Temperature measurement techniques

Temperature measurement is another important variable in any process industry. There have been several measuring techniques implemented in order to accurately measure this variable. These different techniques makes use of a variety of devices for measuring temperature. One of the most devices used from centuries till this day have been associated with fluids (like mercury or alcohol). The sole property of fluids to expand during high temperatures and vice versa, is the main reason for this association. However, there are some other instruments/devices which came to used at a later stage; namely, thermocouples, thermostats and resistive temperature devices (RTD) etc.

Temperature measurement, can be basically differentiated into two methods:

1) Mechanical temperature measurement:

a)Mercury thermometers: This technique makes use of glass tube of very fine bore joined to a reservoir at the bottom and sealed at the top. A measured quantity of mercury is enclosed in the glass tube. When the thermometer is exposed to high temperatures from the reservoir end, the expanding mercury rises up in the thermometer. The markings on the thermometer, helps to scale the temperature and thus aids in measuring.

b)Gas filled thermometers: Besides mercury thermometers, the other types of temperature measuring technique used are gas filled thermometers. This type depends upon the increase in pressure of a confirm gas (constant volume) due to temperature increase. The temperature and pressure in this kind of system follows the charles law and maybe expressed by (P1/P2). The system is filled under high pressure. The increase pressure for each degree of temperature rise is therefore greater than if the filling pressure were low. In such systems, nitrogen is the most commonly used gas, the reason behind this is its character of being inert and its coefficient of thermal expansion.

c)Bimetallic thermometers: In this case, two metal whose coefficient of linear expansion is different are welded and rolled together to the desire thickness. The actual movement of a bi-metal is its flexibility with one end fixed, a straight bi-metal strip deflects in proportion to its temperature, to the square of its length and inversely with its thickness.

2) Electrical temperature measurement:

a)Thermocouples: It is simply a device consisting of dissimilar metal wires joined at their ends. When heat is applied to the junction, a voltage, in the range of milli-volts (mV), is generated. A thermocouple is therefore said to be self-powered. Below are some of the different types of thermocouples, which are differentiated based on the types of wires used: 

i) Iron - constantin (j-type thermocouple)

ii) Chromel alumel (k-type thermocouple)

iii) Platinum rhodium (s-type thermocouple)

The operation of the thermocouple can be understood, by checking this link.

b) Resistive temperature detector (RTD):

RTDs are generally used for precise temperature measurement. Every type of metal has a unique composition and has a different resistance to the flow of electrical current. This is termed the resistively constant for that metal. For most metals the change in electrical resistance is directly proportional to its change in temperature and is linear over a range of temperatures. This constant factor called the temperature coefficient of electrical resistance (TCR) is the basis of RTDs. The RTD can actually be regarded as a high precision wire wound resistor whose resistance varies with temperature. By measuring the resistance of the metal, its temperature can be determined.
Further the working of the RTD can be understood, by checking this link.