Company Details
  • Cangzhou Qirun Detecting Instrument Co., Ltd.

  •  [Hebei,China]
  • Business Type:Manufacturer
  • Main Markets: Africa , Americas , Asia , East Europe , Europe , Middle East , North Europe , Oceania , Other Markets , West Europe , Caribbean , Worldwide
  • Exporter:61% - 70%
  • Certs:GS, CE, GB, ISO9001, ISO9002
Cangzhou Qirun Detecting Instrument Co., Ltd.
Title
  • Title
  • All
Product Categories
Online Service
http://www.qrdetecting.comScan to visit
Home> Industry Information> Thermocouple and thermal resistance difference

Thermocouple and thermal resistance difference

April 16, 2019

Thermocouple and RTD thermometers should be clearly distinguished

Difference between thermocouple and thermal resistance

Although contact thermometers are used, they have different temperature ranges.

Thermocouples are used in higher temperature environments because they have very little output thermoelectric power in the medium and low temperature regions. When the potential is small, the countermeasures against interference and the secondary meter are very demanding. Otherwise, the measurement is inaccurate. In the low temperature region, the relative error caused by the change of the cold junction temperature and the change of the ambient temperature becomes prominent, and it is not easy to obtain full compensation.

At this time in the low temperature, the general use of thermal resistance temperature measurement range of 200 ~ 500 °C, and even can measure lower temperature (such as carbon resistance can be measured to about 1K low temperature). Now the normal use of platinum thermal resistance Pt100 . (There are also Pt50, copper resistance is also used in industry, but the temperature range is small, between 50~~150°C. In some special occasions there are indium resistance, manganese resistance, etc.).

Principle of temperature measurement

The basic principle of the thermocouple to measure the temperature is the thermoelectric effect. The secondary meter is a pedometer or an electronic potentiometer is used to increase the accuracy.

The resistance works based on the characteristics of the resistance of the conductor and the semiconductor as a function of temperature. The secondary meter is an unbalanced bridge.

On-site judgment at work.

1. Thermocouples. Thermocouples have positive and negative poles. Compensation leads also have positive and negative points. First, ensure the connection and configuration. In operation, common short circuits, open circuits, poor contact (with multimeter to judge) and deterioration (according to Surface color to identify.) When checking, separate the thermocouple from the secondary table.

2. Thermal resistance. Is nothing more than a short circuit, and open circuit. With a multimeter to determine. In operation. Suspect the short circuit, as long as the resistance end remove a thread, see the display instrument, such as to the maximum, thermal resistance short circuit. Return to zero, wire Short-circuit. Guaranteed normal connection and configuration, the value of the table shows low or unstable, the possibility of protection tube into the water. Show the maximum, open circuit thermal resistance. Show the minimum, short circuit

Thermocouple and thermal resistance options:

Thermocouple index number is mainly S, R, B, N, K, E, J, T and other types. Among them, S, R, and B belong to noble metal thermocouples, and N, K, E, J, and T belong to inexpensive metal thermocouples.

The T-index number is characterized by the highest level of accuracy among all inexpensive metal thermocouples and is commonly used to measure temperatures below 300°C.

Thermal resistance is one of the most commonly used temperature detectors in the medium and low temperature regions. Its main features are high measurement accuracy and stable performance. Among them, platinum thermal resistance has the highest measurement accuracy. It is not only widely used in industrial temperature measurement, but also used as a standard reference instrument. Most of the thermal resistance is made of pure metal materials. At present, platinum and copper are the most widely used materials. In addition, nickel, manganese, and tantalum have been used to manufacture thermal resistors.

Thermocouple and RTD thermometers must be installed correctly

General requirements for the installation of instrumentation equipment

1. The installation location of on-site installation instruments should meet the following requirements:

★ The light is sufficient, and the operation and maintenance are convenient. It is not suitable to be installed in locations where vibration, moisture, mechanical damage, strong magnetic interference, high temperature, violent temperature change, and corrosive gas.

★ The height of the center of the instrument from the ground should be 1.2 to 1.5 meters.

★ The display instrument installed on-site should be installed at the location where the value of the observer is conveniently displayed when the valve is manually operated.

2. Before the instrument is installed, it should have a complete appearance and complete accessories, and check its model, specifications and materials according to the design regulations.

3. When the instrument is installed, it should not be struck and vibrated, and it should be firm and flat after installation.

4, the design requirements need to degreasing the instrument, should be qualified after degreasing inspection can be installed.

5. The instrument directly installed on the process pipeline should be installed before the pressure test after the process pipeline is purged. When it must be installed together with the process pipeline, the instrument should be removed when the process pipeline is purged. The arrow on the instrument housing should point in the same direction as the measured medium. When the instrument is connected with the process pipe, the axis of the flange on the instrument should be consistent with the axis of the process pipe, and the force should be uniform when fixing.

6. The instrument directly installed on the process equipment or pipeline is installed and should be pressure tested along with the process system.

7. The introduction port of the junction box on the instrument and electrical equipment should not face upwards to prevent oil, water and dust from entering the box. When it is unavoidable, sealing measures should be taken.

8. The characters and terminal numbers on the signs and electrical equipment signs should be written correctly and clearly.

The wiring of instruments and electrical equipment should meet the following requirements:

★ The line should be checked and marked before wiring.

★ The core should not be damaged when stripping the insulation.

