Thermocouple temperature measurement principle and common faults

1. Thermocouple is one of the most commonly used temperature detection elements in
industry. The working principle of thermocouple is based on seeback effect, th
at is, two ends of conductors with different components are connected into a ci
rcuit. If the temperature of two connecting ends is different, thermal current
will be generated in the circuit.The advantages are: ① high measurement accura
cy.Because the thermocouple is in direct contact with the object to be measure
d, it is not affected by the intermediate medium.② The measurement range is wi
de.Common thermocouples can be continuously measured from - 50 ℃ to - 1600 ℃.
Some special thermocouples can be measured from - 269 ℃ (such as gold, iron,
nickel, chromium) to + 2800 ℃ (such as tungsten rhenium).③ Simple structure
and convenient use.Thermocouples are usually composed of two different kinds of
metal wires, and they are not limited by the size and the beginning. There is
a protective sleeve outside, which is very convenient to use.1. The basic princ
iple of thermocouple temperature measurement is to weld the conductors or semic
onductors A and B of two different materials to form a closed circuit, as shown
in the figure.When there is a temperature difference between two rigid points
1 and 2 of conductors A and B, electromotive force will be generated between th
em, thus forming a large and small current in the circuit. This phenomenon is c
alled thermoelectric effect.Thermocouples use this effect to work.Commonly used
thermocouple materials are: thermocouple graduation thermoelectric pole materi
al positive electrode s Platinum Rhodium 10 pure platinum r platinum rhodium 13
pure platinum B platinum rhodium 30 platinum rhodium 6 K nickel chromium nicke
l silicon t pure copper copper nickel J iron copper nickel n nickel chromium si
licon nickel e nickel chromium copper nickel 2. Types and structure formation o
f thermocouples(1) The types of thermocouples commonly used can be divided into
standard thermocouple and non-standard thermocouple.The standard thermocouple
refers to the thermocouple whose relationship between thermal potential and tem
perature, allowable error and unified standard graduation table are specified i
n the national standard. It has its supporting display instrument for selectio

2. n.The non-standard thermocouple is not as good as the standard thermocouple in
the scope of use or order of magnitude, and generally there is no unified scal
e, which is mainly used in some special occasions.Since January 1, 1988, all th
ermocouples and thermal resistances in China have been produced in accordance w
ith IEC international standards, and seven kinds of standardized thermocouples
(s, B, e, K, R, J, t) have been designated as unified design thermocouples in C
hina.(2) In order to ensure the reliable and stable operation of the thermocoup
le, the structural requirements of the thermocouple are as follows: ① the weld
ing of the two thermoelectric poles that make up the thermocouple must be firm;
② the two thermoelectric poles should be well insulated from each other to pr
event short circuit; ③ the connection between the compensation wire and the fr
ee end of the thermocouple should be convenient and reliable;④ The protective
sleeve shall be able to ensure that the thermoelectric pole is fully isolated f
rom the harmful medium.3. The temperature compensation of the cold end of the t
hermocouple is generally expensive (especially when the precious metal is use
d), and the distance from the temperature measuring point to the instrument is
far. In order to save the thermocouple material and reduce the cost, the compen
sation wire is usually used to extend the cold end (free end) of the thermocoup
le to the control room with relatively stable temperature and connect it to the
instrument terminal.It must be pointed out that the function of thermocouple c
ompensation wire is only to extend the thermoelectric pole and move the cold en
d of thermocouple to the instrument terminal in the control room. It can not el
iminate the influence of the cold end temperature change on the temperature mea
surement, and it does not play a compensation role.Therefore, it is necessary t
o use other correction methods to compensate for the influence of cold end temp
erature t0 ≠ 0 ℃.When using thermocouple compensation wire, the model must be
matched, the polarity cannot be connected wrongly, and the temperature between
compensation wire and thermocouple connection end cannot exceed 100 ℃.
Cold junction compensation principle of thermocouple http://www.tiankangcable.com
When measuring the temperature of a thermocouple, the temperature of its cold end (the measuring end is
the hot end, and the end connected with the measuring circuit through the lead wire is called the cold
end) is required to remain unchanged, and its thermal potential is proportional to the measured
temperature.If the temperature of cold end (environment) changes during measurement, it will affect the
accuracy of measurement seriously.It is called the cold junction compensation of thermocouple to take
some measures to compensate the influence caused by the change of cold junction temperature.The cold
junction compensation of thermocouples usually uses a thermal resistance bridge connected in series at
the cold junction.The three arms of the bridge are standard resistors, and the other arm is made of
(copper) thermistor.When the cold end temperature changes (for example, increases), the thermoelectric
potential generated by the thermocouple will also change (decrease), and the thermoelectric resistance

