Common fault analysis and solution of thermocouple

1. Analysis and solution of common faults of
thermocouples
The price of thermal resistance and thermocouple depends on the model. You
need to specify your model, length, relevant specifications and quantity
For example, the price of 350 long platinum resistance of Pt100 230 is about
60 yuan. If it's a fast thermocouple, the price of KS KB thermocouple is generally
4-5 yuan. The price of kW is about 2 yuan. The price of ordinary K-type
thermocouple is about 80 yuan. If the price of anti riot thermocouple is about
200 yuan, the price of integrated thermocouple is about 200 yuanThe price of one
meter long wear-resistant thermocouple and 300 long wear-resistant head is about
400 yuan and the price of one meter long platinum rhodium thermocouple. At
present, the price of thermocouple is about 2000-2500 yuan. Due to the price
change of raw materials, the price of the above thermocouple should go with the
market,
Thermal resistance measurement - 100-550 degrees, thermocouple measurement
0-1300, generally below 500 degrees with thermal resistance, more than 500
degrees with thermal resistance
The correct use of k type 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.If the
installation is not correct, errors such as thermal conductivity and time lag
will be produced, which are the main errors in the use of thermocouples.
1.Error caused by improper installation
The thermocouple shall not be installed too close to the door and heating
place. The insertion depth shall be at least 8-10 times of the diameter of the
protective tube. The installation position and insertion depth cannot reflect
the real temperature of the furnace.The gap between the thermowell and the wall
of the thermocouple is not filled with insulation material, which causes hot
overflow or cold air intrusion in the furnace. Therefore, the gap between the
thermowell and the hole of the furnace wall shall be blocked with insulation
material such as refractory mud or asbestos rope, so as to avoid the influence
of cold and hot air convection on the accuracy of temperature measurement.The
installation of thermocouples shall avoid strong magnetic field and electric

2. field as much as possible, and the thermocouple and power cable shall not be
installed in the same conduit to avoid interference and error.Thermocouples can
not be installed in the area where the measured medium has little flow. When
measuring the gas temperature in the tube with thermocouples, the thermocouples
must be installed against the flow direction and fully contacted with the gas
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2.Error caused by insulation variation
For example, too much dirt or salt slag on thermocouple protection tube and
stay wire plate results in poor insulation between thermal dipole and furnace
wall, which is more serious under high temperature. This will not only cause
loss of 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
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4.Thermal resistance error

3. 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.
Preface
In the existing temperature measurement system, the most commonly used
temperature sensor thermocouple, because of its simple structure, is often
mistaken as "two wires of thermocouple, connect it to finish", in fact, this is
not the case.Although the structure of thermocouple is simple, there are still
various problems in use, such as improper installation or use method, which will
cause large measurement error. Even the certified thermocouple will be out of
tolerance due to improper operation and unqualified in use. In the reductive
atmosphere such as carburizing, if not paid attention, K-type thermocouple will
also be out of tolerance due to selective oxidation.
In order to improve the measurement accuracy, reduce the measurement error
and extend the service life of the thermocouple, the user is required to have
not only instrument operation skills, but also physical, chemical and material
knowledge.Based on many years of practice and related materials, the author
introduces in detail the measurement errors of thermocouples and points for
attention.
II. Main influencing factors of measurement error
1. Effect of insertion depth
(1) Selection of temperature measuring points
The installation position of thermocouple, that is, the selection of
temperature measuring point, is the most important.The location of the
temperature measurement point must be typical and representative for the

