Thermocouples
An interesting phenomenon applied in the
field of instrumentation is the Seebeck effect, which is the
production of a small voltage across the length of a wire
due to a difference in temperature along that wire. This
effect is most easily observed and applied with a junction
of two dissimilar metals in contact, each metal producing a
different Seebeck voltage along its length, which translates
to a voltage between the two (unjoined) wire ends. Most any
pair of dissimilar metals will produce a measurable voltage
when their junction is heated, some combinations of metals
producing more voltage per degree of temperature than
others:
The Seebeck effect is fairly linear; that
is, the voltage produced by a heated junction of two wires
is directly proportional to the temperature. This means that
the temperature of the metal wire junction can be determined
by measuring the voltage produced. Thus, the Seebeck effect
provides for us an electric method of temperature
measurement.
When a pair of dissimilar metals are joined
together for the purpose of measuring temperature, the
device formed is called a thermocouple. Thermocouples
made for instrumentation use metals of high purity for an
accurate temperature/voltage relationship (as linear and as
predictable as possible).
Seebeck voltages are quite small, in the
tens of millivolts for most temperature ranges. This makes
them somewhat difficult to measure accurately. Also, the
fact that any junction between dissimilar metals will
produce temperature-dependent voltage creates a problem when
we try to connect the thermocouple to a voltmeter,
completing a circuit:
The second iron/copper junction formed by
the connection between the thermocouple and the meter on the
top wire will produce a temperature-dependent voltage
opposed in polarity to the voltage produced at the
measurement junction. This means that the voltage between
the voltmeter's copper leads will be a function of the
difference in temperature between the two junctions, and
not the temperature at the measurement junction alone. Even
for thermocouple types where copper is not one of the
dissimilar metals, the combination of the two metals joining
the copper leads of the measuring instrument forms a
junction equivalent to the measurement junction:
This second junction is called the
reference or cold junction, to distinguish it
from the junction at the measuring end, and there is no way
to avoid having one in a thermocouple circuit. In some
applications, a differential temperature measurement between
two points is required, and this inherent property of
thermocouples can be exploited to make a very simple
measurement system.
However, in most applications the intent is
to measure temperature at a single point only, and in these
cases the second junction becomes a liability to function.
Compensation for the voltage generated by
the reference junction is typically performed by a special
circuit designed to measure temperature there and produce a
corresponding voltage to counter the reference junction's
effects. At this point you may wonder, "If we have to resort
to some other form of temperature measurement just to
overcome an idiosyncrasy with thermocouples, then why bother
using thermocouples to measure temperature at all? Why not
just use this other form of temperature measurement,
whatever it may be, to do the job?" The answer is this:
because the other forms of temperature measurement used for
reference junction compensation are not as robust or
versatile as a thermocouple junction, but do the job of
measuring room temperature at the reference junction site
quite well. For example, the thermocouple measurement
junction may be inserted into the 1800 degree (F) flue of a
foundry holding furnace, while the reference junction sits a
hundred feet away in a metal cabinet at ambient temperature,
having its temperature measured by a device that could never
survive the heat or corrosive atmosphere of the furnace.
The voltage produced by thermocouple
junctions is strictly dependent upon temperature. Any
current in a thermocouple circuit is a function of circuit
resistance in opposition to this voltage (I=E/R). In other
words, the relationship between temperature and Seebeck
voltage is fixed, while the relationship between temperature
and current is variable, depending on the total resistance
of the circuit. With heavy enough thermocouple conductors,
currents upwards of hundreds of amps can be generated from a
single pair of thermocouple junctions! (I've actually seen
this in a laboratory experiment, using heavy bars of copper
and copper/nickel alloy to form the junctions and the
circuit conductors.)
For measurement purposes, the voltmeter used
in a thermocouple circuit is designed to have a very high
resistance so as to avoid any error-inducing voltage drops
along the thermocouple wire. The problem of voltage drop
along the conductor length is even more severe here than
with the DC voltage signals discussed earlier, because here
we only have a few millivolts of voltage produced by the
junction. We simply cannot spare to have even a single
millivolt of drop along the conductor lengths without
incurring serious temperature measurement errors.