3. value in the series bridge will also change and the voltage at both ends of the bridge will also change
(increase).If the parameters are selected well and the wiring is correct, the voltage generated by the
bridge is exactly equal to the amount of thermoelectric potential changing with the change of temperature,
and the total output voltage (potential) of the whole thermocouple measuring circuit exactly reflects the
measured temperature value.This is the cold junction compensation principle of thermocouple.
Thermocouple thermometer is the most widely used thermometer at present. Thermocouple
thermometer is a kind of temperature electrical measuring instrument. It usually consists of
three parts: thermocouple, thermocouple cold end temperature compensation device (or element)
and display instrument. The three parts are connected by wires.
Principle of thermocouple temperature measurement
Thermocouples are formed by welding one end of two different conductors or semiconductor
linear materials a and B. A and B are called thermoelectric poles (or thermocouple wires).The
welded end is placed at the temperature T to be measured, which is called the hot end (or
measuring end, working end) of the thermocouple; the non welded end is called the cold end
(or reference end, free end), and the cold end is placed in the environment with the temperature
t0 outside the tested object.
If the two cold ends of the thermocouple are also connected to form a closed circuit, as shown
in Figure 2-1, when the temperature of the hot end and the temperature of the cold end are
not equal, i.e. t ≠ to, there is current flowing through the circuit, which means that the
electromotive force is generated in the circuit. The electromotive force generated due to the
temperature difference between the two contacts of the thermocouple is called thermoelectric
(dynamic) force, and the above-mentioned theory is called thermoelectric effect, orCalled the
Seebeck effect.Thermocouple is to use thermoelectric effect to measure temperature.Further
study shows that thermoelectric potential is composed of contact potential and thermoelectric.

4. 1.Contact potential
When two kinds of homogeneous conductors A and B contact, due to the different density of free
electrons in a and B (set free electron density Na > NB), conductor a will diffuse free
electrons to conductor B through the contact, then a loses electrons, and B accumulates
electrons, so that potential difference is generated on both sides of the contact, and the
electrostatic field E is established, as shown in Figure 2-2, the left side of electrostatic
field E will prevent free electrons from continuing to diffuse.When the interaction of
diffusion force and electric field force is balanced.The diffusion of electrons stops
relatively, and the potential is finally generated between two sides of the contact. This
potential is called contact potential, which is represented by the symbol EAB (T), where t is
the temperature of the contact. The contact potential is related to the temperature of the
contact surface and the properties of the two conductors. The direction is shown in Figure 2-
2, from the electrode with small electronic density to the electrode with large electronic
density http://www.tksiliconerubbercable.com.
2.Thermoelectric potential
Because the free electron density of conductor will increase with the increase of temperature,
when the temperature of both ends of the same conductor is different (as shown in Figure 2-

5. 3), the free electron density of the end with high temperature will be higher than that of
the end with low temperature, so there will be a similar spontaneous electron diffusion process
in contact potential between the two ends, and finally a potential difference will be generated
between the two ends of conductor to establish the potentialThe potential is called
thermoelectric potential, which is expressed by the symbol EA (T, to). Its size is related to
the temperature T, to and conductor properties at both ends of the conductor, as shown in
Figure 2-3, from the low temperature end to the high temperature end.For the convenience of
analysis, the thermoelectric potential is sometimes written in the form of EA (T, to) = EA
(T) - EA (to).
3.Thermal potential
To sum up, in the thermocouple circuit shown in Figure 2-1, when t > T0, Na > Nb, two contact
potentials (T) and EAB (to), two temperature difference potentials (T, to) and EB (T, to) will
be generated in the circuit.The direction of each potential is shown in the figure.
At this time, the total potential of the loop, i.e. the thermal potential EAB (T, to), is the
algebraic sum of these contact potentials and the thermoelectric potential, i.e
EAB(t，to)= eAB(t) - eA(，to) - eAB(to) + eB(t，to)
= eAB(t)-[eA(t) - eA(to)] - eAB(to) + [eB(t) - eB(to)]
= [eAB(t) - eA(t) + eB(t)] - [eAB(tO) - eA(to) + eB(to)]
= fAB(t) – fAB(to)
Because the thermoelectric potential is smaller than the contact potential and T > to, the
contact potential EAB (T, to) accounts for the largest percentage in the total potential EAB
(T, to). Therefore, the direction of the total potential EAB (T, to) depends on the direction
of EAB (T).Because of the high electron density of a, a is the positive electrode and B is
the negative electrode. In the positive hot electrode, the direction of the potential is from
the hot end to the cold end.
The above formula shows that when the materials of the two thermoelectric poles are selected,
the thermoelectric potential is the difference between the two functions related to the contact

