibility;
3. A large temperature coefficient of resistance is used to achieve high sensitivity;
4. High resistivity allows for the creation of small-volume components;
5. The resistance temperature characteristic should be as close to linear as possible;
6. Low price.
Commonly used RTD (Resistant Temperature Detector) components:
Commonly used resistance temperature detectors (RTDs) include: platinum RTDs, copper RTDs, and semiconductor RTDs.
Platinum resistance thermometers (RTMs) are made of high-purity platinum wire and offer advantages such as high temperature measurement accuracy, stable performance, good repeatability, and oxidation resistance. Therefore, they are widely used in reference, laboratory, and industrial applications. However, they are easily contaminated by reducing atmospheres at high temperatures, causing the platinum wire to become brittle and altering its resistance-temperature characteristics. Therefore, they require a protective sheath before use. The purity of the platinum wire is crucial to the accuracy of the thermometer. Higher platinum wire purity results in greater stability, better repeatability, and higher temperature measurement accuracy.
Copper resistance thermometers have a near-linear relationship between resistance and temperature, a relatively large temperature coefficient of resistance, and are inexpensive. Therefore, they are often used in applications where high measurement accuracy is not required. However, they are easily oxidized in atmospheres above 100°C, so they are mostly used for measuring temperatures in the range of -50 to 150°C.
Advantages of semiconductor thermistors: They have a large negative temperature coefficient of resistance, resulting in high sensitivity. Their high resistivity allows for the creation of small-sized, high-resistivity resistive elements, leading to low thermal inertia and the ability to measure point or dynamic temperatures. Disadvantages: The resistance-temperature characteristics of the same type of semiconductor thermistor exhibit significant dispersion and severe nonlinearity, resulting in unstable component performance, poor interchangeability, and lower accuracy.
Resistance temperature detector (RTD) connection method:
Two-wire system: This method involves connecting a wire to each end of the resistance temperature detector (RTD) to extract the resistance signal. While simple, this method inherently involves resistance R, the value of which depends on the material and length of the wire. Therefore, this method is only suitable for applications requiring low measurement accuracy.
Three-wire system: A three-wire system is a method in which one lead is connected to one end of the resistance temperature detector (RTD) and two leads are connected to the other end. This method is usually used in conjunction with a bridge circuit and can effectively eliminate the influence of lead resistance. It is the most commonly used method in industrial process control.
Four-wire system: This system connects two wires to each end of the resistance temperature detector (RTD). Two of these wires provide a constant current (I) to the RTD, converting it into a voltage signal (U). The other two wires then connect U to a secondary instrument. This wiring method completely eliminates the influence of wire resistance and is primarily used for high-precision temperature detection.
Installation requirements:
When installing resistance temperature detectors (RTDs), attention should be paid to ensuring accurate temperature measurement, safety, reliability, and ease of maintenance, without affecting equipment operation and production. The following points should be considered when selecting the installation location and insertion depth of the RTD:
1. In order to ensure sufficient heat exchange between the measuring end of the RTD and the measured medium, the measuring point should be selected reasonably, and the RTD should be installed near valves, elbows, pipes and equipment dead corners as much as possible.
2. Resistance temperature detectors (RTDs) with protective sheaths have heat transfer and dissipation losses. To reduce measurement errors, thermocouples and RTDs should have sufficient insertion depth.
1) For resistance temperature detectors (RTDs) used to measure the temperature of fluid at the center of a pipe, the measuring end should generally be inserted into the center of the pipe (vertical or inclined installation). If the pipe diameter is 200 mm, the insertion depth of the RTD should be 100 mm.
2) For temperature measurement of high-temperature, high-pressure, and high-speed fluids (such as main steam temperature), in order to reduce the resistance of the protective sleeve to the fluid and prevent the protective sleeve from breaking under the action of the fluid, a shallow insertion method of the protective tube or a heat-shrinkable resistance temperature detector (RTD) can be adopted. The insertion depth of the protective sleeve of the shallow insertion RTD into the main steam pipeline should not be less than 75mm; the standard insertion depth of the heat-shrinkable RTD is 100mm.
3) If it is necessary to measure the temperature of the flue gas inside the flue, even if the flue diameter is 4m, the insertion depth of the thermal resistor is only 1m.
4) When the insertion depth of the measuring element exceeds 1m, it should be installed as vertically as possible, or a support frame and protective sleeve should be added.
Thermocouple thermometer
Working principle:
When two conductors of different compositions (called thermocouple wires or thermoelectrodes) are joined at their ends to form a circuit, an electromotive force (EMF) is generated in the circuit when the temperatures at the junctions are different. This phenomenon is called the thermoelectric effect, and the EMF is called the thermoelectric potential. Thermocouples utilize this principle for temperature measurement. The end directly used to measure the temperature of the medium is called the working junction (also known as the measuring junction), and the other end is called the cold junction (also known as the compensating junction). The cold junction is connected to a display instrument or matching instrument, which indicates the thermoelectric potential generated by the thermocouple.
