Working principle and structure of thermocouple

Working principle and structure of thermocouple

1. Working principle and structure of thermocouple
Maintenance post
1.Understand how it works
1.1Thermocouple temperature measurement principle: two conductors with different
electronic density form a closed circuit. If the temperature of the two joints
is different, there will be current in the circuit. This phenomenon is called
thermoelectric phenomenon. The corresponding electromotive force is called
thermoelectric potential or thermoelectric potential. It has a certain functional
relationship with the temperature. Using this relationship, the temperature can
be measured.This phenomenon includes the following principles: Peltier's theorem:
different materials are combined together to generate electric potential at the
joint surface.Thomson's theorem --- electric potential caused by temperature
difference.When the conductor material of the thermocouple is uniform, the size
of its thermal potential has nothing to do with the length and diameter of the
conductor itself, only with the composition of the conductor material and the
temperature of both ends.Therefore, thermocouples of various uses can be made
with different conductors or semiconductors to meet the needs of measurement of
different temperature objects.
1.2Three laws of thermocouples
Law of homogeneous conductor
A closed circuit made of a single
homogeneous metal will not generate
current even if the temperature
changes at every point along the
circuit.That is, e = 0.

2. In the thermocouple circuit formed
by two homogeneous metal materials
a and B, the correlation function
between the thermoelectric
potential E and the temperature T1,
T2 at the junction is not affected
by the intermediate temperature T3
and T4 between a and B.
Law of intermediate metals
Insert a homogeneous third metal C
at any point other than the two
junction points of the thermocouple
circuit formed by a and B. if the
temperature T3 of the junction
points at both ends of C is the
same, e will not be affected by C
insertion.

In the thermocouple circuit formed
by a and B, when the junction of a
and B is opened and inserted into
the homogeneous metal C, if the
temperature of the junction of a
and C is equal to the temperature
of the junction before opening, e
is not affected by the insertion of
C.

3. As shown in the right figure,
insert the intermediate metal C of
the third into the thermocouple
formed by a and B to form two sets
of thermocouples, i.e. a and C, C
and B.EAC + ECB = EAB with junction
temperature T1 and
T2.http://www.chinathermocouple.com

Law of intermediate temperature
As shown in the figure on
the right, the closed loop formed
by any number of dissimilar metals
a, B, C · g. when all or all
joints of the closed loop are kept
at the same temperature, e = 0.

4. As shown in the right
figure, E12 + E23 = E13 for the
thermocouple formed by a and B,
when the temperature of the two
junction points is T1 and T2, e is
E12, and when T2 and T3, e is
E13.At this time, T2 is called the
intermediate temperature.
Choose a standard temperature such
as 0 ℃ with the intermediate
temperature T2, and obtain the
thermoelectric EMF of any
temperature T1, T2, T3 · TN
relative to 0 ℃. The
thermoelectric EMF between any two
points can be calculated.

As shown in the figure on the
right, the above concept is
applicable to thermocouple circuits
using compensation wires.A and B
are thermocouples, C and D are
compensation wires for a and B, and
M is digital voltmeter. After
calculation, the following
relationship can be obtained:
E = EAB (t1) － EAB (t3)
In other words, the potential
difference measured by M is
determined by T1 and T3, and is not
affected by T2.


2.Understand equipment structure

5. 2.1The structure thermocouple of thermocouple is composed of two different
conductors (or electrodes). One end of these two conductors is welded together
to form a hot end (or working end), which is placed in the measured medium during
temperature measurement.The other end is called cold end (or free end), which is
connected to secondary instrument (display instrument) or electrical measuring
equipment.a. Common thermocouple: it is the most widely used thermocouple, mainly
used to measure the temperature of gas, steam, liquid and other media.According
to the difference of temperature measurement range and environment, the materials
of thermocouple electrode and protective sleeve are also different. However, due
to the similar use conditions, this kind of thermocouple has been standardized
and serialized.According to the connection method of its installation, it can be
divided into two types:http://www.tiankangcable.com threaded connection and
flange connection.b. Armored thermocouple: also known as cable thermocouple, it
is a solid whole formed by the combination of hot electrode (mostly platinum
wire, also nickel wire), insulating material (usually magnesium oxide powder)
and metal protection tube.Armored thermocouple can be divided into single (double
core) and double (four core), and its measuring end can be divided into three
basic forms: exposed head type, shell type and insulation type.The reference
terminal (junction box) of armored thermocouple has simple type, waterproof type,
splash proof type, plug-in type and small junction box type.Armored thermocouple
has the advantages of small volume, high precision, rapid response, vibration
resistance, impact resistance, high mechanical strength, good winding performance,
long service life and easy installation.The overall dimensions are φ 5, φ 6
and φ 8, and the length is 10-1000mm.
2.2Common thermocouple types (eight types): type B ------ platinum rhodium
30 - platinum rhodium 6 R ------ platinum rhodium 13 - Platinum s ------ Platinum
Rhodium 10 - Platinum e ------ nickel chromium - constantan J ------ iron
constantan K ------ nickel chromium - nickel aluminum t ------ copper constantan
n ------ nickel chromium silicon - nickel siliconAmong them: B, R, s belong to
noble metal thermocouple; K, e, N, J belong to low metal thermocouple; T-type
thermocouple is used to measure low temperature.

