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Introduction to
Thermocouples (From http://www.capgo.com)
The thermocouple is one of the
simplest of all sensors. It consists of two wires of dissimilar metals
joined near the measurement point. The output is a small voltage measured
between the two wires.
While appealingly simple in
concept, the theory behind the thermocouple is subtle, the basics of which
need to be understood for the most effective use of the sensor.
Thermocouple theory
A thermocouple circuit has at least
two junctions: the measurement junction and a reference junction.
Typically, the reference junction is created where the two wires connect to
the measuring device. This second junction it is really two junctions: one
for each of the two wires, but because they are assumed to be at the same
temperature (isothermal) they are considered as one (thermal) junction. It
is the point where the metals change - from the thermocouple metals to what
ever metals are used in the measuring device - typically copper.
The output voltage is related to
the temperature difference between the measurement and the reference
junctions. This is phenomena is known as the Seebeck
effect. (See the Thermocouple Calculator to get a feel for the magnitude of
the Seebeck voltage).
In practice the Seebeck voltage is made up of two components: the Peltier voltage generated at the junctions, plus the
Thomson voltage generated in the wires by the temperature gradient.
The Peltier
voltage is proportional to the temperature of each junction while the
Thomson voltage is proportional to the square of the temperature difference
between the two junctions. It is the Thomson voltage that accounts for most
of the observed voltage and non-linearity in thermocouple response.
Each thermocouple type has its
characteristic Seebeck voltage curve. The curve
is dependent on the metals, their purity, their homogeneity and their
crystal structure. In the case of alloys, the ratio of constituents and
their distribution in the wire is also important. These potential
inhomogeneous characteristics of metal are why thick wire thermocouples can
be more accurate in high temperature applications, when the thermocouple
metals and their impurities become more mobile by diffusion.
The practical considerations of thermocouples
The above theory of thermocouple
operation has important practical implications that are well worth
understanding:
1. A third metal may be
introduced into a thermocouple circuit and have no impact, provided that
both ends are at the same temperature. This means that the thermocouple
measurement junction may be soldered, brazed or welded without affecting
the thermocouple's calibration, as long as there is no net temperature
gradient along the third metal.
Further, if the measuring circuit
metal (usually copper) is different to that of the thermocouple, then
provided the temperature of the two connecting terminals is the same and
known, the reading will not be affected by the
presence of copper.
2.
The thermocouple's output is generated by the temperature gradient along
the wires and not at the junctions as is commonly believed. Therefore it is
important that the quality of the wire be maintained where temperature
gradients exists. Wire quality can be compromised by contamination from its
operating environment and the insulating material. For temperatures below
400°C, contamination of insulated wires is generally not a problem. At
temperatures above 1000°C, the choice of insulation and sheath materials,
as well as the wire thickness, become critical to the calibration stability
of the thermocouple.
The fact that a thermocouple's output
is not generated at the junction should redirect attention to other
potential problem areas.
3. The voltage generated by a
thermocouple is a function of the temperature difference between the
measurement and reference junctions. Traditionally the reference junction
was held at 0°C by an ice bath:
The ice bath is now considered
impractical and is replace by a reference junction
compensation arrangement. This can be accomplished by measuring the
reference junction temperature with an alternate temperature sensor
(typically an RTD or thermistor) and applying a
correcting voltage to the measured thermocouple voltage before scaling to
temperature.
The correction can be done
electrically in hardware or mathematically in software. The software method
is preferred as it is universal to all thermocouple types (provided the
characteristics are known) and it allows for the correction of the small
non-linearity over the reference temperature range.
4. The low-level
output from thermocouples (typically 50mV full scale) requires that care be
taken to avoid electrical interference from motors, power cable and
transformers. Twisting the thermocouple wire pair (say 1 twist per 10 cm)
can greatly reduce magnetic field pickup. Using shielded cable or running
wires in metal conduit can reduce electric field pickup. The measuring
device should provide signal filtering, either in hardware or by software,
with strong rejection of the line frequency (50/60 Hz) and its harmonics.
5. The operating
environment of the thermocouple needs to be considered. Exposure to
oxidizing or reducing atmospheres at high temperature can significantly
degrade some thermocouples. Thermocouples containing rhodium (B, R and S
types) are not suitable under neutron radiation.
The advantages and disadvantages of thermocouples
Because of their physical
characteristics, thermocouples are the preferred method of temperature
measurement in many applications. They can be very rugged, are immune to
shock and vibration, are useful over a wide temperature range, are simple
to manufactured, require no excitation power, there is no self heating and
they can be made very small. No other temperature sensor provides this
degree of versatility.