Installation requirements:
First, thermocouples and resistance temperature detectors (RTDs) should be installed as vertically as possible to prevent the protective sheath from deforming at high temperatures. However, in the presence of flow, they must be inserted in the direction of the measured medium to ensure full contact between the temperature sensing element and the fluid, thus guaranteeing measurement accuracy.
In addition, thermocouples and RTDs should be installed in pipes with protective layers to prevent heat loss. Secondly, when thermocouples and RTDs are installed in negative pressure pipes, the measuring point must be well sealed to prevent cold air from entering and causing the reading to be too low.
When thermocouples and RTD sensors are installed outdoors, the junction box cover should face upwards and the wiring inlet should face downwards to prevent rainwater or dust from entering the junction box and damaging the wiring inside, thus affecting its measurement accuracy.
The wiring of thermocouples and resistance thermometers should be checked frequently. In particular, the compensating wires of thermocouple thermometers are made of a high-hardness material and are very easy to detach from the terminals, causing an open circuit. Therefore, the wiring should be properly secured, the thermometer wiring should not be touched too much, and it should be checked frequently to obtain accurate temperature measurements.
When installing a thermocouple, it should be placed as close as possible to the control point of the temperature to be measured. To prevent heat loss along the thermocouple or to prevent the protective tube from affecting the measured temperature, the thermocouple should be immersed in the fluid being measured to a depth of at least 10 times its diameter. When measuring the temperature of a solid, the thermocouple should be pressed against or in close contact with that material. To minimize thermal conductivity errors, the temperature gradient near the junction should be reduced.
When using a thermocouple to measure the temperature of a gas in a pipe, if the pipe wall temperature is significantly higher or lower, the thermocouple will radiate or absorb heat, thus significantly altering the measured temperature. In this case, a radiation shield can be used to bring its temperature closer to the gas temperature; this is known as a shielded thermocouple.
When selecting a temperature measurement point, it should be representative. For example, when measuring the temperature of fluid in a pipeline, the measuring end of the thermocouple should be located at the point of maximum flow velocity in the pipeline. Generally, the end of the thermocouple's protective sheath should extend beyond the velocity centerline.
Glass tube liquid thermometer
Working principle:
Glass liquid thermometers use the principle of thermal expansion and contraction: when the temperature changes, the volume of liquid in the glass bulb expands or contracts, causing a change in the height of the liquid column entering the capillary tube, which can be indicated by the scale.
The resolution of a thermometer's scale is related to its sensitivity; higher sensitivity results in higher resolution. To improve sensitivity, the volume of the measuring liquid can be increased or the diameter of the capillary tube can be decreased. However, increasing the volume of the measuring liquid makes it difficult to achieve thermal equilibrium with the measured substance, leading to a larger hysteresis error and potentially deforming the bulb. Conversely, decreasing the capillary tube diameter makes it difficult to manufacture uniformly, resulting in uneven liquid column rise and affecting measurement accuracy. Therefore, an appropriate sensitivity should be chosen.
Furthermore, the sensitivity of a thermometer is also directly proportional to the difference in the coefficients of thermal expansion between the measuring liquid and the glass. Generally, a liquid with a higher coefficient of thermal expansion is chosen as the measuring liquid, while the coefficient of thermal expansion of the glass should be as low as possible. Commonly used measuring liquids include mercury and alcohol.
The main causes of error are:
(1) Zero point permanent displacement
(2) Temporary deformation of the ball
(3) Pressure changes
(4) Inaccurate scale
(5) Incorrect reading method
(6) Thermal hysteresis effect
(7) Special reasons for errors in alcohol thermometers
(8) Special reasons for errors in the highest temperature gauge
Temperature transmitter
Working principle:
A temperature-current transmitter converts the signal from a temperature sensor into a current signal, which is then connected to a secondary instrument to display the corresponding temperature. The temperature transmitter uses thermocouples or resistance temperature detectors (RTDs) as sensing elements. The signal output from the sensing element is sent to the transmitter module, where it undergoes processing through circuits such as voltage regulation and filtering, operational amplification, nonlinear correction, V/I conversion, constant current, and reverse protection before being converted into a 4–20 mA current signal output that is linearly related to the temperature.
Installation requirements:
1. Before installation, check that all accessories are complete and fasteners are secure, and tighten the antenna.
2. Handle with care during installation; do not knock or drop. Once the antenna is tightened, it will function normally.
3. After installation and power-on, non-operators are prohibited from opening the front cover. If an operator accidentally opens it, it must not be kept. Simply turn off the power and reopen it.
The main causes of error are:
If the transmitter output does not change when the temperature of the measured medium rises or falls, this is mostly due to a problem with the temperature transmitter's seal. It may be because the temperature transmitter is not properly sealed or because a small hole was accidentally welded into the sensor during welding. In this case, the transmitter housing usually needs to be replaced to solve the problem.
