Page 179 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

Chapter 3 Scientific principles

celsius or kelvin may be used and, since both scales correspond with each other,
temperature difference is uniform. In other words, a 1°C temperature difference INDUSTRY TIP
is equal to a 1 K temperature difference.
Celsius is named after the
Measuring temperature Swedish astronomer, Anders
Celsius (1701–1744). The
Many methods have been developed for measuring temperature. Most rely Kelvin scale is named after
on measuring some physical property of a working material that varies with the Belfast-born engineer and
temperature. Temperature measuring devices include the following. physicist William Thomson,
l Glass thermometer: one of the most common devices for measuring First Baron Kelvin (1824–1907).
temperature. This consists of a glass tube filled with mercury or some other
liquid. Temperature increases cause the fluid to expand, so the temperature
can be determined by measuring the volume of the fluid. These thermometers
are usually calibrated so that the temperature can be read by observing the
level of the fluid in the thermometer.
l Gas thermometer: this measures temperature by the variation in volume or
pressure of a gas.
l Thermocouple: this device is a connection between two different metals
that produces an electrical voltage when subjected to heat. This senses a
temperature difference. Thermocouples are a widely used type of temperature
sensor for measurement and control when used with digital thermometers
(see below). They can also be used to convert heat into electrical power.
l Thermistor: thermistors are resistors that vary with temperature. They are
constructed of semiconductor material with a resistivity that is especially
sensitive to temperature. When the temperature is measured, the resistance
of the thermistor responds in a predictable way.
l Infrared thermometers: these use infrared energy to detect temperatures.
They detect actual energy levels by the use of an infrared beam and so p Figure 3.12 Glass thermometer
the thermometer does not need to actually touch the surface to take
an accurate temperature measurement.
l Digital thermometers: these are probably the most common thermometer
used in the plumbing industry. Dual digital thermometers can read two
temperatures simultaneously, instantly giving the temperature difference
between two points, which is essential when benchmarking central heating
boilers for reading the temperature of both flow and return pipes.
p Figure 3.13 Digital thermometer
States of matter

Everything around us is made up of matter, which can exist in three classic
states: solid, liquid and gas. Each of the phase changes is associated with either
an increase or decrease in temperature. For example, if heat energy is applied
to ice, it melts to form water and, if more heat energy is applied to the water, it
reaches its boiling point, where it vaporises, evaporating to steam. The process
can also work in reverse. When the heat is given up by the steam, it condenses
back to water. Each of these phase changes is given a name: p Figure 3.14 Infrared thermometer

l ice (solid) to water (liquid) is called melting
l water (liquid) to steam (gas) is called evaporation/vaporisation
l steam (gas) back to a water (liquid) is called condensation

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