14 common questions about thermocouple temperature measurement

1. 14 questions about thermocouple temperature
measurement
In order to facilitate our customers to better understand some
knowledge of thermocouples, the thermocouple engineer has collected
14 common problems of thermocouples, which is convenient for your
use:
1.What is the principle of thermocouple measurement?
The working principle of thermocouples is based on seeback
effect, that is, two ends of conductors with different components are
connected into a circuit. If the temperature of two connecting ends
is different, thermal current will be generated in the circuit.
The thermocouple consists of two different wires (thermoelectric
poles), one end of which is welded to each other, forming the
measuring end (also known as the working end) of the
thermocouple.Insert it into the medium of the temperature to be
measured, and the other end of the thermocouple (reference end or
free end) is connected with the display instrument.If there is a
temperature difference between the measuring end and the reference
end of the thermocouple, the display instrument will indicate the
thermoelectric EMF generated by the thermocouple.
2. What is the measurement principle of thermal resistance?
The thermal resistance measures the temperature by using the
characteristics that the resistance of the metal conductor or
semiconductor changes with the change of the temperature. The heated
part of the thermal resistance (the temperature sensing element) is
formed on the substrate by evenly winding the thin metal wire on the
framework made of the insulating material or by the laser sputtering
process.When the measured medium has a temperature gradient, the
measured temperature is the average temperature of the medium layer
within the range of the temperature sensing element.

2. 3. How to choose thermocouple and thermal resistance?
Select according to the temperature measurement range: thermocouple
is generally selected above 500 ℃, and thermal resistance is
generally selected below 500 ℃;
According to the measurement accuracy selection: select thermal
resistance with high accuracy requirements and thermocouple with low
accuracy requirements;http://www.chinathermocouple.com
According to the selection of measurement range: the temperature
measured by thermocouple generally refers to
"point" temperature, and the temperature measured
by thermal resistance generally refers to the
average space temperature;
4. What is armored thermocouple and what are its advantages?
In iec1515, it is named as "mineral insulated thermocouple
cable",
That is, inorganic mineral insulated thermocouple cable.The outer
surface of the thermoelectric pole, insulator and sheath formed by
overall drawing seems to be covered with a layer of "armour", so it
is called armored thermocouple.Compared with the general assembled
thermocouple, it has the advantages of high pressure resistance, good
bending performance, good oxidation resistance and long service life.
5. What are the graduation numbers of thermocouples? What are their
characteristics?
There are mainly s, R, B, N, K, e, J, t and so on.S, R, B belong
to noble metal thermocouple, N, K, e, J, t belong to low metal
thermocouple.
S scale is characterized by strong anti-oxidation performance,
suitable for continuous use in oxidizing and inert atmosphere, long-
term use temperature 1400 ℃, short-term 1600 ℃.Among all the
thermocouples, the s scale has the highest accuracy level, which is
usually used as standard
thermocouple;http://www.tksiliconerubbercable.com

3. Compared with the s scale, the R scale is about 15% larger than
the s scale, and its other performances are almost the same
The thermoelectromotive force of graduation B is very small at
room temperature, so the compensation wire is not generally used in
measurement.Its long-term use temperature is 1600 ℃, and its short-
term use temperature is 1800 ℃.It can be used in oxidizing or
neutral atmosphere, or in vacuum for a short time.
The characteristics of n-graduation are high temperature oxidation
resistance at 1300 ℃, long-term stability of thermoelectromotive
force and short-term recurrence of thermal cycle, good resistance to
nuclear radiation and low temperature, which can partially replace s-
graduation thermocouple;
K scale is characterized by strong anti-oxidation performance,
suitable for continuous use in oxidizing and inert atmosphere, long-
term use temperature 1000 ℃, short-term 1200 ℃.It is widely used in
all thermocouples;
The characteristic of e-scale is that in common thermocouples,
its thermoelectric EMF is the largest, that is, its sensitivity is
the highest.It should be used continuously in oxidizing and inert
atmosphere at 0-800 ℃;http://www.tiankangcable.com
The characteristics of J scale are that it can be used not only in
oxidizing atmosphere (upper limit of use temperature is 750 ℃), but
also in reducing atmosphere (upper limit of use temperature is
950 ℃), and it is resistant to H2 and CO gas corrosion, and it is
mostly used in oil refining and chemical industry;
The characteristic of the t-scale is that it has the highest
accuracy level among all the low-cost metal thermocouples and is
usually used to measure the temperature below 300 ℃.
6. How many ways are the leads of thermal resistance?What's the
impact?
There are three ways to lead out the thermal resistance: 2-wire
system, 3-wire system and 4-wire system.
2 The wiring of wire made thermistor is simple, but the
additional error of lead wire resistance should be brought.Therefore,