Ideally, then, current in a thermocouple
circuit is zero. Early thermocouple indicating instruments
made use of null-balance potentiometric voltage measurement
circuitry to measure the junction voltage. The early Leeds &
Northrup "Speedomax" line of temperature indicator/recorders
were a good example of this technology. More modern
instruments use semiconductor amplifier circuits to allow
the thermocouple's voltage signal to drive an indication
device with little or no current drawn in the circuit.
Thermocouples, however, can be built from
heavy-gauge wire for low resistance, and connected in such a
way so as to generate very high currents for purposes other
than temperature measurement. One such purpose is electric
power generation. By connecting many thermocouples in
series, alternating hot/cold temperatures with each
junction, a device called a thermopile can be
constructed to produce substantial amounts of voltage and
current:
With the left and right sets of junctions at
the same temperature, the voltage at each junction will be
equal and the opposing polarities would cancel to a final
voltage of zero. However, if the left set of junctions were
heated and the right set cooled, the voltage at each left
junction would be greater than each right junction,
resulting in a total output voltage equal to the sum of all
junction pair differentials. In a thermopile, this is
exactly how things are set up. A source of heat (combustion,
strong radioactive substance, solar heat, etc.) is applied
to one set of junctions, while the other set is bonded to a
heat sink of some sort (air- or water-cooled). Interestingly
enough, as electrons flow through an external load circuit
connected to the thermopile, heat energy is transferred from
the hot junctions to the cold junctions, demonstrating
another thermo-electric phenomenon: the so-called Peltier
Effect (electric current transferring heat energy).
Another application for thermocouples is in
the measurement of average temperature between
several locations. The easiest way to do this is to connect
several thermocouples in parallel with each other. Each
millivoltage signal produced by each thermocouple will tend
to average out at the parallel junction point, the voltage
differences between the junctions' potentials dropped along
the resistances of the thermocouple wire lengths:
Unfortunately, though, the accurate
averaging of these Seebeck voltage potentials relies on each
thermocouple's wire resistances being equal. If the
thermocouples are located at different places and their
wires join in parallel at a single location, equal wire
length will be unlikely. The thermocouple having the
greatest wire length from point of measurement to parallel
connection point will tend to have the greatest resistance,
and will therefore have the least effect on the average
voltage produced.
To help compensate for this, additional
resistance can be added to each of the parallel thermocouple
circuit branches to make their respective resistances more
equal. Without custom-sizing resistors for each branch (to
make resistances precisely equal between all the
thermocouples), it is acceptable to simply install resistors
with equal values, significantly higher than the
thermocouple wires' resistances so that those wire
resistances will have a much smaller impact on the total
branch resistance. These resistors are called swamping
resistors, because their relatively high values overshadow
or "swamp" the resistances of the thermocouple wires
themselves:
Because thermocouple junctions produce such
low voltages, it is imperative that wire connections be very
clean and tight for accurate and reliable operation. Also,
the location of the reference junction (the place where the
dissimilar-metal thermocouple wires join to standard copper)
must be kept close to the measuring instrument, to ensure
that the instrument can accurately compensate for reference
junction temperature. Despite these seemingly restrictive
requirements, thermocouples remain one of the most robust
and popular methods of industrial temperature measurement in
modern use.
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REVIEW:
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The Seebeck Effect is the
production of a voltage between two dissimilar, joined
metals that is proportional to the temperature of that
junction.
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In any thermocouple circuit, there are two
equivalent junctions formed between dissimilar metals. The
junction placed at the site of intended measurement is
called the measurement junction, while the other
(single or equivalent) junction is called the reference
junction.
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Two thermocouple junctions can be
connected in opposition to each other to generate a
voltage signal proportional to differential temperature
between the two junctions. A collection of junctions so
connected for the purpose of generating electricity is
called a thermopile.
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When electrons flow through the junctions
of a thermopile, heat energy is transferred from one set
of junctions to the other. This is known as the Peltier
Effect.
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Multiple thermocouple junctions can be
connected in parallel with each other to generate a
voltage signal representing the average temperature
between the junctions. "Swamping" resistors may be
connected in series with each thermocouple to help
maintain equality between the junctions, so the resultant
voltage will be more representative of a true average
temperature.
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It is imperative that current in a
thermocouple circuit be kept as low as possible for good
measurement accuracy. Also, all related wire connections
should be clean and tight. Mere millivolts of drop at any
place in the circuit will cause substantial measurement
errors.
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