6. 3.Abnormal analysis 3.1 common fault causes and treatment methods of
thermocouple temperature measurement system
Fault phenomenon Possible reasons processing method
Thermocouple is
smaller than the
actual value
Thermoelectric short
circuit
Find out the cause of the short
circuit, if it is due to humidity,
it needs to be dried; if it is due
to the damage of the insulation
terminal, it needs to 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 off the
deteriorated section and weld
again, or replace the
thermocouple with a new one
Compensation conductor
and thermal dipole
connected reversely
Reconnect correctly

7. Compensation wire is
not matched with
thermocouple
Replace the matching compensation
lead
Thermocouple
installation position
is improper or
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
Replace thermocouple or display
instrument to match
The thermoelectric
potential is
larger than the
actual value
Thermocouple is not
matched with display
instrument
Replace thermocouple or display
instrument to match
Compensation wire is
not matched with
thermocouple
Replace the matching compensation
lead
There is DC
interference signal
entering
Eliminate DC interference
Unstable
thermoelectric
potential output
Poor contact between
thermocouple terminal
and thermoelectric
pole
Tighten the terminal screws
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,
Find out the interference source
and take shielding measures

8. electromagnetic
induction, etc.)
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
4.Be able to observe the process discipline
4.1Thermocouple installation and precautions a in pipeline installation, the
working end of temperature sensing element shall be placed at the place with the
maximum flow rate in the pipeline.The end of the thermowell shall cross the
center line of the current beam for 5-10 mm.B. the temperature sensing element
forms counter current with the measured medium, which shall be inserted against
the airflow direction, and at least 90 ° with the flow direction of the measured
medium.In particular, it is not allowed to install temperature measuring elements
along the current, otherwise temperature measurement error will occur.C.
generally, the insertion depth shall not be less than 300 mm. If the insertion
depth is not enough, the exposed part shall be ventilated, so that the measured
temperature is 3-4 degrees lower than the actual temperature.D. during the
installation of temperature measuring elements, the introduction of
electromagnetic interference shall be prevented from affecting the accurate
measurement.
When wiring, make sure good contact, tighten the hollow bolt, and then close
the cover of the junction box. For thermocouples that have to be exposed in the
air, it is better to take rainproof measures to prevent the elements from being
damaged by rain.In order to protect the compensation conductor from external
mechanical damage and the impact on the instrument caused by external magnetic
field, the compensation conductor shall be shielded and there shall be no
twists and turns.
E. thermocouples and thermal resistances shall be installed on horizontal or
vertical pipes as far as possible, and protective sleeves shall be provided
during installation to facilitate maintenance and replacement.

9. F when measuring the temperature in the pipe, the length of the element shall
be on the center line of the pipe (i.e. the insertion depth of the protective
pipe shall be half of the pipe diameter).
G. the cold end of the thermocouple shall be at the same ambient temperature,
the compensation conductor of the same model shall be used, and the positive
and negative shall be connected correctly.
H. high temperature resistant cable or compensation wire shall be used in high
temperature area.http://www.tksiliconerubbercable.com
4.2Process discipline to be observed
Serial
number
must forbid
1 Special tools and instruments must be
used for disassembling, reassembling
and transporting the instrument and
equipment, and they shall be handled
with care
No collision and damage
2 The fasteners of reassembled
instrument and equipment must be
complete, and the bolts shall be
tightened evenly
No shortage of bolts and washers
3 During the static test of important
protection and interlocking system,
it is necessary to simulate the action
from the local instrument; after the
test, the local instrument must be
restored to the normal working state
immediately
It is not allowed to test from
the forced node state in DCS
system
4 The leads of each measuring element
must be firmly fixed and well crimped;
the heat source and mechanical
rotating part must be avoided
Do not connect or lead randomly;
do not squeeze or overheat the
lead
5 For the field equipment of signal
access to DCS system, the wire head
must be wrapped after the wire removal
to prevent the DCS system card from
being damaged due to short circuit or
grounding
No short circuit or grounding at
wire end