6. temperature respectively.If the cold end temperature to remains the same, then FAB (to) = C
(constant), then EAB (T, to) = FAB (T) - C, the thermoelectric potential and the hot end
temperature t have a one-to-one correspondence.Therefore, when the thermoelectric potential
EAB (T, to) is measured, the value of the measured temperature T can be determined, which is
the principle of thermocouple temperature measurement.
For the convenience of use, the corresponding relationship between the hot end temperature
and the thermoelectric potential of the standardized thermocouple can be found in the function
table.This kind of function table is made by experiment under the condition of cold end
temperature of 0 ℃, which is called thermocouple graduation table.The thermocouple scale can
be used to express the thermoelectric characteristics of thermocouples.See table 1-5 for the
graduation table of several common thermocouples.It should be noted that when to is not equal
to 0 ℃, the index table cannot be used to directly check the value of EAB (T, to) or EAB (T,
to)
Attached table 1: graduation table of Pt Rh 10 Pt thermocouple (graduation s; reference end
temperature 0 ℃; MV)
Attached table 2: platinum rhodium 13 platinum thermocouple graduation table (graduation R;
reference end temperature 0 ℃; MV)

7. Attached table 3: Pt Rh 30-pt RH 6 thermocouple graduation table (graduation B; cold end
temperature 0 ℃; MV)
Attached table 4: graduation table of Ni Cr Ni Si thermocouple (graduation number is K; cold
end temperature is 0 ℃; MV)

8. Attached table 5: graduation table of Ni Cr Cu Ni (Constantan) thermocouple (graduation No.
E; cold end temperature 0 ℃; MV)

9. (S-type thermocouple) Platinum Rhodium 10 platinum thermocouple Platinum Rhodium 10 platinum
thermocouple (S-type thermocouple) is a precious metal thermocouple.The diameter of the
coupling wire is specified as 0.5mm, and the allowable deviation is - 0.015mm. The nominal
chemical composition of the positive electrode (SP) is platinum rhodium alloy, including 10%
rhodium, 90% platinum, and the negative electrode (SN) is pure platinum, so it is commonly
known as single platinum rhodium thermocouple.The maximum operating temperature of the
thermocouple is 1300 ℃ in the long term and 1600 ℃ in the short term.S-type thermocouples
have the advantages of the highest accuracy, the best stability, wide temperature measurement
range and long service life.It has good physical and chemical properties, thermoelectric
stability and oxidation resistance at high temperature. It is suitable for oxidation and inert
atmosphere.As S-type thermocouple has excellent comprehensive performance and conforms to the
international temperature standard, it has been used as the interpolation instrument of
international temperature standard for a long time. "ITS-90" stipulates that it will not be
used as the internal inspection instrument of international temperature standard in the future,
but the international temperature Advisory Committee (CCT) recognizes that S-type thermocouple
can still be used to approximate the international temperature standard.The disadvantages of
S-type thermocouples are thermal potential, small thermoelectric potential rate, low sensitive
reading, low mechanical strength at high temperature, very sensitive to pollution, expensive
precious metal materials, so a large one-time investment.(R type thermocouple) platinum rhodium
13 platinum thermocouple platinum rhodium 13 platinum thermocouple (R type thermocouple) is
a precious metal thermocouple.The diameter of the coupling wire is 0.5mm and the allowable
deviation is - 0.015mm. The nominal chemical composition of the positive electrode (RP) is
platinum rhodium alloy, including 13% rhodium, 87% platinum, and the negative electrode (RN)
is pure platinum. The maximum long-term service temperature is 1300 ℃ and the maximum short-
term service temperature is 1600 ℃.The R-type thermocouple has the advantages of the highest
accuracy, the best stability, the wide temperature range and the long service life.It has good
physical and chemical properties, thermoelectric potential stability and oxidation resistance
at high temperature. It is suitable for oxidation and inert atmosphere.As the comprehensive
performance of R-type thermocouple is similar to that of S-type thermocouple, it is difficult
to popularize in China. Except for the application of temperature measurement on imported
equipment, it is rarely used in China.From 1967 to 1971, NPL in the UK, NBS in the US and NRC
in Canada carried out a cooperative study. The results show that the stability and
reproducibility of R-type thermocouple are better than that of S-type thermocouple. At present,
no research has been carried out in China.The disadvantages of R-type thermocouple are thermal
potential, small thermoelectric potential rate, low sensitive reading, low mechanical strength
at high temperature, very sensitive to pollution, expensive precious metal materials, so a
large one-time investment.(type B thermocouple) platinum rhodium 30 - platinum rhodium 6
thermocouple platinum rhodium 30 - platinum rhodium 6 thermocouple (type B thermocouple) is
precious metal thermocouple.The diameter of the coupling wire is specified as 0.5mm, and the
allowable deviation is - 0.015mm. The nominal chemical composition of its positive electrode
(BP) is platinum rhodium alloy, including 30% rhodium, 70% platinum, and the negative electrode

