Thermocouples

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Met 163: Lecture 4
Chapter 4
Thermometry
Thermoelectric Sensors
The junction of two dissimilar metals forms a
thermocouple.
When the two junctions are at different temperatures, a
voltage is developed across the junction.
By measuring the voltage difference between the two
junctions, the difference in temperature between the two
can be calculated.
If the temperature of one junction is known and the
voltage difference is measured, then the temperature of
the second junction can be calculated.
Thermocouples
Thermocouples provide:
A wide useful temperature range,
Are inherently differential,
Are rugged
Reliable and inexpensive
And usually have a fast response.
The main disadvantage of thermocouples:
is the very low output, on the order of 40 μV/ °C.
Slight nonlinearity
And need for calibration.
Thermocouples
There are some observed laws of thermocouple behavior
used as a rule-of-thumb guide to thermocouple circuit
design and construction.
Thermocouples
The thermoelectric effect: when one junction has a
different temperature than the other, an electromotive
force is produced in the circuit and current flows.
The magnitude of the force or potential depends on the
temperature difference between the two junctions.
There are three components of the thermoelectric: The
Seebeck effect, Peltier effect, and Thompson effect.
Thermocouples: Seebeck effect
The Seebeck effect is the conversion of thermal energy to
electrical energy.
This effect measures the ease at which excess electrons
will circulate in an electrical circuit under the influence of
thermal difference.
The change in the voltage is proportional to the
temperature difference between the junctions when the
ends are connected to form a loop.
Seebeck Effect
Thermocouples: Peltier effect
The Peltier effect is closely related to the Seebeck effect.
It represents the thermal effect due to a reversible current
through dissimilar materials or through similar metals due
to an external source of current.
A current flow in one direction might warm the junction of
the two dissimilar materials (and release heat to the
surroundings of that junction), whereas if the current was
reversed, the junction would cool (and absorb heat from
its surroundings).
Thermocouples: Peltier effect
Thermocouples: Thompson effect
The Thompson effect is the absorption or liberation of
heat by a homogeneous conductor due to a current
flowing through it.
It is primarily evident in currents introduced form external
sources and those generated by the thermocouple itself.
The ability of a given material to generate heat with
respect to both a unit temperature gradient and a unit
current, is gauged by the Thompson coefficient.
The importance of the Peltier and Thompson effects is
essentially infinitesimal because the heat evolved is
negligible compared to the amount of thermal energy
available from the environment to the junctions of T1 and
T2.
The (Thermocouple) Thermoelectric Laws
The three fundamental empirical laws behind the
accurate measurement of temperature by
thermoelectric means are the:
1. Law of homogeneous materials
2. Law of intermediate materials
3. Law of intermediate temperatures.
Law #1: the voltage across a thermocouple is unaffected
by temperatures elsewhere in the circuit, provided the
two metals used are each homogeneous.
Thus one can use lead wires made of thermocouple
metals.
The (Thermocouple) Thermoelectric Laws
2. Law of intermediate materials
Law #2: If a third metal is inserted in either A or B and if
the two new junctions are at the same temperature, no
effective voltage is generated by the third metal. This
means that a real voltmeter (or amplifier) can be used.
The terminals of a voltmeter are usually made of a third
metal and can be close together. It is important to
make sure the terminals of the voltmeter are at the
same temperature.
The (Thermocouple) Thermoelectric Laws
3. Law of intermediate materials
Law #3: If a metal C is inserted in one of the AB junctions,
then no net voltage is generated so long as junction AC
and BC are at the same temperature.
This means that the two wires or a junction can be
soldered together and the presence of the third metal,
solder, will not affect the voltage if there is no
temperature gradient across the solder junction.
The (Thermocouple) Thermoelectric Laws
T1
V1
A
V3
+
G
A
B
Ref.
T2
Fig. 4-5 (a)
V2
Thermocouples
Common thermocouple types
Type
Metal
T
J
E
K
Copper and constantan
Iron and constantan
Nickel(10% chromium and constantan
Nickel and Nickel(5% aluminum/silicon)
Thermocouples
A thermocouple is inherently a differential temperature
sensor; it measures the temperature difference
between two junctions.
Absolute temperature measurements can be made only if
one of the junctions is held at a known temperature or
if an electronic reference junction is used.
A block of metal (aluminum, copper, or any highly
conductive metal) can be used for the reference
temperature.
This is done by inserting the reference junction of the
thermocouple in the block and simultaneously
measuring the temperature of the block.
Thermocouples
The Campbell Scientific data loggers have this metal
block underneath the wiring panel.
The CSI data loggers have special instructions in their
programming language that allows for thermocouple
measurements.
See the CR1000 manual.
Thermocouple output voltages
Thermocouples
The most common type of thermocouple used for
meteorology is the copper-constantan (Type-T).
Its range of use varies from -200°C - 350°C, but is mostly
used in the -60°C to 100°C with an accuracy of ±0.5°C.
Another common type used in meteorology is the Type-E.
Thermistors
Thermistor or thermal resistor is a hard, ceramic-like
electronic semi-conductor, commonly made from a
mixture of metallic oxide materials.
Have a very large negative resistance coefficient (i.e., an
increase in T by 1°C yields a decrease of 5% in
resistance).
Thermistors
RTD: Resistance Temperature Detectors
• Platinum is most commonly used for precision
resistance thermometers because it is stable, resists
corrosion, is easily workable, has a high temp melting
point, and can be obtained to a high degree of purity.
• Simple and stable resistance-temperature relationship.
• Platinum is sensitive to strain; bending the sensor can
change the resistance.
RTD: Resistance Temperature Detectors
Resistance of a platinum sensor is given by
RT  R0 (1  aT  bT )
2
with sufficient accuracy for the meteorological
temperature range -50 to 50°C. R0 is resistance at 0°C
RT= resistance of sensor at temperature T°C
Coefficients depend on purity of platinum;
a = 0.00385 or 0.00392°C-1
Because the RTD resistance is fairly low and the change
with temperature is small a bridge circuit is often used.
RTD: Resistance Temperature Detectors
Because the RTD resistance is fairly low and the change
with temperature is small a bridge circuit is often used.
The bridge circuit converts resistance to voltage and can
be amplified to a reasonable level using an instrument
amplifier (CR1000 data loggers have this circuit built
in).
RTD: Resistance Temperature Detectors
RTD: Resistance Temperature Detectors
RTD: Resistance Temperature Detectors
RTD: Resistance Temperature Detectors
Campbell Scientific
Exposure of Temperature Sensors
Unaspirated Radiation Shield
Error inUnaspirated Radiation Shield
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