Thermocouples and RTDs

Thermocouples and RTDs both belong to the contact temperature measurement in temperature measurement, although their role is the same is to measure the temperature of the object, but their principles and characteristics are not the same.

thermocouples

Thermocouple is the most widely used in temperature measurement, the main features are wide temperature range, more stable performance, while the structure is simple, dynamic response is good, more able to transmit 4-20mA signal, easy to automate and centralized control. The principle of temperature measurement is based on the thermoelectric effect.

Two different conductors or semiconductors connected into a closed loop, when the temperature at the two points of contact is different, the circuit will produce a thermal potential, a phenomenon known as the thermoelectric effect, also known as the Seebeck effect.

The thermal potential generated in a closed loop is composed of two potentials: the temperature difference potential and the contact potential.

temperature difference potential

It is the potential generated at the ends of the same conductor due to different temperatures; different conductors have different electron densities and so generate different potentials.

contact potential

Refers to two different conductors in contact, because their electron density is different, so a certain amount of electron diffusion, when they reach a certain equilibrium formed after the potential, the size of the contact potential depends on the material properties of the two different conductors as well as the temperature of their contact point.

At present, the international application of thermocouple has a standard specification, the international regulations thermocouple is divided into eight different degrees, respectively, B, R, S, K, N, E, J and T, and its measurement of the temperature of the minimum can be measured at minus 270 ℃, the highest up to 1800 ℃, of which B, R, S belongs to the platinum series of thermocouples, due to platinum belongs to the precious metals, so it is also known as precious metal thermocouples. And the remaining few are called cheap metal thermocouples.

There are two types of thermocouple construction: plain and armored.

General thermocouple

Generally by the heat electrode, insulating tube, protective casing and junction box and other parts of the composition, and armored thermocouple is the thermocouple wire, insulating materials and metal protective casing three combination of assembly, after stretching and processing of a solid combination of body. But the electrical signal of the thermocouple needs a special kind of wire to be transmitted, this kind of wire we call the compensation wire.

Different thermocouples require different compensation wires, whose main function is to connect to the thermocouple to keep the reference end of the thermocouple away from the power source, thus stabilizing the temperature of the reference end.

Compensation wire is also divided into compensation type and extension type two kinds, the chemical composition of the extension wire and the thermocouple to be compensated the same, but in practice, the extension of the wire is not and thermocouple and the same material metal, generally use and thermocouple has the same electron density of the wire instead. Compensation wire and thermocouple connection is generally very clear, the positive terminal of the thermocouple connected to the red wire of the compensation wire, while the negative terminal is connected to the rest of the color. Most of the compensation wires are made of copper-nickel alloy.

RTDs are not only widely used in industrial temperature measurement, but also made into standard reference instruments. However, due to his temperature measurement range makes his application is somewhat limited, the temperature measurement principle of RTD is based on the conductor or semiconductor resistance value with the change of temperature and the change of characteristics. Its advantages are also many, can also transmit electrical signals, high sensitivity, high stability, interchangeability and accuracy are better, but the need for power supply excitation, not able to measure the temperature change instantaneously.

Industrial RTD is generally used Pt100, Pt10, Cu50, Cu100, platinum RTD temperature measurement range is generally minus 200-800 ℃, copper RTD for minus 40 to 140 ℃. RTDs are distinguished from thermocouples in the same way as thermocouples, but they do not require compensation wires and are cheaper than thermocouples. Platinum RTDs are available in a wide range of mounting formats, including fixed thread mounting, movable thread mounting, fixed flange mounting, movable flange mounting, movable pipe fitting mounting, in-line pipe fitting mounting, and so on.

The biggest difference between RTD and thermocouple selection

--Selection of temperature range

RTD is a temperature sensor to measure low temperature, generally measuring temperature at -200~800℃, while thermocouple is a temperature sensor to measure middle and high temperature, generally measuring temperature at 400~1800℃, in the selection if the measuring temperature is around 200℃ should choose RTD to measure, if the measuring temperature is at 600℃ should choose the K-type thermocouple, if the measuring temperature is at 1200~1600℃ should choose S-type or B-type thermocouple. If the measuring temperature is 1200~1600℃, you should choose S-type or B-type thermocouple.

RTDs have the following characteristics compared to thermocouples.

(1) The output signal is larger at the same temperature, making it easier to measure.

(2) The resistance must be measured with the aid of an applied power supply.

(3) The size of the temperature-sensing part of the RTD is large, while the working end of the thermocouple is a very small soldering point, and thus the response speed of the RTD temperature measurement is slower than that of the thermocouple.

(4) RTDs made of similar materials do not have as high an upper temperature limit as thermocouples.