4. production process, otherwise it will lose the significance of measurement and
control.
(2) Insertion depth
When the thermocouple is inserted into the measured site, heat flow will be
generated along the length direction of the sensor.When the ambient temperature
is low, there will be heat loss, resulting in the temperature difference between
the thermocouple and the object to be measured, resulting in temperature
measurement error.In a word, the error caused by heat conduction is related to
the insertion depth.The insertion depth is related to the material of the
protection pipe.Because of its good thermal conductivity, the insertion depth of
metal protective tube should be deeper (about 15-20 times of the diameter), and
the thermal insulation of ceramic material should be better, which can be inserted
shallower (about 10-15 times of the diameter).For engineering temperature
measurement, the insertion depth is also related to the static or flowing state
of the measurement object. For example, the measurement of the temperature of
flowing liquid or high-speed air flow will not be subject to the above
restrictions, and the insertion can be shallower. The specific value shall be
determined by the experiment.
2. Impact of response time
The basic principle of contact temperature measurement is to achieve thermal
balance between the temperature measuring element and the measured
object.Therefore, it is necessary to keep the temperature measurement for a
certain period of time in order to achieve the heat balance between the two.The
holding time is related to the thermal response time of the temperature measuring
element.However, the thermal response time mainly depends on the sensor structure
and measurement conditions.For gas medium, especially static gas, it should be
kept for at least 30 minutes to reach equilibrium; for liquid, it should be kept
for at least 5 minutes.
The response time of the sensor is required to be in millisecond level when
the whole process is only 1s for the measured place with constant temperature
change, especially for the instantaneous change process.Therefore, the common
temperature sensor not only can't keep up with the temperature change speed
of the measured object, but also can't reach the thermal balance and produce

5. the measurement error.It's better to choose a sensor with fast response.In
addition to the influence of the protective tube, the diameter of the measuring
end of the thermocouple is also the main factor, that is, the finer the coupling
wire, the smaller the diameter of the measuring end, and the shorter the thermal
response time.The thermal response error of the temperature measuring element
can be determined by the following formula.
Δθ=Δθ0exp(-t/t) （1）
In style
Δ θ - error caused by temperature measuring element at time t, K or ℃
Error caused by temperature measuring element at Δ θ 0 - "t = 0", K or ℃
T -- measurement time, s
τ - time constant, s
ε - base of natural logarithm (2.718)
Therefore, when t = τ, Δθ = Δθ 0 / E
That is 0.368,
If t = 2 τ, then Δθ = Δθ 0 / E2
That is 0.135.
When the temperature of the measured object rises or falls at a certain speed
α (K / s or ℃ / s), after enough time, the response error can be expressed as
follows:
Δθ∞=-ατ (2)
In style
Δ θ∞ - error caused by temperature measuring element after enough time
It can be seen from equation (2) that the response error is directly
proportional to the time constant (τ).In order to improve the efficiency of
verification, many enterprises use automatic verification device to verify
thermocouple, but the device is not perfect.It is found in the heat treatment
workshop of the second steam transmission plant that if the constant temperature

6. time at 400 ℃ is not enough and the heat balance cannot be achieved, it is easy
to misjudge.
3. Effects of thermal radiation
The thermocouple inserted into the furnace for temperature measurement will
be heated by the heat radiation from the high temperature object.It is assumed
that the gas in the furnace is transparent, and when the temperature difference
between the thermocouple and the furnace wall is large, the temperature
measurement error will be generated due to the energy exchange.
In unit time, the radiant energy exchanged by the two is p, which can be
expressed as follows:
P=σε(Tw4-Tt4) (3)
In style
σ - Stefan Boltz constant
ε - emissivity
TT -- temperature of thermocouple, K
Tw - temperature of furnace wall, K
In unit time, the energy of heat exchange between thermocouple and
surrounding gas (temperature T) through convection and heat conduction is p '
P'=αA(T-Tt) (4)
Where α - thermal conductivity
A - surface area of thermocouple
In normal state, P = P ', the error is:
Tt-T=σε(Tt4-Tw4)/ αA (5)
For unit area, the error is