4. it is not suitable to manufacture class a precision thermal
resistance, and the lead and wire should not be too long when used.
3 The wire system can eliminate the influence of the lead
resistance, and the measurement accuracy is higher than that of the
two wire system.As a process detection component, it is widely used.
4 The wire system can not only eliminate the influence of the
lead resistance, but also eliminate the influence of the resistance
when the resistance of the connecting wire is the same.In high-
precision measurement, 4-wire system shall be adopted.
7. What are the advantages and disadvantages of n-type thermocouple
compared with K-type thermocouple?
Advantages of n-type thermocouple:
-High temperature oxidation resistance, long-term stability.The
selective oxidation of Cr and Si elements in the positive electrode
of K-type thermocouple leads to the inhomogeneous composition and
thermoelectric potential drift of the alloy. The increase of Cr and
Si content in n-type thermocouple changes the oxidation mode of Ni Cr
alloy from internal oxidation to external oxidation, resulting in the
oxidation reaction only on the surface;
-The stability of low temperature short-term thermal cycle is
good, and the magnetic transformation is restrained;
-Strong resistance to nuclear radiation.The n-type thermocouples
eliminate the easily changed elements Mn and CO in the K-type
thermocouples and further enhance the neutron radiation resistance;
-In the range of 400 ~ 1300 ℃, the linearity of n-type
thermocouple is better than that of K-type thermocouple.
Disadvantages of n-type thermocouple:
-The material of n-type thermocouple is harder than that of K-type
thermocouple, so it is difficult to process;
-The price is relatively expensive.The thermal expansion
coefficient of n-type thermocouple is 15% lower than that of
stainless steel, so nicrsi / NiSi alloy should be used as the outer
tube of n-type armored thermocouple; - the nonlinear error is large
in the range of - 200 ~ 400 ℃.

5. 8. How to choose the right heat installation sleeve?
The shape of the heat installation sleeve is mainly determined by
the temperature, pressure, density and flow rate of the medium and
the required insertion length.ASME / ANSI ptc19.3 fully stipulates
this, and the casing strength analysis software can be used to
calculate whether the casing design meets the process
requirements.The heat sleeve installed on site needs to be calculated
The strength of the pipe, which affects the strength of the
casing, mainly includes the following three points:
1. The vibration caused by the flow; the liquid passing through
the casing produces a certain frequency of vortex, which is called
vortex frequency, and the frequency and velocity are proportional to
each other.If this frequency is close to or consistent with the
natural frequency of the thermowell, resonance will be generated,
which will absorb a large amount of heat energy, resulting in high
stress and may damage the thermowell and the sensor in the
thermowell.ASME technical standards require that the ratio of vortex
frequency and natural frequency of thermal casing shall be less than
0.8.
2. Flow induced stress; fluid flow changes with flow rate and
density, and a force is applied to the hot casing, and the pressure
caused by this flow can be calculated.
3. Process pressure; the maximum static pressure that the
thermowell can withstand can be calculated."
Generally, there are three connection methods of hot installation
sleeve: threaded connection, flange connection and welding.
9.How to choose a suitable bimetal thermometer?
When installing horizontally, choose axial or universal bimetal
thermometer;
When installing vertically, choose radial or universal bimetal
thermometer;
When inclined installation, select axial, radial or universal
bimetal thermometer according to the actual needs;

6. If it is necessary to set the upper and lower limit alarm control
for the measuring point, the bimetal thermometer with electric
contact can be selected
10. What are the advantages and disadvantages of bimetal thermometer?
Bimetallic thermometer has the advantages of relatively low price,
intuitive reading, and the disadvantages of small temperature
measurement range and relatively low accuracy.It is usually used as a
local measurement and display instrument.
11.What are the characteristics of the temperature transmitter?
Temperature transmitter is characterized by low static power
consumption, safety and reliability, no maintenance, long service
life.-Small size, can be integrated with thermocouple and thermal
resistance, not only convenient to install, but also save the
installation cost of temperature transformer.
-The transmission signal is 4-20mA standard signal, which not only
has strong anti-interference ability and long transmission distance,
but also can save expensive compensation wire.
-HART protocol and FF, profbus communication protocol can be
provided.
12. What is the principle of pressure thermometer?
According to the law of liquid expansion, the pressure and
temperature of a certain mass of liquid are linear under the
condition of constant volume.The pressure of gas and steam is also a
function of temperature,
Therefore, the scale of pressure thermometer should be evenly
divided.The pressure type thermometer is composed of a temperature
bag filled with a temperature sensing medium, a pressure transmitting
element (capillary) and a pressure sensing element (spring tube).
13. What is the principle of infrared thermometer?

7. The infrared thermometer consists of optical system,
photoelectric detector, signal amplifier, signal processing, display
output and other parts.The optical system gathers the infrared
radiation energy of the target in its field of view. The infrared
energy is focused on the photodetector and transformed into the
corresponding electrical signal, which is then converted into the
temperature value of the measured target.
14. How to choose the right compensation wire or cable?
The compensating wires and cables of thermocouples are mainly
used to extend the thermoelectric EMF of thermocouples to the
secondary instrument or control room.There are mainly two types of
compensation wires: extension type and compensation type. The
extension type uses the same material as the thermoelectric pole, so
the accuracy is higher; the compensation type uses the material with
the phase potential characteristic of the thermoelectric pole, so the
accuracy is not as high as the extension type.