★ Multi-strand core ends should be hot tin or use lugs. When the lug is used, the connection between the wire and the lug should be crimped or welded, and the joint should be even and firm and conductive.

★ Non-corrosive flux should be used for soldering.

★ The connection between the cable (line) and the terminal should be fixed firmly and leave an appropriate margin.

★ The wiring should be correct and the arrangement should be neat and beautiful.

★ When the instrument and electrical equipment are vulnerable to vibration, spring washers should be added to the terminal block.

★ The line compensation resistor should be firmly installed and easily assembled and disassembled, and its allowable error is ±0.1 ohms.

Thermocouple, RTD temperature meter installation

1. The location where the temperature source component is to be installed should be selected where the temperature of the medium is sensitive and representative, and should not be selected in the vicinity of the valve and other resistance components and where the media stream is at a dead angle and where the vibration is large.

2. The location where the thermocouples take the source parts should be away from strong magnetic fields.

3. The installation of temperature source components on the process piping should meet the following requirements:

★ When installed perpendicular to the process pipe, the axis of the source component should intersect the process pipe axis perpendicularly.

★ When tilting with process piping, the flow direction of the medium should be reversed. The axis of the source part should intersect with the axis of the art pipeline.

4. The temperature measuring element installed on the multi-dust process pipeline should be protected against wear.

5. The thermocouple or RTD should be installed in places where it is susceptible to strong impact from the measured medium, and when it is installed horizontally with an insertion depth greater than 1 m or when the measured temperature is greater than 700 °C, anti-bending measures should be taken.

6. The surface temperature of the surface thermometer should be in close contact with the surface to be measured and securely fastened.

7. Install a thermometer on the elbow. Make sure that the axis of the thermometer coincides with the centerline of the straight section of the elbow.

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

9. When thermocouples and thermal resistances are used to measure temperature, the introduction of interference signals should be prevented. At the same time, the holes in the junction box should be directed downwards to prevent moisture and dust from entering and affecting the measurement.

10. If the process piping is too small, the expansion tube may be installed at the installation of the temperature measurement assembly.

Thermocouples, thermal resistance, and bimetal thermometers are installed on pipes with DN < 80mm.

11. The thermocouple should be placed as close as possible to the temperature control point to be measured. To prevent heat transfer along the thermocouple or prevent the protection tube from affecting the measured temperature, the thermocouple should be immersed in the fluid under test, at a depth of at least 10 times the diameter. When measuring the solid temperature, the thermocouple should be against or in intimate contact with the material. In order to minimize the thermal conduction error, the temperature gradient near the contact should be reduced.

12. When using a thermocouple to measure the temperature of a gas in a pipe, if the wall temperature is significantly higher or lower, the thermocouple will radiate or absorb heat, significantly changing the measured temperature. At this time, a radiation shield can be used to bring its temperature close to the gas temperature, using a so-called shield thermocouple.

13. It should be representative when selecting the temperature measurement point. For example, when measuring the fluid temperature in the pipeline, the measurement end of the thermocouple should be at the maximum flow rate in the pipeline. In general, the end of the thermowell's protective sleeve should cross the flow centerline.

Thermocouple, RTD thermometer installation method,

1. Installation and use notes:

According to the characteristics of the measured medium and operating conditions,

Use suitable materials, thickness and structure of protective sleeves and gaskets.

The location, depth, direction, and wiring of the thermocouple installation should meet the requirements of measurement technology.

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

For thermocouples used below 0°C, a wax seal should be made under the terminal block to isolate it from the outside world.

2. Thermocouple, RTD thermometer installation

a. straight connector: straight line.

B.45° angle connector: oblique insertion.

c. flange: line.

d. High-voltage bushings (fixed bushings and replaceable bushings).

3. Thermal resistance, thermocouple installation drawings on acid-resistant steel expansion tube

a. Gasket

B.45 degree connection diagram

c. Thermometer expansion tube

4. Installation of thermal resistance and thermocouple on steel elbow

a. Gasket

B.45° connection diagram

5. Surface thermocouple installation diagram

The material is a surface thermocouple straight connector

6. Installation of the thermocouple thermal resistance fixed on the aluminum pipe with the flanging loose flange

a. Aluminum protective sleeve

b. Flanged loose flange flange

Thermocouple and Thermal Resistance Thermometer Maintenance


Thermal Resistance Temperature Measurement Troubleshooting Inspection Ideas



Thermocouple temperature measurement troubleshooting check ideas


Click for larger image

Thermocouple measurement circuit common faults and treatment methods

Thermal potential is smaller than the actual value (indicating that the indicator value of the meter is low)


Thermal potential is greater than actual value (indicating that the indicated value of the meter is high)


Thermoelectric output is not stable


The above is the Thermocouple and thermal resistance difference we have listed for you. You can submit the following form to obtain more industry information we provide for you.

You can visit our website or contact us, and we will provide the latest consultation and solutions

Share to:

Send Inquiry

Home> Industry Information> Thermocouple and thermal resistance difference

Mobile Website Index. Sitemap


Subscribe To Our Newsletter:
Get Updates, Discounts, Special
Offers and Big Prizes!

Copyright © 2024 Cangzhou Qirun Detecting Instrument Co., Ltd. All rights reserved.
Communicate with Supplier?Supplier
Wang Mr. Wang
What can I do for you?
Contact Supplier