7. Tt-T=σε(Tt4-Tw4)/ α (6)
Therefore, in order to reduce the thermal radiation error, the heat
conduction should be increased and the furnace wall temperature TW should be as
close to the thermocouple temperature TT as possible.In addition, attention shall
be paid during installation: the thermocouple installation position shall avoid
the thermal radiation from the solid as far as possible, so that it cannot
radiate to the thermocouple surface; the thermocouple is better to be equipped
with a thermal radiation shield.
4. Effect of increasing thermal impedance
For the thermocouple used at high temperature, if the measured medium is in
gas state, the dust deposited on the surface of the protection tube will be
burned and melted on the surface, which will increase the thermal impedance of
the protection tube; if the measured medium is a melt, there will be slag
deposition in the use process, which not only increases the response time of the
thermocouple, but also makes the indicated temperature low.Therefore, in addition
to regular verification, in order to reduce the error, it is necessary to carry
out spot check frequently.For example, imported copper smelting furnace is not
only equipped with continuous temperature measuring thermocouple, but also
equipped with consumption type thermocouple temperature measuring device to
calibrate the accuracy of continuous temperature measuring thermocouple in time.
III. precautions for thermocouple temperature measurement
1. Influence of thermocouple wire heterogeneity
(1) Thermocouple material is not homogeneous
When the thermocouple is verified in the measuring room, according to the
regulations, the depth inserted into the verification furnace is only 300
mm.Therefore, the verification results of each thermocouple can only reflect or

8. mainly reflect the thermoelectric behavior of 300 mm long thermocouple wires
from the measurement end. However, when the thermocouple is long, most of the
thermocouple wires are in the high temperature zone. If the thermocouple wires
are homogeneous, the measurement results are independent of the length according
to the uniform circuit rule.However, thermocouple wires are not homogeneous,
especially in the case of poor homogeneity and temperature gradient of low-metal
thermocouple wires, then local thermoelectric EMF will be generated, which is
called parasitic EMF.The error caused by parasitic potential is called
inhomogeneous error.
In the existing verification regulation of precious metal and low-cost metal
thermocouples, the heterogeneity of thermocouples has not yet been regulated.
Only in the standard of thermocouple wire material, there are certain
requirements for the heterogeneity of thermocouple wire.The non-uniform
thermoelectric EMF of low-cost metal thermocouples is calculated by the head
tail verification method.Normal thermocouple wire production plants, according
to the requirements of national standards, produce non-uniform thermoelectric
EMF products that meet the requirements.
(2) Heterogeneity of thermocouple wire after use
For the new thermocouple, even if the non-uniform thermoelectric potential
energy meets the requirements, but repeated processing and bending will cause
processing distortion and loss of homogeneity of the thermocouple; and the
thermocouple in use will also cause thermoelectric potential change due to the
degradation of the thermocouple wire for a long time under high temperature. For
example, the thermocouple inserted into the industrial furnace will degrade along
the length direction of the thermocouple wire and increase with the
temperatureWhen the degraded part is in the place with temperature gradient, the
parasitic electromotive force will be superposed in the total thermoelectric
force and the measurement error will appear.
In practice, the author found that some thermocouples (mostly low-cost metal
thermocouples) that have passed the verification by the Metrology Department are
unqualified when they are used on site, and they are still qualified when they
are returned to the Metrology Department for verification. The main reason is
the inhomogeneity of the coupling wire.The technicians who produce thermocouples
have realized that the unqualified rate of thermocouples also increases with the
increase of their length, which is the influence of the heterogeneity of heating
thermocouple wire.In a word, the error caused by inhomogeneity, i.e. parasitic
electromotive force, depends on the degree of inhomogeneity of thermocouple wire

9. itself and the magnitude of temperature gradient, so it is very difficult to
quantify it.
2. Shunt error of armored thermocouple
(1) Shunt error
The armored thermocouple used in carburizing furnace of Wazhou group is not
allowed for only one week.In order to explore the reasons, the author has visited
the site and found no abnormality, and took it from the furnace and passed the
verification by the measuring room.So what's the problem?Finally, according
to the field installation characteristics of the thermocouple, it is found that
the above problems are caused by the shunt error of armored thermocouple.
The so-called shunt error is that when the temperature of the furnace is
measured by armored thermocouple, when there is a temperature distribution of
more than 800 ℃ in the middle part of the thermocouple, because of the decrease
of its insulation resistance, the thermocouple indication appears abnormal
phenomenon.According to the rule of homogeneous circuit, the temperature
measurement with thermocouple is only related to the temperature at both ends of
the measuring end and the reference end, but not to the temperature distribution
in the middle.However, because the insulation material of armored thermocouple
is powder like MgO, the insulation resistance will drop by an order of magnitude
every 100 ℃ when the temperature rises. When the temperature of the middle part
is high, there must be leakage current, which will cause shunt error in the
output potential of thermocouple.
(2) Conditions for the generation of shunt error
The armored thermocouple is horizontally inserted into the furnace. Its
specification and experimental conditions are as follows: diameter of Φ 4.8mm,
length of 25m, length of heating belt in the middle part of 20m, and temperature
of 1000 ℃.In this experiment, the temperature difference between the measuring
end and the middle part of the thermocouple is 200 ℃.If the temperature of the
measuring end is higher than the middle part, a negative error will be generated;
on the contrary, a positive error will be generated.If the temperature difference
between the two is 200 ℃, then the shunt error is about 100 ℃.This is absolutely

