Resistance thermocouples. Scheme, description

ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Sensors. Resistance thermocouples. Encyclopedia of radio electronics and electrical engineering

Free technical library

Encyclopedia of radio electronics and electrical engineering / Electrician's Handbook

Operating principle thermal converters resistance (thermistors) is based on the change in the electrical resistance of conductors and semiconductors depending on temperature (discussed earlier).

Platinum thermistors are designed to measure temperatures within -260...+1100 °C. Cheaper copper thermistors, which have a linear dependence of resistance on temperature, have become widespread in practice.

The disadvantage copper is its low resistivity and easy oxidation at high temperatures, as a result of which the final limit of the use of copper resistance thermometers is limited to a temperature of 180 ° C. In terms of stability and reproducibility of characteristics, copper thermistors are inferior to platinum ones. Nickel is used in inexpensive sensors for measurements in the room temperature range.

Semiconductor thermistors (thermistors) have a negative or positive temperature coefficient of resistance, the value of which at 20 ° C is (2-8) / 10^-2 (°C)^-1, i.e., an order of magnitude greater than that of copper and platinum. Semiconductor thermistors with very small sizes have high resistance values (up to 1 MΩ). As a semiconductor material, metal oxides are used: semiconductor thermistors of types KMT - a mixture of oxides of cobalt and manganese and MMT - copper and manganese.

Semiconductor temperature sensors have a high stability of characteristics over time and are used to change temperatures in the range from 100 to 200 °C.

Thermoelectric converters (thermocouples) are devices whose operating principle is based on the thermoelectric effect. This effect consists in the fact that in the presence of a temperature difference between the junctions (junctions) of two dissimilar metals or semiconductors, an electromotive force arises in the circuit, called thermoelectromotive force (abbreviated thermoEMF).

In a certain temperature range, we can assume that thermoEMF is directly proportional to the temperature difference T = T₁ - T₀ between the junction and the ends of the thermocouple.

The interconnected ends of the thermocouple, immersed in the medium whose temperature is measured, is called the working end of the thermocouple. The ends that are in the environment and are usually connected by wires to the measuring circuit are called free ends. The temperature of these ends must be kept constant. Under this condition, thermoEMF E_T will depend only on the temperature T₁ working end:

where C is a coefficient depending on the material of the thermocouple conductors.

The EMF generated by thermocouples is relatively small. It does not exceed 8 mV for every 100 °C and usually does not exceed 70 mV in absolute value. Thermocouples allow you to measure temperatures in the range from -200 to +2200 °C.

Platinum, platinum-rhodium, chromel, and alumel are most widely used for the manufacture of thermoelectric converters.

Thermocouples have the following benefits:

ease of manufacture and reliability in operation;
cheapness;
lack of power sources;
possibility of measurements in a wide range of temperatures.

Along with this, thermocouples are also characterized by some shortcomings:

lower than that of thermistors, measurement accuracy;
the presence of significant thermal inertia;
the need to correct for the temperature of the free ends;
the need to use special connecting wires.

Infrared sensors (pyrometers) - These are devices that use the radiation energy of heated bodies, which allows you to measure the surface temperature at a distance.

Pyrometers are divided into three categories:

radiation;
brightness;
color.

Radiation pyrometers are used to measure temperatures from 20 °C to 2500 °C, and the device measures the integral radiation intensity of a real object.

Brightness (optical) pyrometers are used to measure temperatures from 500 °C to 4000 °C. The principle of their operation is based on comparison in a narrow part of the spectrum of the brightness of the object under study with the brightness of a reference emitter (photometric lamp).

Color pyrometers based on measuring the ratio of radiation intensities at two wavelengths, usually chosen in the red or blue part of the spectrum; are used to measure temperatures in the range from 800 °C.

Pyrometers allow you to measure the temperature in hard-to-reach places and the temperature of moving objects, high temperatures, where other sensors no longer work.

Author: Koryakin-Chernyak S.L.

See other articles Section Electrician's Handbook.

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