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Chapter 6
Electrical Thermometers
Thermocouples • Resistance
Temperature Detectors (RTDs) •
Thermistors • Semiconductor
Thermometers
Chapter 6 — Electrical Thermometers
A thermocouple
creates an electrical
potential when the
junction is at an
elevated temperature.
Chapter 6 — Electrical Thermometers
The Seebeck effect
causes an electrical
potential when two
dissimilar wires are
joined and the end
is heated.
Chapter 6 — Electrical Thermometers
The Peltier effect can be used to build thermoelectric coolers.
Chapter 6 — Electrical Thermometers
The Thomson effect causes heating or cooling when
there is current flow through a temperature gradient
in a wire.
Chapter 6 — Electrical Thermometers
The law of intermediate
temperatures states that
the temperature at the end
of the wires determines
the electrical potential
regardless of the intermediate temperatures.
Chapter 6 — Electrical Thermometers
The law of intermediate metals
states that other
metals may be used
in a thermocouple
circuit as long as the
junctions are at the
same temperature.
Chapter 6 — Electrical Thermometers
A complete thermocouple circuit includes extra
junctions from the copper wire in the leads to the
voltmeter.
Chapter 6 — Electrical Thermometers
A 32°F ice bath is the
reference temperature
for thermocouple tables.
Chapter 6 — Electrical Thermometers
An isothermal block can
be used to establish a
reference temperature
for the cold junction.
Chapter 6 — Electrical Thermometers
A modern digital thermocouple system includes a
voltage to temperature
conversion, cold junction
compensation, and a digital
readout of the temperature.
Chapter 6 — Electrical Thermometers
Conventional thermocouple construction uses
insulator beads to isolate the two thermocouple wires.
Chapter 6 — Electrical Thermometers
Thermocouple
designations require
that the wires follow a
particular voltagetemperature curve.
Chapter 6 — Electrical Thermometers
Thermocouple
color codes have
been standardized
in many countries.
Chapter 6 — Electrical Thermometers
Sheathed thermocouples can be wired
in several ways for
different applications.
Chapter 6 — Electrical Thermometers
Difference thermocouples are made of
two thermocouples
wired in series with
reversed polarity.
Chapter 6 — Electrical Thermometers
A thermopile consists of
several thermocouples
wired in series in order to
amplify the signal.
Chapter 6 — Electrical Thermometers
A swamping box uses resistors in each thermocouple
circuit to eliminate errors when measuring an average
reading of a set of thermocouples.
Chapter 6 — Electrical Thermometers
A thermocouple
pyrometer uses a
variable potentiometer to balance
loop resistance.
Chapter 6 — Electrical Thermometers
A resistance temperature detector (RTD)
contains a resistor with
a resistance that varies
with temperature.
Chapter 6 — Electrical Thermometers
RTDs consist of
precision wires
wrapped around an
insulator and
encapsulated in a
protective sheath.
Chapter 6 — Electrical Thermometers
Thermistors are
available in a variety of
shapes and sizes.
Chapter 6 — Electrical Thermometers
The changing
resistance of a
thermistor can be
used as a temperature switch.
Chapter 6 — Electrical Thermometers
PTC thermistors have a
resistance that increases
with increases in
temperature.