10. not to be ignored. The generation conditions of shunt error are related to the
type and diameter of armored thermocouple, as shown in the table.
3. Influencing factors and Countermeasures of shunt error
The phenomenon of shunt error of armored thermocouple under high temperature
is attracting people's attention. Therefore, it is necessary to understand
the influencing factors of shunt error and take appropriate measures to reduce
or eliminate the influence of shunt error.
(1) Diameter of armored thermocouple
For the K-type armored thermocouple (MgO insulated) with a length of 9m,
only the middle part of the thermocouple is heated.The experimental results show
that the size of the shunt error is inversely proportional to the square root of
its diameter (the diameter is too thin, and this rule is not observed), that is,
the finer the diameter is, the larger the shunt error is.
When the temperature of the middle part is higher than 800 ℃, the shunt
error will be produced for Φ 3.2mm armored thermocouple.But for Φ 6.4mm and Φ
8mm armored thermocouples, when the temperature of the middle part is 900 ℃, no
shunt error is found.For armored thermocouples with diameter of Φ 6.4mm
(diameter of thermal wire is Φ 1.4mm) and Φ 8mm (diameter of thermal wire is
Φ 2.0mm), when the temperature of the middle part is 1100 ℃, the shunt error
of armored thermocouple with diameter of Φ 8mm is only half of that of Φ
6.4mm.This value (50%) is approximate to the square ratio (1.42 / 2.02) of the
wire diameter of two armored thermocouples, while the square ratio of the wire
diameter is the resistance ratio of the wire.Therefore, in order to reduce the
shunt error, we should try to choose the armored thermocouple with the coarse
diameter.
(2) Temperature in the middle
If the temperature of the middle part exceeds 800 ℃, it may produce shunt
error, and its size will increase exponentially with the increase of
temperature.Therefore, in addition to the measuring end, other parts should be
avoided to exceed 800 ℃.

11. When the temperature of the heating belt in the middle part is higher than
800 ℃, the longer the length of the heating belt is, the farther it is from the
measuring end, and the larger the shunt error is.Therefore, the length of heating
belt should be shortened as much as possible, and heating should not be far away
from the measuring end to reduce the shunt error.
(3) Resistance of thermocouple wire
When the diameter of armored thermocouple is the same, the shunt error will
increase with the increase of resistance of thermocouple wire.Therefore, it is
better to use thermocouple wire with small resistance.For example, the shunt
error of S-type armored thermocouple with the same diameter is reduced by 40%
compared with K-type thermocouple.Therefore, S-type thermocouple can be used to
measure the temperature field distribution in the furnace. Although the cost is
high, it is more accurate.
(4) Insulation resistance
At high temperature, the oxide resistivity will decrease exponentially with
the increase of temperature, and the shunting error mainly depends on the
insulation performance of the high temperature part. The lower the insulation
resistance is, the easier the shunting error will occur.When the insulation
resistance is increased 10 times or reduced to 1 / 10, the shunt error is also
reduced to 1 / 10 or increased 10 times.In order to reduce shunt error, armored
thermocouple with thick diameter should be used as much as possible to increase
the thickness of insulation layer.If the above measures are invalid, the
assembled thermocouple shall be used.
4. The influence of short range ordered structure change (k state)
When K-type thermocouples are used in the temperature range of 250-600 ℃, due to
the change of their microstructure and the formation of short-range ordered structure, they
will affect the thermoelectric potential and produce errors, which is called K-state.It is a
unique lattice change of Ni Cr alloy. When the content of Cr is in the range of 5-30%, there
is an error caused by the order disorder transition of atomic lattice, which varies with the
content of Cr and the temperature.Heat the K-type thermocouple from 300 ℃ to 800 ℃,
take a point every 50 ℃, and measure the potential at this point.When the temperature is
450 ℃, the maximum deviation is 4 ℃. In the range of 350-600 ℃, all of them are positive