Thermocouples are wonderful
sensors to experiment with because of their robustness, wide temperature
range and unique properties.
On the down side, the
thermocouple produces a relative low output signal that is non-linear.
These characteristics require a sensitive and stable measuring device that
is able provide reference junction compensation and linearization. Also the
low signal level demands that a higher level of care be taken when
installing to minimise potential noise sources.
The measuring hardware requires
good noise rejection capability. Ground loops can be a problem with
non-isolated systems, unless the common mode range and rejection is
adequate.
Types of thermocouple
About 13 'standard' thermocouple
types are commonly used. Eight have been given an
internationally recognised letter type
designators. The letter type designator refers to the emf
table, not the composition of the metals - so any thermocouple that matches
the emf table within the defined tolerances may
receive that table's letter designator.
Some of the non-recognised thermocouples may excel in particular niche
applications and have gained a degree of acceptance for this reason, as
well as due to effective marketing by the alloy manufacturer. Some of these
have been given letter type designators by their manufacturers that have
been partially accepted by industry.
Each thermocouple type has
characteristics that can be matched to applications. Industry generally
prefers K and N types because of their suitability to high temperatures,
while others often prefer the T type due to its sensitivity, low cost and
ease of use.
A table of standard thermocouple
types is presented below. The table also shows the temperature range for
extension grade wire in brackets.
|
Type
|
Positive Material
|
Negative
Material
|
Accuracy***
Class 2
|
Range
°C
(extension)
|
Comments
|
|
B
|
Pt,
30%Rh
|
Pt,
6%Rh
|
0.5%
>800°C
|
50 to
1820
(1 to 100)
|
Good
at high temperatures, no reference junction compensation required.
|
|
C**
|
W,
5%Re
|
W,
26%Re
|
1%
>425°C
|
0 to
2315
(0 to 870)
|
Very
high temperature use, brittle
|
|
D**
|
W,
3%Re
|
W,
25%Re
|
1%
>425°C
|
0 to
2315
(0 to 260)
|
Very
high temperature use, brittle
|
|
E
|
Ni,
10%Cr
|
Cu,
45%Ni
|
0.5%
or 1.7°C
|
-270
to 1000
(0 to 200)
|
General
purpose, low and medium temperatures
|
|
G**
|
W
|
W,
26%Re
|
1%
>425°C
|
0 to
2315
(0 to 260)
|
Very
high temperature use, brittle
|
|
J
|
Fe
|
Cu,
45%Ni
|
0.75%
or 2.2°C
|
-210
to 1200
(0 to 200)
|
High
temperature, reducing environment
|
|
K*
|
Ni,
10%Cr
|
Ni,
2%Al
2%Mn
1%Si
|
0.75%
or 2.2°C
|
-270
to 1372
(0 to 80)
|
General
purpose high temperature, oxidizing environment
|
|
L**
|
Fe
|
Cu,
45%Ni
|
0.4%
or 1.5°C
|
0 to
900
|
Similar
to J type. Obsolete - not for new designs
|
|
M**
|
Ni
|
Ni,
18%Mo
|
0.75%
or 2.2°C
|
-50
to 1410
|
.
|
|
N*
|
Ni,
14%Cr
1.5%Si
|
Ni,
4.5%Si
0.1%Mg
|
0.75%
or 2.2°C
|
-270
to 1300
(0 to 200)
|
Relatively
new type as a superior replacement for K Type.
|
|
P**
|
Platinel II
|
Platinel II
|
1.0%
|
0 to
1395
|
A
more stable but expensive substitute for K & N types
|
|
R
|
Pt,
13%Rh
|
Pt
|
0.25%
or 1.5°C
|
-50
to 1768
(0 to 50)
|
Precision,
high temperature
|
|
S
|
Pt,
10%Rh
|
Pt
|
0.25%
or 1.5°C
|
-50
to 1768
(0 to 50)
|
Precision,
high temperature
|
|
T*
|
Cu
|
Cu,
45%Ni
|
0.75%
or 1.0°C
|
-270
to 400
(-60 to 100)
|
Good
general purpose, low temperature, tolerant to moisture.
|
|
U**
|
Cu
|
Cu,
45%Ni
|
0.4%
or 1.5°C
|
0 to
600
|
Similar
to T type. Obsolete - not for new designs
|
|
* Most commonly used thermocouple types, ** Not ANSI
recognized types. *** See IEC 584-2 for more details. Materials codes:-
Al = Aluminum, Cr = Chromium, Cu = Copper, Mg = Magnesium, Mo =
Molybdenum, Ni = Nickel, Pt = Platinum, Re = Rhenium, Rh
= Rhodium, Si = Silicon, W = Tungsten
|
Accuracy of thermocouples
Thermocouples will function over a wide temperature range - from
near absolute zero to their melting point, however they are normally only
characterized over their stable range. Thermocouple accuracy is a difficult
subject due to a range of factors. In principal and in practice a
thermocouple can achieve excellent results (that is, significantly better
than the above table indicates) if calibrated, used well below its nominal
upper temperature limit and if protected from harsh atmospheres. At higher
temperatures it is often better to use a heavier gauge of wire in order to
maintain stability (Wire Gauge below).