10. (BN) is platinum rhodium alloy, containing 6% rhodium. Therefore, it is commonly known as
double platinum rhodium thermocouple.The maximum operating temperature of the thermocouple is
1600 ℃ in the long term and 1800 ℃ in the short term.B-type thermocouple has the advantages
of the highest accuracy, the best stability, wide temperature measurement range, long service
life, high temperature upper limit, etc.It can be used in oxidizing and inert atmosphere, or
in vacuum for a short time, but not in reducing atmosphere or atmosphere containing metal or
non-metal vapor.An obvious advantage of B-type thermocouple is that it does not need
compensation wire, because the thermal potential is less than 3 μ V in the range of 0 ~
50 ℃.The disadvantages of B-type thermocouple are thermal potential, small thermoelectric
potential rate, low sensitive reading, low mechanical strength at high temperature, very
sensitive to pollution, expensive precious metal materials, so a large one-time investment.(K-
type thermocouple) Ni Cr Ni Si thermocouple Ni Cr Ni Si thermocouple (K-type thermocouple) is
the most widely used low-cost metal thermocouple at present, and its consumption is the sum
of other thermocouples.The nominal chemical composition of the positive electrode (KP) is Ni:
Cr = 90:10, the nominal chemical composition of the negative electrode (KN) is Ni: Si = 97:3,
and the service temperature is - 200 ~ 1300 ℃.K-type thermocouple has the advantages of good
linearity, large thermoelectric potential, high sensitivity, good stability and uniformity,
strong oxidation resistance, low price, etc. it can be used in the oxidizing inert
atmosphere.Widely used by users.K-type thermocouple can not be directly used in the atmosphere
of sulfur, reducibility or reduction, oxidation alternation and vacuum at high temperature,
nor is it recommended to be used in the atmosphere of weak oxidation.(n-type thermocouple) Ni
Cr Si Ni Si thermocoupleNi Cr Si Ni Si thermocouples (n-type thermocouples) are low-cost metal
thermocouples. They are the latest internationally standardized thermocouples. They were
developed by the laboratory of the Australian Ministry of defense in the early 1970s. They
overcome two important shortcomings of K-type thermocouples: the instability of thermoelectric
potential caused by the short-range ordering of Ni Cr alloy lattice in the range of 300-500 ℃;
the instability of thermoelectric potential caused by the short-range ordering of Ni Cr alloy
lattice in the range of 800 ℃Thermoelectric potential instability caused by preferential
oxidation of Ni Cr alloy.The nominal chemical composition of the positive electrode (NP) is
Ni: Cr: Si = 84.4:14.2:1.4, the nominal chemical composition of the negative electrode (NN)
is Ni: Si: Mg = 95.5:4.4:0.1, and the service temperature is - 200 ~ 1300 ℃.The n-type
thermocouple has the advantages of good linearity, large thermoelectric potential, high
sensitivity, good stability and uniformity, strong oxidation resistance, low price, not
affected by short-range ordering, and its comprehensive performance is better than the K-type
thermocouple,N-type thermocouple can not be directly used in the atmosphere of sulfur,
reducibility or reduction, oxidation alternation and vacuum, nor in the atmosphere of weak
oxidation.(E-type thermocouple) nichrome copper nickel thermocouple (E-type thermocouple),
also known as nichrome constantan thermocouple, is also a kind of low-cost metal thermocouple.
The positive electrode (EP) is: nichrome 10 alloy, the chemical composition is the same as
KP, the negative electrode (EN) is copper nickel alloy, the nominal composition is: 55% copper,
45% nickel and a small amount of manganese, cobalt, iron and other elements.The temperature

