Temperature Measurements
Interesting Information about
Temperatures
Kelvin
K
Absolute zero
0
Celsius
°C
−273.15
Lowest recorded natural temperature on Earth
(Vostok, Antarctica - 21 July 1983)
184
−89
−128
Celsius / Fahrenheit's "cross-over" temperature
233.15
−40
–40
Fahrenheit's ice/salt mixture
Water freezes (at standard pressure)
Average surface temperature on Earth
Average human body temperature
Highest recorded surface temperature on Earth
(Al 'Aziziyah, Libya - 13 September 1922)
But that reading is queried.
Water boils (at standard pressure)
Gas flame
Titanium melts
The surface of the Sun
255.37
273.15
288
310.0 ±0.7
−17.78
0
15
36.8 ±0.7
0
32
59
98.2 ±1.3
331
58
136
373
~1773
1941
5800
100
~1500
1668
5526
212
~2732
3034
9980
Comment
Fahrenheit
°F
−459.67
Glass Thermometers
• Glass Thermometer: Consists of a
glass tube filled with mercury or
some other liquid, which acts as the
working fluid. Temperature increase
causes the fluid to expand, so the
temperature can be determined by
measuring the volume of the fluid.
Such thermometers are usually
calibrated so that one can read the
temperature simply by observing the
level of the fluid in the thermometer
Alcohol Thermometers
• Alcohol thermometer is an alternative to the mercury-in-glass
thermometer, and functions in a similar way.
• But unlike mercury-in-glass thermometer, the contents of an alcohol
thermometer are less toxic but will evaporate away fairly quickly.
• The liquid can be pure ethanol or toluene or kerosene or Isoamy
acetate, depending on manufacturer and working temperature
range. Since these are transparent, the liquid is made more visible
by the addition of a red or blue dye. One half of the glass containing
the capillary is usually enamelled white or yellow to give a
background for reading the scale.
• The range of usefulness of the thermometer is set by the boiling
point of the liquid used. In the case of the ethanol-filled
thermometer the upper limit for measurement is 78°C
Bi-Metallic Thermometor
• A bimetallic strip is used to convert a
temperature
change
into
mechanical
displacement. The strip consists of two strips of
different metals which expand at different rates
as they are heated, usually steel and copper, or
in some cases brass instead of copper. The strips
are joined together throughout their length
by riveting, brazing or welding. The different
expansions force the flat strip to bend one way if
heated, and in the opposite direction if cooled
below its initial temperature
How does Bi-Metallic Thermometer
work?
• A
direct
indicating
dial thermometer (such as
a patio thermometer or a
meat thermometer) uses a
bimetallic strip wrapped
into a coil. One end of the
coil is fixed to the housing
of the device and the
other drives an indicating
needle
Resistance Thermometers
• Also called resistance temperature
detectors or resistive thermal devices (RTDs),
are temperature sensors that exploit the
predictable change in electrical resistance of
some materials with changing temperature. As
they are almost invariably made of platinum,
they are often called platinum resistance
thermometers (PRTs).
Resistance Thermometers
Thermocouples
• A thermocouple is a device
consisting
of
two
different
conductors (usually metal alloys) that
produce a voltage proportional to
a temperature difference between
either end of the pair of conductors.
Thermocouples are a widely used
type of temperature sensor for
measurement and control and can
also be used to convert a heat
gradient into electricity
Theory of Thermocouples
• Seebeck Effect
T  S V
Types of Thermocouples
Type
Alloys
S
Temperatur Temperature
e range °C
range °C
(continuous) (short term)
K
Chromel (90 percent nickel and 10 percent chromium) Alumel (95% nickel, 2% manganese, 2% aluminium and 41 µV/°C
1% silicon)
0 to +1100 −180 to +1300
J
Iron – Constantan
0 to +750
N
Nicrosil (Nickel-Chromium-Silicon)–Nisil ( Nickel-Silicon) 39 µV/°C
R
S
B
Platinum–rhodium alloy containing 13% rhodium for
one conductor and pure platinum for the other
conductor
90% Platinum and 10% Rhodium (the positive or "+"
wire) and a second wire of 100% platinum
platinum–rhodium alloy for each conductor. One
conductor contains 30% rhodium while the other
conductor contains 6% rhodium
55 µV/°C
−180 to +800
0 to +1100 −270 to +1300
10 µV/°C
0 to +1600 −50 to +1700
10 µV/°C
0 to 1600
−50 to +1750
10 µV/°C
+200 to
+1700
0 to +1820
T
Copper–Constantan
43 µV/°C
−185 to
+300
−250 to +400
E
Chromel - Constantan
(68 µV/°C)
0 to +800
−40 to +900
Infrared Thermometers
• Infrared (IR) light is electromagnetic radiation with
a wavelength longer than that of visible light
measured from the nominal edge of visible red light
at 0.74 micrometers (740 nm), and extending
conventionally to 300 micrometres (300,000 nm).
These wavelengths correspond to a frequency range
of approximately 1 to 400 THz and include most of
the thermal radiation emitted by objects near room
temperature. Microscopically, IR light is typically
emitted or absorbed by molecules when they
change their rotational-vibrational movements.
Infrared thermometer (Pyrometer)
• Infrared thermometer (Pyrometer) has an
optical system and detector. The optical
system focuses the thermal radiation onto the
detector. The output signal of the detector
(Temperature T) is related to the thermal
radiation or irradiance j* of the target object
through the Stefan–Boltzmann law,
the constant of proportionality σ, called
the Stefan-Boltzmann constant and
the emissivity ε of the object.
j*  T
4
Specs of IR Thermometer
• Specifications of portable handheld sensors available
will include ratings of temperature accuracy (usually
with measurement uncertainty of ±2 °C/±4 °F) and
other parameters.
• The distance-to-spot ratio (D:S) is the ratio of the
distance to the object and the diameter of the
temperature measurement area. For instance if the D:S
ratio is 12:1, measurement of an object 12 inches
(30 cm) away will average the temperature over a 1inch-diameter (25 mm) area. The sensor may have an
adjustable emissivity setting, which can be set to
measure the temperature of reflective (shiny) and nonreflective surfaces.
Liquid Crystal Thermometers
• A liquid crystal thermometer or plastic strip
thermometer is a type of thermometer that contains heatsensitive (thermochromic) liquid crystals in a plastic strip
that change color to indicate different temperatures. Liquid
crystals possess the mechanical properties of a liquid, but
have the optical properties of a single crystal. Temperature
changes can affect the color of a liquid crystal, which makes
them useful for temperature measurement. Disposable
liquid crystal thermometers have been developed for home
and medical use. For example if the thermometer is black
and it is put onto someone's forehead it will change colour
depending on the temperature of the person.