As mentioned previously, the temperature and voltage scales were
redefined in 1990. The eight main thermocouple types - B, E, J, K, N, R, S
and T - were re-characterised in 1993 to reflect
the scale changes. (See: NIST Monograph 175 for details). The remaining
types: C, D, G, L, M, P and U appear to have been informally re-characterised.
Thermocouple wire
grades
There are different grades of thermocouple wire. The principal
divisions are between measurement grades and extension grades. The
measurement grade has the highest purity and should be used where the
temperature gradient is significant. The standard measurement grade (Class
2) is most commonly used. Special measurement grades (Class 1) are
available with accuracy about twice the standard measurement grades.
The extension thermocouple wire grades are designed for connecting
the thermocouple to the measuring device. The extension wire may be of
different metals to the measurement grade, but are chosen to have a
matching response over a much reduced temperature range - typically -40°C
to 120°C. The reason for using extension wire is reduced cost - they can be
20% to 30% of the cost of equivalent measurement grades. Further cost
savings are possible by using thinner gauge extension wire and a lower
temperature rated insulation.
Note: When temperatures within the extension wire's rating are being
measured, it is OK to use the extension wire for the entire circuit. This
is frequently done with T type extension wire, which is accurate over the
-60 to 100°C range.
Thermocouple wire
gauge
At high temperatures, thermocouple wire can under go irreversible
changes in the form of modified crystal structure, selective migration of
alloy components and chemical changes originating from the surface metal
reacting to the surrounding environment. With some types, mechanical stress
and cycling can also induce changes.
Increasing the diameter of the wire where it is exposed to the high
temperatures can reduce the impact of these effects.
The following table can be used as a very approximate guide to wire
gauge:
|
Type
|
8
Gauge
4.06mm
|
16
Gauge
1.63mm
|
20
Gauge
0.91mm
|
24
Gauge
0.56mm
|
28
Gauge
0.38mm
|
30
Gauge
0.32mm
|
|
B
|
1820
|
-
|
-
|
1700
|
1700
|
-
|
|
C
|
2315
|
2315
|
2315
|
2315
|
2315
|
-
|
|
D
|
2315
|
2315
|
2315
|
2315
|
2000
|
-
|
|
E
|
870
|
620
|
540
|
430
|
400
|
370
|
|
G
|
2315
|
2315
|
2315
|
2315
|
2315
|
-
|
|
J
|
760
|
560
|
480
|
370
|
370
|
320
|
|
K
|
1260*
|
1000*
|
980
|
870
|
820
|
760
|
|
M
|
1260*
|
1200*
|
-
|
-
|
-
|
-
|
|
N
|
1260*
|
1000*
|
980
|
870
|
820
|
760
|
|
P
|
1395
|
-
|
1250
|
1250
|
1250
|
-
|
|
R
|
1760
|
-
|
-
|
1480
|
1480
|
-
|
|
S
|
1760
|
-
|
-
|
1480
|
1480
|
-
|
|
T
|
400
|
370
|
260
|
200
|
200
|
150
|
|
* Upper temperature limits only apply in a protective
sheath
|
At these higher temperatures, the thermocouple wire should be
protected as much as possible from hostile gases. Reducing or oxidizing
gases can corrode some thermocouple wire very quickly. Remember, the purity
of the thermocouple wire is most important where the temperature gradients
are greatest. It is with this part of the thermocouple wiring where the
most care must be taken.
Other sources of wire contamination include the mineral packing
material and the protective metal sheath. Metallic vapour
diffusion can be significant problem at high temperatures. Platinum wires
should only be used inside a nonmetallic sheath, such as high-purity
alumna.
Neutron radiation (as in a nuclear reactor) can have significant permanent
impact on the thermocouple calibration. This is due to the transformation
of metals to different elements.
High temperature measurement is very difficult in some situations.
In preference, use non-contact methods. However this is not always possible,
as the site of temperature measurement is not always visible to these types
of sensors.
Colour coding of
thermocouple wire
The colour coding of thermocouple wire is
something of a nightmare! There are at least seven different standards.
