Page 255 - Instrumentation Reference Book 3E

14 Temperature measurement

C. H AGI ART- AL E XANDE R

14.1 Temperature and heat concepts of temperature and other heat-related
phenomena.
14.1.1 Application considerations
Temperature is one of the most frequently used 14.1.2.1 Ternperatwe
process measurements. Almost all chemical pro- The first recorded temperature measurement was
cesses and reactions are temperature-dependent. carried out by Galileo at the end of the sixteenth
Not infrequently in chemical plants, temperature century. His thermometer depended on the
is the only indication of the progress of the pro- expansion of air. Some form of scale was attached
cess. Where the temperature is critical to the reac- to his apparatus, for he mentions "degrees of heat"
tion, a considerable loss of product may result in his records.
from incorrect temperatures. In some cases, loss As with any other measurement, it is necessary
of control of temperature can result in cata- to have agreed and standardized units of mea-
strophic plant failure with the attendant damage surement. In the case of temperature the inter-
and possible loss of life. nationally recognized units are the Kelvin and
Another area where accurate temperature mea- the degree Celsius. The definitions of these units
surement is essential is in the metallurgical indus- are set out in Section 14.2.
tries. In the case of many metal alloys, typically Temperature is a measure of stored or potential
steel and aluminum alloys, the temperature of the energy in a mass of matter. It is the state of
heat treatment the metal receives during manu- agitation, both lateral and rotational oscillation,
facture is a crucial factor in establishing the of the molecules of the medium. The higher the
mechanicai properties of the finished product. temperature of a body, the greater the vibrational
There Ere many other areas of industry where energy of its molecules and the greater its poten-
temperature measurement is essential. Such tial to transfer this molecular kinetic energy to
applications include steam raising and electricity another body. Temperature is the potential to
generation, plastics manufacture and molding, cause heat to move from a point of higher tem-
milk and dairy products, and many other areas perature to one of lower temperature. The rate of
of the food industries. heat transfer is a function of that temperature
Where most of us are most aware of tempera- difference.
ture is in the heating and air-conditioning systems
which make so much difference to people's per-
sona! comfort. 14.1.2.2 Heat
Instruments for the measurement of tempera-
ture, as with so many other instruments, are Heat is thermal energy. The quantity of heat in a
available in a wide range of configurations. body is proportional to the temperature of that
Everyone must be familiar with the ubiquitous body. i.e.. it is its heat capacity multiplied by its
liquid-in-glass thermometer. absolute temperature.
However, a number of other temperature- Heat is measured in joules. (Before the inter-
dependent effects are used for measurement pur- national agreements on the SI system of units
heat was measured in calories. One calorie was
poses and readout can be local to the point of
measurement 0: remote in a control room or else- approximately 4.2 joules.)
where.
14.1.2.3 Specific heat capacity
Different materials absorb different amounts of
14.1.2 Definitions
heat to produce the same temperature rise. The
For the understanding of temperature measure- specific heat capacity, or more usually the specific
ment it is essential to have an appreciation of the heat, of a substance is the amount of heat which.