11. of the thermocouple is - 200 ~ 900 ℃.The thermoelectric force and sensitivity of e-type
thermocouple are the highest among all thermocouples. It is suitable to make thermopile to
measure small temperature change.It is not sensitive to the corrosion of high humidity
atmosphere, so it is suitable for the environment with high humidity.E thermocouple also has
the advantages of good stability, better oxidation resistance than copper constantan, iron
constantan thermocouple, low price, etc., which can be used in oxidizing and inert atmosphere
and widely used by users.E-type thermocouple can not be directly used in sulfur at high
temperature, reducing atmosphere, thermal potential uniformity is poor.(J-type thermocouple)
Fe Cu Ni thermocouple (J-type thermocouple), also known as fe-constantan thermocouple, is also
a cheap low-cost low-metal thermocouple.The nominal chemical composition of its positive
electrode (JP) is pure iron, and its negative electrode (JN) is copper nickel alloy, which is
often vaguely called constantan. Its nominal chemical composition is 55% copper and 45% nickel,
as well as a small amount of very important elements such as manganese, cobalt and iron.
Although it is called constantan, it is different from that of nickel chromium constantan and
copper constantan, so it cannot be replaced by en and TN.The temperature range of iron
constantan thermocouple is - 200 ~ 1200 ℃, but the commonly used temperature range is 0 ~
750 ℃.J-type thermocouples can be used in vacuum, oxidation, reduction and inert atmosphere,
but the positive iron oxidizes faster at high temperature, so the use temperature is limited,
and it can not be directly used in sulfurization atmosphere at high temperature without
protection.(T-type thermocouple) copper copper copper nickel thermocouple copper nickel
thermocouple (T-type thermocouple) is also known as copper constantan thermocouple, which is
also the best low-temperature low-cost metal thermocouple.Its positive electrode (TP) is pure
copper, its negative electrode (TN) is copper nickel alloy, often constantan. It is common
with constantan en of nichrome constantan, but not with constantan jn of iron constantan.
Although they are all called constantan, the measuring temperature range of the cover of
copper copper nickel thermocouple is - 200 ~ 350 ℃.T-type thermocouple has the advantages of
good linearity, large thermoelectric potential, high sensitivity, good stability and
uniformity, low price, etc., especially in the temperature range of - 200 ~ 0 ℃, better
stability, annual stability can be less than ± 3 μ V, which can be used as the second-class
standard for low-temperature value transfer after low-temperature verification.The oxidation
resistance of positive copper of T-type thermocouple is poor at high temperature, so the upper
limit of use temperature is limited.
The basic knowledge of thermocouples - the causes of common faults
of thermocouples and their solutions

12. 4、 Causes of common faults of thermocouples and Solutions
See table 4-1 for common fault causes and treatment methods of thermocouples
Table 4-1 common fault causes and treatment methods of thermocouples
Fault phenomenon Possible reasons processing method
The thermoelectric
potential is smaller
than the actual value
(the indicated value
of the display
instrument is lower)
Thermoelectric short circuit
Find out the cause of short
circuit. If it is caused by
humidity, dry it; if it is
caused by insulator damage,
replace the insulator
Dust accumulated at the terminal of
thermocouple, causing short circuit
Ash cleaning
Short circuit between compensating wires
Find out the short circuit
point, strengthen the insulation
or replace the compensation wire
Thermocouple thermal electrode
deterioration
If the length allows, cut out
the metamorphic section and weld
again, or replace the
thermocouple with a new one
Compensation conductor and thermal
dipole connected reversely
Reconnect correctly
Compensation wire is not matched with
thermocouple
Replace the matching
compensation lead
The installation position of
thermocouple is not recorded or the
insertion depth does not meet the
requirements
Re install as required
Temperature compensation of thermocouple
cold end does not meet the requirements
Adjust the cold end compensator
Thermocouple is not matched with display
instrument
More thermocouple or display
instrument to match
The thermoelectric
potential is larger
than the actual value
(the indicated value
of the display
instrument is higher)
Thermocouple is not matched with display
instrument
More thermocouple or display
instrument to match
Compensation wire is not matched with
thermocouple
Replace the compensation lead to
match
There is DC interference signal entering Eliminate DC interference
Poor contact between thermocouple
terminal and thermoelectric pole
Tighten the terminal screws

13. Unstable
thermoelectric
potential output
The insulation of thermocouple measuring
line is damaged, causing intermittent
short circuit or grounding
Find out the fault point and
repair the insulation
Thermocouple installation is not firm or
external vibration
Fasten thermocouple, eliminate
vibration or take shock
absorption measures
The thermoelectric pole will not break Repair or replace thermocouple
External interference (AC leakage,
electromagnetic induction, etc.)
Find out the interference source
and take shielding measures
Thermal potential
error of thermocouple
is large
Thermoelectric pole metamorphism Replace the hot electrode
Improper installation position of
thermocouple
Change installation position
Surface area of protective pipe grey Ash removal
Correct use of thermocouples
Xue Fu Lian
The correct use of thermocouples can not only accurately get the temperature value, ensure the
product is qualified, but also save the material consumption of thermocouples, save money and ensure
the product quality.Incorrect installation, thermal conductivity and time lag are the main errors of
thermocouple in use.
1. Error caused by improper installation
For example, the installation position and insertion depth of thermocouples cannot reflect the
real temperature of furnace, in other words, thermocouples should not be installed too close to doors
and heating places, and the insertion depth should be at least 8-10 times of the diameter of the
protective tube; the gap between the protective sleeve and the wall of thermocouples is not filled
with insulation material, which causes the hot overflow or cold air intrusion in the furnace, so the
thermocouple protective tube and the wall holeThe gap between them shall be blocked with insulating
materials such as fire-resistant mud or asbestos rope to avoid the influence of cold and hot air
convection on the accuracy of temperature measurement; the cold end of thermocouple shall be too
close to the furnace body to make the temperature exceed 100 ℃; the installation of thermocouple
shall avoid strong magnetic field and strong electric field as far as possible, so the thermocouple
and power cable shall not be installed in the same conduit to avoid interference and error;
thermocouple cannot be installedIn the area where the measured medium rarely flows, when measuring
the gas temperature in the tube with thermocouple, the thermocouple must be installed against the
flow direction and fully contacted with the gas.
2. Error caused by insulation variation