12. deviation.Due to the existence of K-state, the verification results of K-type thermocouples
in temperature rise or temperature fall are inconsistent, so the verification sequence is
clearly specified in the verification regulation of low-temperature to high-temperature point
by point temperature rise verification; moreover, at the verification point of 400 ℃, not only
the heat transfer effect is poor, it is difficult to achieve heat balance, but also just in the
maximum range of K-state error.Therefore, it should be very careful to judge whether the
point is qualified or not.
The change of short-range ordered structure of Ni Cr alloy exists not only in the K-
type, but also in the E-type thermocouple positive pole, but as a change, the E-type
thermocouple is only 2 / 3 of the K-type.In a word, K state is related to temperature and
time. When the temperature distribution or thermocouple position changes, its deviation
will also change greatly, so it is difficult to make an accurate evaluation of the deviation.
5. Influence of use atmosphere
(1) Selective oxidation
For the Ni Cr alloy containing Fe, if the oxygen partial pressure is lower
than a certain value, the CR with high affinity to O2 will undergo selective
oxidation, which is the unique grain boundary oxidation of Ni Cr alloy.If the
oxide layer on the outer surface is observed with a microscope, green precipitates
can be seen, which is usually called "green corrosion".Especially when the
temperature is in the range of 800-1050 ℃ and there are reductive gases such as
CO and H2 in the system, the positive electrode of K-type thermocouple is more
likely to undergo selective oxidation.This kind of low thermal potential caused
by the decrease of Cr content has become the limiting factor for the long-term
use of K-type thermocouple in the heat treatment industry.
If the gas used is very pure and the system does not contain oxygen, the
service life of the thermocouple can be prolonged; if there is an oxide layer on
the surface of the thermocouple wire, enough oxygen can still be provided for
the selective oxidation of CR.Therefore, clean and polished even wire should be
used in non oxidizing atmosphere.At the same time, try to avoid using in inert
gas with trace oxygen or air with low oxygen partial pressure.When the length
and diameter of the protective tube are large (i.e. the protective tube is very
thin), due to the poor air circulation, the residual small amount of oxygen can
still provide conditions for the selective oxidation of CR.

13. (2) Countermeasures of selective oxidation
In order to prevent or slow down the deterioration of K-type thermocouples
caused by selective oxidation, in addition to improving the materials,
corresponding countermeasures should be taken in the structure of thermocouples:
(a) select the metal with stronger affinity for oxygen than Cr as the getter,
seal it in the protective tube to prevent selective oxidation of Cr, or increase
the diameter of the protective tube or air blowing method to increase the oxygen
content.(b) Assembly thermocouple materialization.The patent product developed
by the author - thermocouple for solid carburizing furnace, i.e. the assembled
thermocouple with sealed structure, can prevent selective oxidation of Cr. the
scheme has been proved effective by more than ten enterprises, such as wachaft
group, FAW, Erqi, Epson industrial furnace, Shenzhong, Shenchi, Qianjiang
Motorcycle, etc.The service life is more than 12 months, the user is very
satisfied.
(3) Influence of use atmosphere
The stability of thermocouples, due to the use of different temperatures and
atmospheres, for the same sensor, such as K-type thermocouples, the maximum use
temperature also varies with the diameter, and the stability of K-type
thermocouples with the same diameter also varies with the structure.When
selecting thermocouples, it is necessary to consider the service conditions:
common temperature and maximum service temperature; use atmosphere such as
oxidation-reduction; anti vibration performance.
For the assembled thermocouple, the influence of atmosphere depends on the
material of protection tube and thermocouple structure. Therefore, it is
necessary to be familiar with and master the physical and chemical properties of
various protection tube materials.For example, in the powder metallurgy industry,
molybdenum tube is commonly used as thermocouple protection tube, and the
application effect is better under the H2 atmosphere of 1600 ℃.However,
molybdenum tube is damaged by oxidation in a very short time in an oxidizing
atmosphere.Secondly, the appropriate thermocouple should be selected according
to the use atmosphere. In the oxidizing atmosphere above 1300 ℃, the platinum
rhodium thermocouple should be selected, and the tungsten rhenium thermocouple
is better under the reducing and vacuum conditions.
For K-type thermocouple, it is suitable to work in air, O2 and other
atmospheres, but when it is used in H2 atmosphere, its surface is reduced by H2,
and there is no effect in a short time. For example, when it is exposed in H2
for a long time, at the same time of accelerated reduction, the even wire will

