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Dec 24, 2018

# The difference between thermocouple and thermal resistance

In the process of power equipment temperature monitoring control or instrument temperature data acquisition, we generally use temperature measuring equipment: thermal resistance and thermocouple.

In the process of power equipment operation and maintenance, we often encounter temperature measurement of power equipment. In many cases, the temperature parameter of power equipment is an important indicator to judge whether the power equipment is running normally. Then, what are we generally measuring? What is the principle of measurement?

## First, the measurement principle of thermal resistance and thermocouple

1, the measurement principle of thermocouple

The working principle of the thermocouple is based on the Seeb effect, that is, the two ends of the conductors of different components are connected into a loop. If the temperature of the two terminals is different, the physical phenomenon of generating a thermal current in the loop is obtained.

The thermocouple consists of two different wires (hot electrodes), one end of which is welded to each other to form the measuring end (also called the working end) of the thermocouple. Insert it into the medium to be tested at the temperature; the other end (reference or free end) of the thermocouple is connected to the display meter. If there is a temperature difference between the measuring end of the thermocouple and the reference junction, the display meter will indicate the thermoelectromotive force generated by the thermocouple.

2, the principle of measurement of thermal resistance

The thermal resistance is measured by the characteristic that the electrical resistance of the metal conductor or the semiconductor changes with the temperature change. The heated portion of the thermal resistor (temperature sensing element) is a skeleton made of a thin metal wire uniformly wound around the insulating material. The substrate is formed on or by a laser sputtering process. When the measured medium has a temperature gradient, the measured temperature is the average temperature of the dielectric layer within the range of the temperature sensing element.

## Second, thermocouple and thermal resistance secondary side measurement table performance comparison

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

2. The resistance is based on the characteristic that the resistance value of the conductor and the semiconductor changes with temperature, and the secondary meter is an unbalanced bridge.

## Third, the basic line system of thermocouples and thermal resistance

1. It is known from the thermocouple temperature measurement principle that the measured temperature is a single-valued function relationship with the thermoelectric potential only when the cold junction temperature is constant. In practical use, an inexpensive connecting wire (also called a compensating wire) having thermoelectric characteristics similar to that of the corresponding thermocouple is used to extend the cold end of the thermocouple to a relatively constant temperature (preferably 0 degree). If copper--Constantan is used as the compensation wire to extend the nickel-chromium-nickel silicon thermal resistance. Therefore, the extension of the thermocouple to the secondary meter is two.

2. The thermal resistance and the secondary meter are connected by copper wires. In order to reduce the measurement error caused by environmental changes, the three-wire connection method is generally adopted. Two of the wires have the thermal resistance connected in series to the adjacent two. On the bridge arm, the other wire is the power source.

In practical applications, the thermal resistance generally uses a three-core copper wire for removing the influence of the resistance value of the wire.

The thermocouple uses a two-core dedicated compensation wire to remove the effects of the thermocouple's on-site temperature.

## Fourth, the basic choice of thermal resistance and thermocouple

1. Select according to the temperature range:

Thermocouples are generally selected above 500 °C, and thermal resistors are generally selected below 500 °C;

2. Select according to measurement accuracy:

Selecting a thermal resistance for higher accuracy, and selecting a thermocouple for low accuracy;

3. Select according to the measurement range:

Thermocouples generally refer to "point" temperatures, which are generally measured as spatial average temperatures.

## Five, the key difference between thermal resistance and thermocouple

A thermocouple is a temperature-measuring sensor that is a temperature sensor like a thermal resistor.

1, the nature of the signal

The thermal resistance itself is a resistance, a change in temperature that causes the resistance to produce a positive or negative resistance change;

The thermocouple measurement process produces a change in the induced voltage that changes with temperature.

2. The temperature range detected by the two sensors is different.

The thermal resistance is generally measured in the 0-150 degree temperature range, and the highest measurement range is up to 600 degrees (of course, the negative temperature can be detected).

Thermocouples can detect a temperature range of 0-1000 degrees (or even higher), so the former is low temperature detection and the latter is high temperature detection.

3, from the material

A thermal resistor is a metallic material that has a temperature-sensitive metal material.

Thermocouples are bimetallic materials that have two different metals that create a potential difference across the two different wires due to temperature changes.

4, PLC corresponding thermal resistance and thermocouple input module is also different, this sentence is no problem, but the general PLC directly access 4 ~ 20ma signal, and the thermal resistance and thermocouple generally have a transmitter Only access to the PLC. If you are connected to the DCS, you don't have to use the transmitter!

The thermal resistance is the RTD signal and the thermocouple is the TC signal!

5. PLC also has a thermal resistance module and a thermocouple module, which can directly input resistance and galvanic signals.

6, thermocouples have J, T, N, K, S and other models, have higher than the resistance, but also cheaper than the resistance, but count the compensation wire, the comprehensive cost of the thermocouple is relatively high.

7, the type of measurement signal is different

Thermistance measurement is a resistance signal. Thermocouple measurement is a voltage signal.

8, the temperature measurement principle is different

(1), the principle of thermal resistance temperature measurement is based on the resistance of the conductor (or semiconductor) with temperature changes, the measurement range is negative 00 ~ 500 degrees, commonly used platinum resistance (Pt100, Pt10), copper resistance Cu50 (negative 50-150 degrees).

(2), the thermocouple temperature measurement principle is based on the thermoelectric effect to measure the temperature, commonly used platinum iridium - platinum (index S, measuring range 0 ~ 1300 degrees), nickel chrome - nickel silicon (index K, measuring range 0~900 degrees), nickel-chromium-constantan (index E, measuring range 0~600 degrees), platinum-iridium 30-platinum 铑6 (index B, measuring range 0~1600 degrees ).

Thermal resistance and thermocouple are the temperature measurement components we often encounter in power maintenance and operation and maintenance. It is very necessary to familiarize with the basic application and difference of thermocouple and thermal resistance.