14. For example, if the thermocouple is insulated, the dirt or salt slag between the protective tube
and the stay wire plate are too much, resulting in poor insulation between the thermal dipole and the
furnace wall, which is more serious under high temperature. This will not only cause the loss of the
thermal potential, but also introduce interference, resulting in errors sometimes up to Baidu.
3. Error introduced by thermal inertia
Because of the thermal inertia of the thermocouple, the indicating value of the instrument lags
behind the change of the measured temperature, especially in the rapid measurement.Therefore,
thermocouples with thin thermoelectric pole and small diameter of protective tube should be used as
far as possible.When the temperature measurement environment permits, the protective pipe can even be
taken away.Because of the measurement lag, the amplitude of temperature fluctuation detected by
thermocouple is smaller than that of furnace temperature fluctuation.The larger the measurement lag
is, the smaller the amplitude of thermocouple fluctuation is, and the greater the difference with the
actual furnace temperature is.When a thermocouple with a large time constant is used to measure or
control the temperature, although the temperature displayed by the instrument fluctuates very little,
the actual furnace temperature may fluctuate a lot.In order to measure temperature accurately,
thermocouple with small time constant should be selected.The time constant is inversely proportional
to the heat transfer coefficient and directly proportional to the diameter of the hot end of the
thermocouple, the density of the material and the specific heat. To reduce the time constant, in
addition to increasing the heat transfer coefficient, the most effective way is to minimize the size
of the hot end.In use, the protective casing with thin wall and small inner diameter is usually made
of materials with good thermal conductivity.In the more precise temperature measurement, the bare
wire thermocouple without protective sleeve is used, but the thermocouple is easy to be damaged, so
it should be corrected and replaced in time.
4. Thermal resistance error
In case of high temperature, if there is a layer of coal ash on the protective tube and dust is
attached to it, the thermal resistance will increase and block the heat conduction. At this time, the
temperature indication is lower than the true value of the measured temperature.Therefore, the
outside of the thermocouple protection tube should be kept clean to reduce the error.
Remember not to connect the thermocouple compensation wire reversely
Li Qiang (Longkou metrological verification and Measurement Institute)
All the comrades who are engaged in temperature measurement know the working principle of
thermocouples - two different conductors form a closed circuit. When the temperature at both ends of
the circuit is equal, the thermoelectric potential of the circuit is zero.When the temperature of the
working end and the overstock end of the thermocouple is not equal, the heat generated in the rebel
loop is potential.The thermoelectric potential is related to the temperature difference between the
two ends.The free end temperature of the thermocouple is not a constant value.All manufacturers use
thermocouple special wire compensation wire.By extending the thermocouple to a constant temperature,
the thermoelectric potential of the thermocouple is compensated.
In the process of daily inspection, many electricians do not know the function of compensation wire,
only know that compensation wire is the special wire connecting thermocouple and instrument.It is
wrong to connect the compensation wire with thermocouple and instrument at will, which will cause
great error.Take a machining factory in our city as an example. At that time, the process of

15. workpiece required 920 ℃.The operator also set the value according to the process requirements, and
the instrument indication is normal.After the liquid fire, the workpiece appears striation, which
makes a furnace of workpiece scrapped.In the analysis of the accident, it is found that the
compensation wire is connected reversely. After correction, the temperature in the furnace is
973 ℃.At that time, the free end temperature of the mass transformer couple was 45 ℃, and the
indoor temperature was 18 ℃.The temperature error caused by reverse connection is as follows:
=1.817mV =0.718mV
Equivalent to 53.13 ℃.The above example shows that the
thermocouple compensation wire can cause considerable temperature error after being connected
reversely.Therefore, do not connect the compensation wire in reverse.
1. Principle of thermocouple temperature measurement
Two conductors (or semiconductors) of different materials form a closed circuit.
When the temperature of two contacts is different from 0, electromotive force
will be generated in the circuit. This phenomenon is called thermoelectric effect,
and the electromotive force is called thermoelectric potential.The combination
of conductors or semiconductors of these two different materials is called
thermocouple, and conductors A and B are called thermoelectric poles.Two contacts,
one is called hot end, also known as measuring end or working end. When measuring
temperature, it is placed in the measured medium; the other is called cold end,
also known as reference end or free end, which is connected with display
instrument through wire.TT
Contact potential is the electromotive force formed at the contact due to the
different free electron density of two different conductors.When two kinds of
conductors contact, the free electron diffuses from the conductor with high density
to the conductor with low density, and one side of the free electron loses positive
charge at the contact, and the other side of the free electron has negative charge.

