Thermocouples and thermistors—do you really understand them?

Differences

Thermocouples and thermistors are common temperature sensors, but they have different temperature measurement ranges.

Thermocouples are used in high-temperature environments because their thermoelectric potential output is very small at medium and low temperatures. When the potential is small, high requirements are placed on anti-interference measures and secondary instruments, otherwise the measurement will be inaccurate. Additionally, in lower temperature regions, the relative error caused by changes in cold-end temperature and ambient temperature becomes prominent and is difficult to fully compensate for.

In such cases, thermistors with a temperature measurement range of 200–500°C are generally used at medium and low temperatures, and even lower temperatures can be measured (e.g., carbon resistors can measure temperatures as low as approximately 1 K). Currently, platinum thermistors (Pt100) are commonly used. (There are also Pt50 thermistors, and copper thermistors are used in industry, but their temperature measurement range is limited to -50°C to 150°C. In some special applications, indium thermistors and manganese thermistors are also used.)

PART.1

Temperature measurement principle

The basic principle of thermocouple temperature measurement is the thermoelectric effect. A secondary meter is a voltmeter or, for higher accuracy, an electronic potentiometer.

Resistors work based on the characteristic that the resistance values of conductors and semiconductors change with temperature, and a secondary meter is an unbalanced bridge.

PART.2

On-site Judgment at Work

Thermocouple

Thermocouples have positive and negative terminals, and compensation wires also have positive and negative distinctions. First, ensure proper connection and accurate configuration. During operation, common issues include short circuits, open circuits, poor contact (which can be diagnosed with a multimeter), and degradation (identified by surface color). When inspecting, separate the thermocouple from the secondary meter.

Thermoresistors

Issues are limited to short circuits and open circuits. These can be diagnosed using a multimeter. During operation, if a short circuit is suspected, simply disconnect one wire terminal from the resistance end and observe the display instrument. If the reading reaches maximum, the thermoresistor is short-circuited. If it returns to zero, the wires are short-circuited. If the connection and configuration are normal but the meter reading is low or unstable, water may have entered the protective tube. If the reading is maximum, the thermoresistor is open-circuited. If the reading is minimum, it is short-circuited.

PART.3

Selection of Thermocouples and Thermistors

Thermocouple calibration codes primarily include S, R, B, N, K, E, J, and T. Among these, S, R, and B belong to precious metal thermocouples, while N, K, E, J, and T belong to base metal thermocouples.

The T calibration code is characterized by the highest accuracy grade among all base metal thermocouples and is typically used to measure temperatures below 300°C.

Thermoresistors are the most commonly used temperature sensors in the medium-to-low temperature range. Their main features include high measurement accuracy and stable performance. Among these, platinum thermoresistors offer the highest measurement accuracy and are widely used in industrial temperature measurement, as well as being manufactured into standard reference instruments. Thermoresistors are primarily made from pure metal materials, with platinum and copper being the most commonly used today. Additionally, materials such as nickel, manganese, and rhodium are now being employed in the production of thermoresistors.

PART.4

Thermocouples and resistance thermometers must be installed correctly.

General Provisions for the Installation of Instrumentation Equipment

1. The installation location of locally installed instruments shall comply with the following provisions:

★Adequate lighting, convenient for operation and maintenance; shall not be installed in locations subject to vibration, humidity, mechanical damage, strong magnetic field interference, high temperatures, extreme temperature fluctuations, or corrosive gases.

★The center of the instrument shall be 1.2 to 1.5 meters above the ground.

★Local display instruments should be installed in a position where the instrument readings can be easily observed when manually operating valves.

2. Before installation, instruments should be inspected to ensure they are intact, with all accessories complete, and their model, specifications, and materials should be checked in accordance with design requirements.

3. Instruments should not be struck or subjected to vibration during installation and should be securely and levelly installed after installation.

4. Instruments requiring degreasing as specified in the design should only be installed after passing a degreasing inspection.

5. Instruments directly installed on process pipelines should be installed after the process pipeline has been purged but before pressure testing. If installation must be done simultaneously with the process pipeline, the instrument should be removed during pipeline purging. The arrow on the instrument housing should point in the same direction as the flow of the measured medium. When connecting the instrument to the process pipeline, the flange axis of the instrument should align with the pipeline axis, and it should be secured to ensure even force distribution.

6. Instruments directly installed on process equipment or pipelines should be subjected to pressure testing along with the process system after installation.  

7. The entry points of junction boxes on instruments and electrical equipment should not face upward to prevent oil, water, and dust from entering the box. If this cannot be avoided, sealing measures should be taken.  

8. The text and terminal numbers on the nameplates of instruments and electrical equipment should be written correctly and clearly.

Wiring of instruments and electrical equipment shall comply with the following provisions:

★ Wires shall be identified and labeled before wiring.

★ Insulation shall not be damaged when stripping.

★ The ends of multi-strand wires shall be tinned or use terminal strips. When using terminal strips, the connection between the wire and the terminal strip shall be crimped or welded, ensuring a uniform, secure, and good electrical connection.

