Temperature is probably the most fundamental parameter, and is a widely measured and controlled industrial variable. It is required in the routine control of an industrial plant. The conditions under which temperature has to be measured, differ so widely that no fixed rule can be followed. All that is essential is that one should select the most appropriate method of temperature measurement for a particular use, considering the points such as, sources of error and limitations, precautions to be observed, the exact location of the sensing probe, etc. The steps to be taken to check the accuracy of instruments before, during, and after the test are also of extreme importance.

The temperature of a substance is a measure of the hotness, or coldness, of that substance. It is the thermal state of a body or a substance which determines whether it will give heat to, or receive heat from, other bodies. If two bodies are placed in contact then heat tends to flow from a body at a higher to lower levels. The terms, heat and temperature, are closely related. Temperature may be defined as "degree of heat" but heat is usually taken to mean "quantity of heat". Temperature and heat flow are related quantitatively by the second law of thermodynamics, which states that heat flows, of its own accord, from a body at a higher temperature to be body at a lower temperature. It is therefore important to remember that in temperature measurement, two bodies in intimate contact are at the same temperature only if there is no heat flow between them.

International Practical Temperature Scale

An international practical temperature scale (IPTS) was adopted in 1927 and revised in 1948 by all industrialized nations to ensure accurate and uniform temperature measurements. This scale is based on number of basic and secondary fixed points that can be used for calibrating temperature measuring instruments. These fixed points are given below:
Basic Fixed Points The following are the basic fixed points, all corresponding to the normal atmospheric pressure of 760 mm of mercury, and are used to standardize thermometers in laboratories:
(i) Boiling Point The boiling point is the temperature at which the substance changes physical state and becomes a gas.
(ii) Freezing Point The freezing point is the temperature at which the substance changes physical state and becomes a solid.
(iii) Triple Point A particular temperature and pressure at which three different phases of one substance can exist in equilibrium is known as 'triple point'. In common uses, the three phases are normally solid, liquid, and gas, although triple points can also occur with two solid phases and one liquid phase, with two solid phases and gas phase or with three solid phases.