16. When the diffusion reaches dynamic balance, a stable contact potential is formed at
both sides of the contact surface.The value of contact potential depends on the
properties of two different conductors and the temperature of the contact point.The
contact potentials AB (T) and ab (0) of the two contacts can be expressed asEET
Where: K - Boltzmann constant; - electric quantity per unit charge; NAT, and at0,
BT0 - electronic density of the two materials when the temperature is and 0,
respectively.ENBTNNTTA, B
Thermoelectric potential is a kind of electromotive force produced by the
difference of temperature between two ends of the same conductor.When the temperature
at both ends of the same conductor is different, the electronic energy at the high
temperature end is greater than that at the low temperature end, so the number of
electrons from the high temperature end to the low temperature end is more than that
from the low temperature end to the high temperature end. As a result, the high
temperature end is positively charged due to the loss of electrons, and the low
temperature end is negatively charged due to the gain of redundant electrons.
Therefore, the contact potential is formed at both ends of the conductor, and its
size is determined by the following factorsThe formula is:
Where, NAT and are the electronic density of conductor and conductor respectively,
which are functions of temperature.NBtAB
The total thermal potential generated in the thermocouple loop is
EAB(T, T0)=eAB(T)+eB(T,T0)-eAB(T0)-eA(T,T0)
In the total thermal potential, the thermoelectric potential of the
thermocouple is much smaller than that of the contact potential, which can be

17. ignored. Then the thermoelectric potential of the thermocouple can be expressed
as: (T, t0) = EAB (T) - EAB (T0)EAB
For the selected thermocouple, when the reference terminal temperature is 0
constant, (t0 = C) is constant, then the total thermoelectric EMF is only a
function of temperature, that is, (T, t0 = EAB (T) - C = f (T)TEABTEAB
This relation is very useful in practical measurement, that is, as long as the
magnitude of (, t0) is measured, the measured temperature can be obtained, which is
the principle of using thermocouple to measure temperature.EABTT
2、 Basic law of thermocouple
1Law of homogeneous Conductor: The thermoelectric EMF of a thermocouple composed
of two kinds of homogeneous conductors is only related to the temperature of two
materials and two contacts, but not to the size, shape and temperature
distribution along the electrode.That is, if the material is not uniform, when
there is a temperature gradient on the conductor, there will be additional
electromotive force.This theorem shows that thermocouples must be composed of
two homogeneous materials with different properties.Inside
2, law of intermediate conductor:When using thermocouple to measure temperature,
it is necessary to introduce connecting wires and instruments into the circuit.
Will the connecting wires and instruments affect the thermoelectric potential in
the circuit?The law of intermediate conductor states that when the third
conductor is connected in the thermocouple temperature measuring circuit, as
long as the temperature at both ends of the third conductor is the same, there
is no effect on the total thermal potential of the circuit.
3, law of intermediate temperatureIn the thermocouple temperature measuring
circuit, C is the temperature of a certain point on the thermoelectric pole. The
thermoelectric potential (T, t0) of the thermocouple at the contact temperature
of 0 is equal to the algebraic sum of the thermoelectric potential (T, TC) and

18. (T, t0) of the thermocouple at the contact temperature of C and 0, that is, EAB
(T, t0) = EAB (T, TC) + EAB (TC, t0)TABT, tEABABT, tTCTEABEAB
This law is the theoretical basis of the reference terminal temperature
calculation correction method. In the actual thermocouple temperature measuring
circuit, the thermoelectric potential with the reference terminal temperature not
0 ℃ can be corrected by using the thermocouple property.
3、 Structure of thermocouple
In order to meet the temperature measurement requirements and conditions of
different production objects, the structure of thermocouples includes common
type thermocouple, armored type thermocouple and thin film thermocouple.Inside
1. ordinary thermocouple: the common structure thermocouple is the most widely
used in the industry. It is generally composed of thermal electrode, insulating
sleeve, protective tube and junction box.Common thermocouple can be divided into
fixed thread connection, fixed flange connection, movable flange connection, no
fixed device and other forms according to the connection form when it is installed.
2Armored thermocouple: armored thermocouple is also called thermowell.It is a
solid combination of thermocouple wire, insulating material and metal sleeve.
The main advantages of armored thermocouple are small thermal capacity, fast
dynamic response, high mechanical strength and good flexibility. It can be
installed on the device with complex structure, so it is widely used in many
industrial sectors.
3Membrane thermocoupleThin film thermocouple is a kind of special thermocouple
made of two kinds of thin film thermoelectric materials, which are evaporated to
the insulating substrate by vacuum plating and chemical coating,The thermal
contact of thin film thermocouple can be made very small (can be thin to 0.01-
0.1 μ m), has the characteristics of small thermal capacity, fast reaction speed
and so on. The thermal response time reaches the microsecond level, which is

