Working principle of thermocouple?

Seebeck effect: If the two ends of a metal rod are at different temperatures, free electrons will diffuse from the high-temperature region to the low-temperature region, resulting in the phenomenon of heat flux and current flowing from the high-temperature region to the low-temperature region.

Thermocouple: When two contacts are in contact with different temperatures, the diffusion rate of electrons in different metals varies, resulting in different diffusion currents between the two metals. As a result, a small net current (about 10 μ V) is formed in the connecting circuit between the two metals. This practice can also be done by connecting two metal wires of different materials together and observing the reading of a multimeter.

But in many practical situations, the cold end temperature is not 0 ℃, but a certain temperature tn, so when using a scale table, the measured electromotive force must be corrected by the following formula:

ε(T,0) = ε(T, Tn) + ε(Tn, 0)

Thermocouple electromotive force=instrument measurement value+room temperature correction value

Thermocouple signal detection:

It is not possible to directly measure the voltage of Seebeck effect with a multimeter, as the wiring of the multimeter and thermocouple will generate new thermocouple junction voltage.

The thermocouple junction voltage of the meter wiring and thermocouple wiring is offset by the common mode gain of the "differential amplifier".

The junction voltage of the thermocouple to be tested and the reference thermocouple junction voltage are amplified by a differential amplifier with differential mode gain.

Characteristics of Thermocouples:

1. The measurable temperature range is wide, and most sensors have been standardized.

2. The operating temperature of thermocouples is related to the size and material of the metal wire diameter

3. No need to add other power sources to drive the sensor

4. The degree that can be obtained from the design of the circuit

Types and operating temperatures of thermocouples:

The category name of thermocouple thermometer (type of thermocouple) for measuring temperature range (℃), thermoelectric potential (mV), advantages and disadvantages, materials+- for high temperature K -200~1200-5.89/-200 ℃ 48.8/1200 ℃

1. Widely used in industry

2. Good acid resistance

Has linear properties

1. Not applicable to CO sulfuric acid gas

2. Deterioration of chromium nickel aluminum, manganese, silicon and other nickel alloys in high-temperature reducing air at temperatures ranging from E-200~800~8.82/-200 ℃

61.02/800 ℃ 1. Possess thermoelectric potential

1. Cannot tolerate use in reducing air

2. Chromium nickel copper J-200~350-7.89/-200 ℃

72.28/750 ℃ 1. Can withstand use in reducing air 1. Easy to rust Iron Nickel Copper Low temperature use T -200~350-5.6/-200 ℃

17.82/350 ℃ 1. Very stable in weakly acidic and reducing air. Copper will oxidize above 1.300 ℃. Copper nickel copper ultra-high temperature uses B 500~1700 1.24/500 ℃

12.4/1700 ℃ 1. Can withstand acidic air 1. Cannot withstand reducing air using 30% rhodium platinum 6% rhodium platinum R 0~1600 0/0 ℃

18.84/1600 ℃ 13% rhodium platinum S 0~1600-7.89/200 ℃

72.28/750 ℃ 10% rhodium platinum

Relationship between working temperature and wire diameter of thermocouple:

Thermocouple type, wire diameter (mm), commonly used temperature (℃), commonly used temperature (℃)

K：0.65 650 850 1.00 750 950 1.60 850 1050 2.30 900 1100 3.20 1000 1200

E：0.65 450 500 1.00 500 550 1.60 550 650 2.30 600 750 3.20 700 800

J：0.65 400 500 1.00 450 550 1.60 500 650 2.30 550 750 3.20 600 750

T：0.32 200 250 0.65 200 250 1.00 250 300 1.6 300 350

B：0.50 1500 1700

RS：0.50 1400 1600

Exploration of Temperature Measurement Error Values by Thermocouples

1, Error of the measuring device:

The temperature reading is displayed by a temperature controller, recorder, voltmeter (mv), etc. And these instruments all have varying degrees of tolerance. If the error of the voltage divider is 0.001% plus 0.01 Μ v or more. The error of the digital temperature controller should be at least 0.25% multiplied by the full range plus one digit of error.

2, Error of thermocouple wire:

Thermocouple wire materials have different degrees of error due to varying national standards. Such as American ANSI TYPE K ordinary grade ± 2.2 ℃ or 0.75%, precision grade ± 1.1 ℃ or 0.4%, Japanese JIS TYPE K ordinary grade ± 2.5 ℃ or 0.75%, precision grade ± 1.5 ℃ or 0.4%.

3, Error of reference point:

The benchmark contact (cold contact) may have an error of 0.05 ℃ to 1 ℃ when using an ice water tank in calibration operations. When using a thermocouple on site, as long as the thermoelectric effect of the compensating wire and the thermocouple wire is the same, and both contacts are at the same temperature, it will not affect the magnitude of the electromotive force in this circuit.

4, Compensate for the error of the wire:

The calibration unreliability of extension wire material is about twice that of thermocouple wire. Due to different wire materials, it is divided into two types: plain wire level (thermocouple wire) and wire level. The error degree is roughly the same as that of thermocouple wire. However, when using extension wire, it is necessary to maintain an appropriate low temperature, otherwise it will cause significant errors.

5, Error caused by thermal conduction: (commonly known as thermal short circuit)

One end of the thermocouple is on the heat flow plate, and the other end is outside the heat flow plate. If the length of the thermocouple insertion is not sufficient, the thermal energy inside the heat flow plate is determined by the heat transfer coefficient of the protective tube, which is about 10-20 times the diameter of the protective tube. The temperature outside the heat flow plate is transmitted to the heat flow plate, causing measurement errors due to thermal convection.

6, Poor insulation error: (commonly known as electrical short circuit)

At high temperatures, the insulation resistance of thermocouples decreases, causing a short circuit between two wires. The error value can reach more than 1% to 10% of the measured temperature, sometimes positive and sometimes negative, depending on the location of the short circuit.

7, Error caused by magnetic effect:

Thermocouple wires or compensating wires, when disturbed by magnetic fields, affect the movement of electrons in metal materials and alter the electromotive force of the material.

Induced magnetic field: The magnetic field generated by transformers and motors, and the wires are shielded with copper or iron mesh.

Current induced magnetic field: The magnetic field generated by the current in an electrical cable, and the wire is shielded with copper platinum.

8, Errors caused by friction:

The thermal energy generated by the friction between thermocouples and high-speed fluids (gases, liquids) causes errors.

9, Error caused by thermal radiation:

The thermocouple is too close to the heat source, and the measured temperature is higher than the actual temperature due to radiation heat.

10, Error caused by self heating:

Power: square of current x resistance, due to the current generated in the closed circuit, the sensor (thermocouple wire, platinum thermometer) generates thermal energy and produces errors. This effect is more pronounced in resistance thermometers, and low current devices are used when purchasing to improve measurement precision