Transcript Thermometry - Texas A&M University

Thermocouples
• Thomas Johann Seebeck, 1821, discovered
that a circuit of two different metals with
junctions at different temperatures produced
an electromotive force (EMF), a voltage.
• No external source of power is required.
Power is self-generated.
• Total Seebeck EMF is the manifestation of
four distinct EMFs
– Fourier Heat Conduction
– Joule Heating
- Peltier effect
- Thomson effect
• The net Seebeck EMF is given by:
dT
• dx Is the change in temperature along the
wire.
• eA and eB are Seebeck coefficients and are
different for each type of wire.
• When the wires are homogeneous,
uniform in calibration across their
length, then:
TRe f
Es 

T Jc t
TJc t
A
dT 

T Re f
B
dT
• If both wires begin at Tref and end at TJct,
then:
• For small temperature differences, we can
use the average calibration:
Es   A  B TJct  T Re f  ABTJct  T Re f 
• The electromotive force can also be
approximated by:
Es     T1  T2   T1  T2  ..... T1  T2 
2
• And, if the reference temperature, T2, is
0oC and higher order constants are
negligible, it becomes:
Es  T  T
2
n
• The most common thermocouple alloys
for moderate temperatures are:
– Iron-Constantan (Type J),
– Copper-Constantan (Type T), Constantan
(55% copper, 45% Ni), and
– Chromel-Alumel (Type K). Chromel
(90%Ni, 10% Chrome). Alumel (95% Ni, 3%
Mn, 2% Al)
– Chromel-Constantan [Type E].
Seebeck Coefficients for Various Thermocouples at
Reference Temperature, ()oC
Thermocouple Type
Coefficient V/oC
E
58.7
J
50.4
K
39.5
R
5.3
S
5.4
T
38.7
• The first named element of the pair is the
positive element.
• The negative wire is color coded red (current
U.S. standards)
• Type T is coded Red-blue, U.S. standard.
•
USA/CANADA
UK
FRANCE
GERMANY
• The calibration of Precision Grade
thermocouple wire is guaranteed within
±3.8% or 1oC (2oF) which ever is the larger.
• Most meteorological types are Type T
(Copper-Constantan). Useful range: -60 to
100oC
• They are slightly non-linear in output.
• Typical output for Type T is about 40 V
• Response time is 1 second or less.
• Thermocouples require a Reference
Temperature.
• There are several different systems for
establishing a reference temperature.
– Ice Baths: A drug-store thermos flask will
maintain 0oC for several hours if filled with
finely crushed ice and then flooded with water.
– Electronically Controlled Reference:
Temperature of the reference connection is
electronically controlled. These devices are
available, both for high temperature and ice
point. They require periodic calibration and
generally are not as stable as ice-baths, but
more convenient.
– Compensated Reference Temperature System:
Each wire from a thermocouple probe is
terminated at a connection panel inside the
chassis of the instrument.
– The temperature at this point is measured by
another means, e.g., a PRT or thermistor.
– A compensation network is used to inject a
signal which compensates for the temperature
of the panel being different than 0oC before
calculating the temperature at the probe.
• Peltier Effect:
– Reported by Jean Peltier in 1834.
– Current forced to flow in one direction would warm the
junction and release heat.
– Current forced to flow in the opposite direction would
cool the junction and absorb heat.
– The heat transfer can be described by:
qP  d ABI
– qP is the heat transfer in Watts
– dAB is the Peltier coefficient for particular thermocouple
materials.
– Useful for heating and cooling material.
– Used in moisture measuring instruments.
• Thompson Effect:
– This relates to the absorption or liberation of
heat by a homogeneous conductor that has a
temperature gradient in it due to a current
flowing through it. qT  I T1  T2 
– qT = Thompson coefficient dependent on
material of wire.
– It has a small effect on a thermocouple due to
the small currents produced by the
thermoelectric effect.
Thermoelectric Laws
• Law of Homogeneous Materials: The
thermal EMF of a thermocouple is
unaffected by temperatures elsewhere in
the circuit if the two metals used are
homogeneous.
– Thermocouple leads can be made out of
thermocouple material.
– EMF is determined by temperatures at the
junctions, not elsewhere in the circuit.
• Law of Intermediate Materials
– If a third metal is inserted in either wire A or
B and if the two junctions are at the same
temperature, there will be no effect on the
generated EMF by the addition of the third
metal.
• Allows the connection of the thermocouple to a
voltmeter or an amplifier.
• Allows the soldering of the joints of the
thermocouple.
• Law of Successive Intermediate
Temperatures