There are some inconsistencies between standards, which seem to have been
designed to confuse. For example the colour red
in the USA standard is always used for the
negative lead, while in German and Japanese standards it is always the
positive lead. The British, French and International standards avoid the
use of red entirely!
Thermocouple
mounting
There are four common ways in which thermocouples are mounted with
in a stainless steel or Inconel sheath and
electrically insulated with mineral oxides. Each of the methods has its
advantages and disadvantages.
Sealed and Isolated from Sheath: Good relatively trouble-free arrangement.
The principal reason for not using this arrangement for all applications is
its sluggish response time - the typical time constant is 75 seconds
Sealed and Grounded to Sheath: Can cause ground loops and
other noise injection, but provides a reasonable time constant (40 seconds)
and a sealed enclosure.
Exposed Bead: Faster response time constant (typically 15 seconds),
but lacks mechanical and chemical protection, and electrical isolation from
material being measured. The porous insulating mineral oxides must be
sealed
Exposed Fast Response: Fastest response time constant, typically 2
seconds but with fine gauge of junction wire the time constant can be
10-100 ms. In addition to problems of the exposed bead type, the protruding
and light construction makes the thermocouple more prone to physical
damage.
Thermocouple
compensation and linearization
As mentioned above, it is possible to provide reference junction
compensation in hardware or in software. The principal is the same in both cases:
adding a correction voltage to the thermocouple output voltage,
proportional to the reference junction temperature. To this end, the
connection point of the thermocouple wires to the measuring device (i.e.
where the thermocouple materials change to the copper of the circuit
electronics) must be monitored by a sensor. This area must be design to be
isothermal, so that the sensor accurately tracks both reference junction
temperatures.
The hardware solution is simple but not always as easy to implement as
one might expect.
The circuit needs to be designed for a specific thermocouple type
and hence lacks the flexibility of the software approach.
The software compensation technique simplifies the hardware
requirement, by eliminating the reference sensor amplifier and summing
circuit (although a multiplexer may be required).
The software algorithm to process the signals needs to be carefully
written. A sample algorithm details the process.
A good resource for thermocouple emf
tables and coefficients is at the US Commerce
Dept's NIST web site. It covers the B, E, J, K, N, R, S and T types.
The thermocouple as a heat pump
The thermocouple can function in reverse. If a current is passed
through a thermocouple circuit, one junction will cool and the other warm.
This is known as the Peltier Effect and is used
in small cooling systems. The effect can be demonstrated by alternately
passing a current through a thermocouple circuit and then quickly measuring
the circuit's Seebeck voltage. This process has
been used, with very fine thermocouple wire (0.025 mm with about a 10 mA current), to measure humidity by ensuring the cooled
junction drops below the air's dew point. This causes condensation to form
on the cooled junction. The junction is allowed to return to ambient, with
the temperature curve showing an inflection at the dew point caused by the
latent heat of vaporization.
Measuring
temperature differences
Thermocouples are excellent for measuring temperatures differences,
such as the wet bulb depression in measuring humidity. Sensitivity can be
enhanced by constructing a thermopile - a number of thermocouple
circuits in series.
In the above example, the thermopile output is proportional to the
temperature difference T1 - T2,
with a sensitivity three times that of a single junction pair. In practice,
thermopiles with two to hundreds of junctions are used in radiometers, heat
flux sensors, flow sensors and humidity sensors. The thermocouple materials
can be in wire form, but also printed or etched as foils and even
electroplated.
An excellent example of the thermopile is in the heat flux sensors
manufactured by Hukseflux Thermal Sensors. Also
see RdF Corp.
The thermocouple is unique in its ability to directly measure a
temperature difference. Other sensor types require a pair of closely
matched sensors to ensure tracking over the entire operational temperature
range.
The thermoelectric generator
While the Seebeck voltage is very small
(in the order of 10-70µV/°C), if the circuit's electrical resistance is low
(thick, short wires), then large currents are possible (e.g. many amperes).
An efficiency trade-off of electrical resistance (as small as possible) and
thermal resistance (as large as possible) between the junctions is the
major issue. Generally, electrical and thermal resistances trend together
with different materials. The output voltage can be increased by wiring as
a thermopile.
The thermoelectric generator has found its best-known application as
the power source in some spacecraft. A radioactive material, such as
plutonium, generates heat and cooling is provided by heat radiation into
space. Such an atomic power source can reliably provide many tens of watts
of power for years. The fact that atomic generators are highly radioactive
prevents their wider application.
NOTES:
1) "Chromel" and "Alumel" are trademarks of Hoskins Mfg
2) "Constantan" is a trademark of Wilbur B. Driver Co.
3) "Platinel" is a trademark of Englehard Industries
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