2. TEMPERATURE SCALES
• There are three temperature scales in use
today.
• They are as follows:
1. Celsius,
2. Fahrenheit,
3. Kelvin.
3. KELVIN TEMPERATURE SCALE
• Kelvin temperature scale is the base unit of thermodynamic
temperature measurement in the International System (SI) of
measurement.
• It is defined as 1/ 273.16 of the triple point (equilibrium among the
solid, liquid, and gaseous phases) of pure water. The kelvin (symbol
K without the degree sign) is also the fundamental unit of the Kelvin
scale, an absolute temperature scale named for the British physicist
William Thomson, Baron Kelvin.
• Such a scale has as its zero point absolute zero, the theoretical
temperature at which the molecules of a substance have the lowest
energy.
• Many physical laws and formulas can be expressed more simply
when an absolute temperature scale is used; accordingly, the Kelvin
scale has been adopted as the international standard for scientific
temperature measurement.
4. CELSIUS TEMPERATURE SCALE
• Celsius temperature scale also called
centigrade temperature scale, is the
scale based on 0 for the freezing
point of water and 100 for the boiling
point of water.
• Invented in 1742 by the Swedish
astronomer Anders Celsius, it is
sometimes called the centigrade
scale because of the 100-degree
interval between the defined points.
• The Celsius scale is in general use
wherever metric units have become
accepted, and it is used in scientific
work everywhere.
5. FAHRENHEIT TEMPERATURE SCALE
• Fahrenheit temperature scale is a scale based on 32 for the freezing
point of water and 212 for the boiling point of water, the interval
between the two being divided into 180 parts.
• The 18th-century German physicist Daniel Gabriel Fahrenheit
originally took as the zero of his scale the temperature of an equal
ice-salt mixture and selected the values of 30 and 90 for the
freezing point of water and normal body temperature, respectively;
these later were revised to 32 and 96, but the final scale required
an adjustment to 98.6 for the latter value.
6. • Until the 1970s the Fahrenheit
temperature scale was in general
common use in English-speaking
countries; the Celsius, or
centigrade, scale was employed in
most other countries and for
scientific purposes worldwide.
Since that time, however, most
English-speaking countries have
officially adopted the Celsius
scale. The conversion formula for
a temperature that is expressed
on the Celsius (C) scale to its
Fahrenheit (F) representation is: F
= 9/5C + 32.
7. METHORDS OF TEMPERATURE
• The methods of temperature we are going to
discuss here are as follows:
1. Bimetallic Strip Thermometer,
2. Liquid in Glass Thermometer,
3. Liquid in Metal Thermometer.
8. BIMETALLIC STRIP THERMOMETER
• Bimetallic strip thermometers are mechanical
thermometers. They are widely used in industry for
temperature control because of their robustness,
temperature range and simplicity.
• It consists of two strips made of dissimilar metals and
bonded together with one end fixed and the other
free. The principle is that as the temperature changes
one strip expands more than the other, causing the
pair to bend at the free end.
• Most bimetallic strips use a high thermal expansion
alloy, such as steel or stainless steel, coupled with a
low thermal expansion alloy such as Invar.
9. • Two constructions are available:
-A spiral strip: The bimetallic strip is coiled into a spiral attached to a
dial that indicates temperature
-A cantilever strip: the bimetallic strips are bonded together in a
cantilever. The deflection is used to indicate
temperature.
10. Bimetallic strip advantages and
disadvantages
• Power source not required
• Robust, easy to use and cheap but not very
accurate. Can be used to 500 °C
• Limited to applications where manual reading
is acceptable, e.g. a household thermometer
• Not suitable for very low temperatures
because the expansion of metals tend to be
too similar, so the device becomes a rather
insensitive thermometer
