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Home::Reference & Education
How Does A Thermocouple Work?
Author : Joe Crew
Thermocouples
Measurement and control of
temperature is one of the most common requirements of
industrial instrumentation and the thermocouple is by far the
most widely used temperature sensor. Its characteristics include
good inherent accuracy, suitability over a broad temperature
range, fast thermal response, ruggedness, high reliability and
low cost.
How does a thermocouple work?
T.J Seebeck discovered in the 1820s that an electric current
flows in a closed circuit of two dissimilar metals when one of
the two junctions is heated with respect to the other. In a
thermocouple circuit the current continues to flow as long as
the two junctions are at different temperatures. The magnitude
and direction of the current depends on the temperature
difference between the junctions and the properties of the
metals used in the circuit. This is known as the Seebeck effect.
Click
here to see an example of the circuit.
If the circuit is broken at the center, the net open circuit
voltage (the Seebeck voltage) is a function of the junction
temperature and the composition of the two metals.
If the hot and cold junctions are reversed, current will flow in
the opposite direction. Any two dissimilar metals can be used
and the thermocouple circuit will generate a low voltage output
that is almost (but not exactly) proportional to the temperature
difference between the hot junction and the cold junction. The
voltage output is between 15 and 40µV per degree C, dependant on
the thermocouple conductor metals used. The actual metals used
in industrial thermocouples depend on the application and
temperature measurement range required.
Thermocouple failure prediction
Like any other metal object, thermocouples are subject to metal
fatigue wear and tear; they have a finite life. Many users of
thermocouples are not aware of thermocouple deterioration until
the sensor breaks, often causing an expensive interruption of a
process. Removing a thermocouple from a furnace when at
operating temperature can be difficult and dangerous. In fact
the thermocouple, a simple and generally inexpensive sensor, can
cause inaccurate readings for some time before any errors are
detected. The errors usually cause low readings due to the
thermocouple wires becoming thinner.
Impurities induced by any handling during manufacture or
installation can accelerate chemical deterioration of the
thermocouple. For base metal thermocouples, deterioration occurs
slowly due to contact with the atmosphere, which in turn causes
oxidation. As the surface of the thermocouple wires oxidises the
current carrying cross sectional area is reduced. Nobel metal
thermocouple deterioration is also well documented.
In "Principals and Method of Temperature Measurement", Thomas D
McGee explains that the usual result of deterioration is the
gradual reduction in the Seebeck voltage, often extended over
several weeks and not frequently detected. If the Seebeck
voltage is low, the measured temperature will also be low, so
the actual process temperature will be increased to produce the
required Seebeck voltage. The net result will be excessive
temperature generation with resulting damage to material and
processes. Those who use thermocouples should be aware of the
possibilities of slow deterioration and its consequences.
A temperature
controller, for example, would actually compensate for the
thermocouple's loss of thermoelectric power by putting more heat
into the process with all the energy, environmental and process
plant costs that would be incurred. Fortunately, while Mr Thomas
Johann Seebeck was experimenting with his wires in the 1820s,
his contemporary and fellow countryman, Mr Georg Ohm, was also
conducting his own experiments. Fortuitously because as the
thermocouple conductors become thinner, their resistance changes
as described in "Practical Temperature Measurement" by Peter R.
N. Childs.
"The loop resistance of a thermocouple depends on its length,
type and diameter of the thermocouple wire, the length type and
diameter of extension wires, temperatures along the circuit and
the contact resistance at any connections. If on installation,
and at regular intervals in use, a measurement is made of this
loop resistance, then a change in this value can be used to
indicate wire thinning due to chemical attack, loose or corroded
connections, contact resistance due to broken but touching wires
or electrical shunting due to loss of insulation at some
location along the wire."
Regular measurements of the thermocouple loop can indicate that
the sensor should be replaced for reasons of accuracy and can
also be used to predict its complete failure (sensor break). As
thermocouple conductors oxidise they become brittle, making them
more susceptible to breakage due to bending or vibration.
Replacing thermocouples during a planned maintenance period is
easier and more cost effecting than replacing thermocouples
while the plant is running.
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