★ When soldering, use non-corrosive flux.  

★ The connection between cables (wires) and terminals should be securely fixed and have appropriate clearance.  

★ Wiring should be correct, and the arrangement should be neat and aesthetically pleasing.  

★ When instruments and electrical equipment are susceptible to vibration, spring washers should be added to the terminal connectors.

★Line compensation resistors should be securely installed, easy to install and remove, with an allowable resistance value error of ±0.1 ohms.

Thermocouple and thermoresistor temperature measurement instrument installation methods

1. The installation location of the temperature sensing component should be selected at a point where the medium temperature changes sensitively and is representative, and should not be selected near valves or other resistance components, in areas where the medium flow forms dead angles, or in areas with significant vibration.

2. The installation location of thermocouple temperature-sensing components should be kept away from strong magnetic fields.

3. The installation of temperature-sensing components on process pipelines should comply with the following provisions:

★When installed perpendicular to the process pipeline, the axis of the temperature-sensing component should intersect the axis of the process pipeline at a right angle.

When installing process pipelines at an angle, it is advisable to install them against the direction of the medium flow, with the axis of the source component intersecting the axis of the process pipeline.

4. Temperature measurement elements installed on process pipelines with high dust levels should be protected against wear.

5. When thermocouples or thermistors are installed in locations prone to strong impacts from the measured medium, or when the insertion depth exceeds 1 meter in horizontal installations or the measured temperature exceeds 700°C, measures should be taken to prevent bending.

6. The sensing surface of surface temperature meters should be in close contact with the measured surface and securely fixed.

7. When installing a thermometer on an elbow pipe, the thermometer's axis must align with the centerline of the straight section of the elbow pipe during installation.

8. When measuring furnace temperature with a thermocouple, avoid direct contact between the temperature measurement assembly and the flame, and avoid installing it too close to the flame or near the furnace door. The junction box should not touch the furnace wall to prevent the cold end of the thermocouple from overheating.

9. When using thermocouples or thermistors for temperature measurement, interference signals should be prevented from entering. Additionally, the opening of the junction box should face downward to prevent moisture, dust, or other contaminants from entering and affecting the measurement.

10. If the process pipe is too small, an expansion tube can be installed at the location where the temperature measurement component is mounted.

Thermocouples, thermistors, and bimetallic thermometers are installed on pipes with a diameter of less than 80 mm.

11. Thermocouples should be installed as close as possible to the temperature control point being measured. To prevent heat from being conducted away along the thermocouple or to prevent the protective tube from affecting the measured temperature, the thermocouple should be immersed in the measured fluid to a depth of at least 10 times its diameter. When measuring solid temperatures, the thermocouple should be in direct contact with the material or tightly against it. To minimize thermal conductivity errors, the temperature gradient near the junction should be reduced.

12. When measuring the temperature of gas in a pipe using a thermocouple, if the pipe wall temperature is significantly higher or lower, the thermocouple will radiate or absorb heat from it, thereby significantly altering the measured temperature. In such cases, a radiation shielding cover can be used to bring its temperature closer to that of the gas, employing what is known as a shielded thermocouple.

13. When selecting a measurement point, it should be representative. For example, when measuring the temperature of a 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 flow velocity centerline.

Thermocouple and Thermoresistor Temperature Sensor Installation Methods

1. Installation and Usage Precautions:

Based on the characteristics of the measured medium and operating conditions,

select protective sheaths and gaskets of appropriate material, thickness, and structure.

The installation location, depth, direction, and wiring of the thermocouple should comply with measurement technical requirements.

The ambient temperature at the junction of the thermocouple and compensation lead should not exceed 100°C.

For thermocouples used below 0°C, wax should be poured into the junction box to seal it and isolate it from the external environment.

2. Installation methods for thermocouples and resistance thermometers

a. Straight connector: Direct insertion.

b. 45° angle connector: Angled insertion.

c. Flange: Direct insertion.

d. High-pressure sleeves (fixed sleeves and replaceable sleeves).

3. Installation diagram of thermistors and thermocouples on acid-resistant steel expansion tubes

a. Gasket

b. 45-degree angle connection diagram

c. Thermometer expansion tube

4. Installation diagram of thermoresistors and thermocouples on steel elbow pipes

a. Gasket

b. 45° angle connection diagram

5. Surface thermocouple installation diagram

The material is a straight surface thermocouple connector.

6. Installation diagram of thermocouple thermoresistors fixed with flanged loose flanges on aluminum pipes

a. Aluminum protective sleeve

b. Flanged loose flange pipe connection

PART.2

Thermocouple and Thermoresistor Temperature Measurement Instrument Fault Maintenance

Troubleshooting and handling of thermoresistor temperature measurement faults

Troubleshooting Approach for Thermocouple Temperature Measurement Faults

Thermocouple Emf is Lower Than Actual Value (Displayed Instrument Reading is Low)

The thermoelectric potential is greater than the actual value (the display instrument indicates a higher value than it should).

Thermocouple output instability

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