19. suitable for the surface temperature of small area and the dynamic temperature
measurement with rapid change.
4、 Compensation wire of thermocouple and compensation method of cold end
temperature
When the thermocouple material is selected, the thermoelectric EMF is only
related to the temperature of the hot end and the cold end.Therefore, only when
the cold end temperature is constant, the thermoelectric potential of the
thermocouple has a single value function relationship with the hot end
temperature.The graduation table of thermocouple is based on the cold end
temperature of 0 ℃, but in the actual use process, the cold end temperature is
often not 0 ℃, so the cold end temperature must be treated to eliminate the
influence of cold end temperature.
Then, when the hot end temperature is, the relationship between the
thermoelectric potential (T, 0) corresponding to the graduation table and the
thermoelectric potential (T, t0) actually generated by the thermocouple can be
obtained according to the intermediate temperature law as follows: (T, 0) = EAB (T,
t0) + EAB (T0, 0)TEABEABEAB
It can be seen that (T0, 0) is a function of cold junction temperature 0, so it
is necessary to treat the cold junction temperature of thermocouple.EABT
1. thermocouple compensation wire: In the actual temperature measurement, it is
necessary to transmit the potential signal output by the thermocouple to the
display instrument or control instrument in the control room tens of meters away
from the site, so that the cold end temperature 0 is relatively
stable.Thermocouples are generally made short, generally 350-2000mm, and wires
are needed to extend the cold end of the thermocouple.In the project, a kind of
compensation wire is used, which is usually made of two kinds of cheap metal
wires with different properties. In the temperature range of 0-100 ℃, the

20. compensation wire and the matched thermocouple are required to have the same
thermoelectric characteristics.T
2. cold end temperature correction method: The cold end of the thermocouple can
be extended to the place where the temperature is relatively stable by using the
compensation wire, but as long as the cold end temperature 0 is not equal to
0 ℃, the measured potential value (0) of the thermocouple loop needs to be
corrected.When the working end temperature is, the relationship between the
thermoelectric potential (T, 0) corresponding to the graduation table and the
thermoelectric potential (T, t0) actually generated by the thermocouple can be
obtained according to the intermediate temperature law as follows: (T, 0) = EAB
(T, t0) + EAB (T0, 0)TEABTTTEABEABEAB
It can be seen that the corrected value of the measured potential value (T,
t0) is (T0, 0).(T0, 0) is a function of the reference end temperature 0, the
corrected thermoelectric potential is (, 0), and the actual temperature value to
be measured can be found out from the graduation table.In the
worldEABEABEABTEABTT
For example, a nichrome nickel silicon thermocouple is used to measure the
furnace temperature.It is known that the cold end temperature is 0 = 30 ℃, and the
measured thermal potential (0) is 33.29mv, and the temperature of the heating furnace
is calculated.TEABT,T
solution(30,0) 1.203 MV was obtained by checking the graduation table of Ni
Cr Ni Si thermocouple.According to equation (15-13), EAB (, 0) = EAB (T, t0) +
EAB (T0, 0) = 33.29 + 1.203 = 34.493mv, and from the graduation table of Ni Cr
Ni Si thermocouple = 829.5 ℃.EABTT
3Cold end 0Constant temperature method at constant temperature: In laboratory
and precise measurement, the cold end is usually put into a 0 ℃ thermostat or
a container filled with ice water mixture to keep the cold end temperature at

21. 0 ℃, which is also called ice bath method.This is an ideal compensation method,
but it is very inconvenient to use in industry.
4. cold end temperature automatic compensation method (compensation bridge
method): The compensation bridge method uses the unbalanced voltage generated
by the unbalanced bridge as the compensation signal to automatically compensate
the change value of the thermal potential caused by the temperature of the cold
end not being 0 ℃ or the change in the measurement process of the
thermocouple.The compensation bridge is composed of three resistances 1, 2, 3
which are wound by manganese copper wire with smaller temperature coefficient of
resistance, and the resistances Cu which are wound by copper wire with larger
temperature coefficient of resistance and the regulated power supply.The
compensation bridge and the cold end of the thermocouple are in the same ambient
temperature. When the thermoelectric potential (T, t0) changes due to the change
of the cold end temperature, the resistance value of Cu changes with the change
of the cold end temperature. If the resistance of the bridge arm and the bridge
current are properly selected, the unbalanced voltage AB generated by the bridge
can compensate the thermoelectric potential change due to the change of the cold
end temperature 0, so as to achieve the purpose of automatic
compensation.UabRRRREABRUT