The output signal is unstable. This is due to the temperature source itself, which is inherently unstable. If the instrument display is unstable, it is because the instrument's anti-interference capability is weak.
There are many reasons why a transmitter might have a large output error. It could be that the resistance wire of the temperature transmitter is incorrect, causing an incorrect measurement range, or that the transmitter was not properly calibrated at the factory.
Temperature switch
Working principle:
A temperature switch is a type of temperature switch that uses a bimetallic strip as its temperature sensing element. When the appliance is operating normally, the bimetallic strip is in a free state, and the contacts are in a closed/open state. When the temperature rises to the operating temperature, the bimetallic element generates internal stress due to heat and quickly actuates, opening/closing the contacts to cut off/connect the circuit, thus providing thermal protection. When the temperature gradually drops to the reset temperature, the contacts automatically close/open, restoring normal operation.
Installation requirements:
1. When using contact temperature sensing installation, the metal cover should be in close contact with the mounting surface of the controlled appliance. To ensure the temperature sensing effect, thermal grease or other similar thermally conductive media should be applied to the temperature sensing surface.
2. Do not crush, loosen or deform the top of the cover during installation, so as not to affect performance.
3. Do not allow liquid to seep into the temperature controller, do not allow the outer casing to crack, and do not arbitrarily change the shape of the external terminals.
4. When using the product in a circuit with a current of no more than 5A, a copper core wire with a cross-sectional area of 0.5-1 mm² should be selected for connection; when using the product in a circuit with a current of no more than 10A, a copper core wire with a cross-sectional area of 0.75-1.5 mm² should be selected for connection.
5. The product should be stored in a well-ventilated, clean, dry warehouse with a relative humidity of less than 90% and an ambient temperature of less than 40℃, free from corrosive gases.
Optical and radiation pyrometers
Working principle:
A radiation pyrometer is designed and manufactured based on the functional relationship between the radiant energy of an object and its temperature across the entire wavelength range. It uses a radiation temperature sensor as the primary instrument and an electronic potentiometer as the secondary instrument. It is a lens-focusing temperature sensor with an aluminum alloy shell. The objective lens is located at the front, and a thermopile compensation aperture is installed inside the shell. There is an adjustment plate on the field aperture close to the thermopile. The function of the adjustment plate is to adjust the radiant energy irradiated onto the thermopile, so that the product has a uniform scale value. An eyepiece is installed on the removable rear cover to observe the image of the object being measured.
The radiation thermometer focuses the radiant energy of the object being measured onto a thermistor through a lens. The thermistor converts the radiant energy into electrical parameters. The thermoelectric potential is measured by a secondary instrument based on the known relationship between the thermoelectric potential and the object's temperature, and the temperature value is displayed. This temperature value must be corrected using the object's total radiation black system number or by directly inserting a platinum-rhodium 10-platinum thermocouple into a high-temperature salt bath furnace with a DC potentiometer to measure the temperature. The result is then compared with the temperature displayed on the instrument to calibrate the accuracy of the pyrometer's temperature measurement.
Fiber optic thermometers, using optical fibers for short: optical fibers transmit large amounts of information at high speed and with high reliability. They have advantages such as being unaffected by electromagnetic interference, having good insulation, being safe and explosion-proof, having low loss, wide transmission bandwidth, large capacity, small diameter, light weight, flexibility, and corrosion resistance, and are used in the field of signal detection. Currently, the most commonly used optical fiber is glass fiber, which is made of quartz glass fibers that are thinner than a human hair.
It consists of a light-guiding fiber core and its surrounding cladding, with the cladding often covered by a protective sleeve made of plastic or rubber.
Installation requirements:
The instrument is a fixed installation type, and the temperature sensor can be used in environments ranging from 10 to 80°C. When the ambient temperature exceeds 80°C or the air medium contains water vapor or smoke, auxiliary devices such as water cooling and ventilation can be used to lower the ambient temperature and blow away the smoke in the measurement channel to reduce measurement errors.
Temperature sensor auxiliary devices are divided into light-duty and heavy-duty types. Heavy-duty devices are used in harsh environments to prevent flames or high-temperature furnace gases from spraying out of the measuring channel and burning the instrument. They are equipped with flame protection devices that can automatically activate in case of danger, protect the instrument, and issue an alarm signal.
Non-contact infrared thermometer
Working principle:
Non-contact infrared thermometers (hereinafter referred to as "thermometers") can determine surface temperature by measuring the infrared energy radiated by the target surface.
This non-contact infrared thermometer features an ultra-low power intelligent design. This design ensures the product can operate for longer periods, reducing the hassle of frequent battery changes and running out of power. The intelligent design helps users test more conveniently and quickly capture the true values of the object being measured, while the instrument can intelligently select between battery or USB connection power supply.
