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Structure And Working Principle Of Thermocouple (I)

Dec 04, 2018

What is a thermocouple and how does it work?

A thermocouple is one of the most commonly used temperature detection elements in the industry.

The working principle of thermocouple,

is based on setback effect,

that is, two conductors with different components are connected,

into a circuit at both ends.

If the temperature of the two connection ends is different,

the physical phenomenon of thermal current will be generated in the circuit.

What is a thermocouple and how does it work?

As industrial temperature, one of the most widely used temperature sensor- thermocouple,

with platinum thermal resistance,

about 60% of the total temperature sensor,

thermocouple and display instrument,

usually measured directly in the production of various

- 40 ~ 1800 ℃ in the scope of liquid,

vapor and gas medium, as well as the surface temperature of the solid.

Its advantages are:

(1) high measurement accuracy;

(2) wide range of measurement;

(3) simple structure, thermocouple easy to use.


Type and structure of thermocouple

I. types:

A thermocouple can be divided into standard thermocouple and non-standard thermocouple two categories.

The so-called standard thermocouple,

refers to the national standard stipulates the relationship,

between its thermoelectric potential and temperature, ,

allows errors,

and has a unified standard dividing meter thermocouple,

it has its supporting display instrument for selection.

Non-standardized thermocouple in the range of use or order of magnitude,

are less than the standard thermocouple,

generally, there is no unified scale,

mainly used for some special occasions of measurement.

Ii. The basic structure of thermocouple:

The basic structure of thermocouple,

for industrial temperature measurement,

includes thermocouple wire, insulation pipe,

protection pipe and junction box, etc.

Common thermocouple wire and its properties:

1. Platinum-rhodium 10-platinum thermocouple

(the index number is S, also known as the single platinum-rhodium thermocouple)

the positive electrode of the thermocouple is a platinum-rhodium alloy containing 10% rhodium,

and the negative pole is pure platinum;

Its features are:

(1) thermal stable performance,

strong oxidation resistance,

appropriate continuous use in an oxidizing atmosphere,

the long-term use of temperature can reach 1300 ℃,

and above 1400 ℃, even in the air,

or pure platinum wire recrystallization, coarse grains, and fracture;

(2) high precision, it is in all thermocouples,

accuracy level  high,

usually used as a standard or measurement of high temperature;

(3) wide range of use, good uniformity and interchangeability;

(4) the main disadvantages are:

the differential thermoelectric potential is small,

so the sensitivity is low;

The price is more expensive, the mechanical strength is low,

is not suitable in the reducing atmosphere or has the metal steam condition to use.

2. Platinum-rhodium 13-platinum thermocouple 

(degree R, also known as a single platinum-rhodium thermocouple)

This thermocouple positive pole contains 13% platinum-rhodium alloy,

negative pole pure platinum, compared with s-type,

its potential rate is about 15% higher,

other properties are almost the same,

this kind of thermocouple in the Japanese industry,

as a high-temperature thermocouple used more,

but in China, it is used less;

What is the function of a thermocouple?

3. Platinum-rhodium 30-platinum-rhodium 6 thermocouple

(the index number is B, also known as a double platinum-rhodium thermocouple)

The positive pole of the thermocouple is a platinum-rhodium alloy containing rhodium 30%,

the negative pole is a platinum-rhodium alloy containing rhodium 6%,

at room temperature,

its thermoelectric potential is very small,

so in the measurement of the general need not compensate the wire,

can ignore the impact of temperature changes at the cold end;

Long-term use of temperature is 1600 ℃,

short-term is 1800 ℃, for thermoelectric power is small,

so the need to display instrument with high sensitivity.

Type B thermocouples are suitable for use in oxidizing,

or neutral atmospheres or in vacuum atmospheres for short-term use.

Even in the reduction atmosphere,

its life is R or S type of 10 to 20 times;

Because its electrode is made of platinum-rhodium alloy,

so there is no platinum-rhodium-platinum thermocouple on the negative pole all the shortcomings,

at high temperature, there is little tendency of large crystallization,

and has a greater mechanical strength;

At the same time, because it has less impact on the absorption of impurities or rhodium migration,

its thermoelectric potential change is not serious after long-term use,

and its disadvantages are expensive (compared with the single platinum-rhodium).

4. Nickel-chrome-nickel-silicon (nickel-aluminum) thermocouple (index K)

The thermocouple is made of ani-cr alloy,

with a positive electrode containing 10% chromium,

and a ni-si alloy with a negative electrode containing 3% silicon

(some countries have products with a negative electrode containing pure nickel).

Measurement of 0 ~ 1300 ℃ medium temperature,

suitable for continuous use in the oxidizing and inert gas,

short-term use temperature is 1200 ℃,

the long-term use of temperature is 1000 ℃,

the approximately linear relationship between thermoelectric potential and temperature, cheap,

is the most consumable thermocouples.

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