14. grow up and break; in reducing atmospheres such as CO or gas, its degradation
will be significantly accelerated and out of tolerance.
For Sheathed Thermocouples, the radius of hydrogen atom is very small, and
it is easy to enter the inner part of the thermocouple through the outer shell,
which will also accelerate the degradation, resulting in a significant reduction
of the thermoelectric potential.
(4) Influence of insulation resistance
Under high temperature, the insulation resistance of the insulation material
used for thermocouple decreases rapidly with the increase of temperature.
Therefore, leakage current will be generated. The current flows into the
instrument through the insulation whose insulation resistance has decreased,
which makes the indication of the instrument unstable or produces measurement
error, and the recorder may also be disordered.
Deterioration and service life of thermocouples
1. Degradation of thermocouples
The service life of thermocouples is related to their deterioration. The so-
called deterioration of thermocouples is the phenomenon of aging and
deterioration of thermocouples after use.The internal grains of thermocouple
made of metal or alloy grow up gradually at high temperature.At the same time,
there are a small amount of impurities in the alloy, its position or shape will
also change, and the reduction or oxidation gas in the surrounding environment
will also react.With these changes, the thermoelectric EMF of the thermocouple
will also change very sensitively.So the degradation of thermocouple is
inevitable.
2. Thermocouple service life

15. The degradation of thermocouple is a quantitative process, which is difficult
to quantify. It will change with the type, diameter, temperature, atmosphere and
time of thermocouple.The service life of thermocouples refers to the time when
the deterioration of thermocouples exceeds the allowable error, or even the
broken wires cannot be used.
(1) Life of assembled thermocouple
Only the stability of thermocouples is required in Chinese standards.That is
to say, the change of thermoelectric EMF before and after use is regulated after
200H at a certain temperature.However, no provision for service life has been
found.The requirements for thermocouple service life in Japan are based on the
continuous use time of thermocouple specified in JIS (c-1602-1995) standard.2000h
for B, R, s type thermocouples, 10000h for K, e, J, T type thermocouples.
In practical use, the assembled thermocouple usually has a protective tube,
which can only be used as bare wire under special circumstances.Therefore, in
most cases, the life of the protection tube determines the life of the
thermocouple.To judge the actual service life of thermocouple, it is necessary
to collect and accumulate the data in actual use for a long time, so as to give
a more accurate result.
(2) Life of armored thermocouple
Since the armored thermocouple is protected by a sleeve and isolated from
the external environment, the material of the sleeve has a great influence on
the service life of the armored thermocouple. The thermocouple wire and metal
sleeve must be selected according to the purpose.When the material is selected,
its life increases with the increase of the diameter of armored
thermocouple.Compared with assembled thermocouple, armored thermocouple has many
advantages, but its service life is often lower than assembled thermocouple.
Five conclusion
Thermocouple is the most commonly used temperature sensor in scientific
research and production. Although its structure is simple, if it is not noticed
in use, it will still produce large measurement error.In view of the problems
that are easy to occur in use, the author discusses in detail the main causes of
errors in the selection of temperature measuring points, the insertion depth,

16. response time, thermal radiation and thermal impedance of thermocouples, and
points out the precautions in the use of thermocouples, such as the heterogeneity
of thermocouples, the use atmosphere, the insulation resistance, the selective
oxidation of K-type thermocouples, the K-state and the shunt error of armored
thermocouples.It is helpful to improve the accuracy of measurement and prolong
the service life of thermocouple.