Temperature Sensors

TEMPERATURE SENSORS

Temperature conveys the state of a mechanical system in terms of expansion or contraction of solids, liquids or gases, change in electrical resistance of conductors, semiconductors and thermoelectric emfs. Temperature sensors such as bimetallic strips, thermocouples, thermistors are widely used in monitoring of manufacturing processes such as casting, molding, metal cutting etc. The construction details and principle of working of some of the temperature sensors are discussed in following sections.

1. Bimetallic strips

Bimetallic strips are used as thermal switch in controlling the temperature or heat in a manufacturing process or system. It contains two different metal strips bonded together.

The metals have different coefficients of expansion. On heating the strips bend into curved strips with the metal with higher coefficient of expansion on the outside of the curve. Figure shows a typical arrangement of a bimetallic strip used with a setting‒up magnet. As the strips bend, the soft iron comes in closer proximity of the small magnet and further touches. Then the electric circuit completes and generates an alarm. In this way bimetallic strips help to protect the desired application from heating above the pre‒set value of temperature.

2. Resistance temperature detectors (RTDs)

RTDs work on the principle that the electric resistance of a metal changes due to change in its temperature. On heating up metals, their resistance increases and follows a linear relationship as shown in Figure 8.29 The correlation is

R_t = R₀ (1+αT)

Where R_t is the resistance at temperature T (°C) and R₀ is the temperature at 0°C and α is the constant for the metal termed as temperature coefficient of resistance. The sensor is usually made to have a resistance of 100Ω at 0°C.

Fig. 940 shows the construction of a RTD. It has a resistor element connected to a Wheatstone bridge. The element and the connection leads are insulated and protected by a sheath. A small amount of current is continuously passing though the coil. As the temperature changes the resistance of the coil changes which is detected at the Wheatstone bridge.

RTDs are used in the form of thin films, wire wound or coil. They are generally made of metals such as platinum, nickel or nickel‒copper alloys. Platinum wire held by a high‒ temperature glass adhesive in a ceramic tube is used to measure the temperature in a metal furnace. Other applications are:

• Air conditioning and refrigeration servicing

• Food Processing

• Stoves and grills

• Textile production

• Plastics processing

• Petrochemical processing

• Micro electronics

• Air, gas and liquid temperature measurement in pipes and tanks

• Exhaust gas temperature measurement

Thermistors

Thermistors follow the principle of decrease in resistance with increasing temperature. The material used in thermistor is generally a semiconductor material such as a sintered metal oxide (mixtures of metal oxides, chromium, cobalt, iron, manganese and nickel) or doped polycrystalline ceramic containing barium titanate (BaTiO₃) and other compounds. As the temperature of semiconductor material increases the number of electrons able to move about increases which results in more current in the material and reduced resistance. Thermistors are rugged and small in dimensions. They exhibit nonlinear response characteristics.

Thermistors are available in the form of a bead (pressed disc), probe or chip. Fig 8.31 shows the construction of a bead type thermistor. It has a small bead of dimension from 0.5 mm to 5 mm coated with ceramic or glass material. The bead is connected to an electric circuit through two leads. To protect from the environment, the leads are contained in a stainless steel tube.

Applications of Thermistors

• To monitor the coolant temperature and/or oil temperature inside the engine

• To monitor the temperature of anincubator

• Thermistors are used in modern digital thermostats

• To monitor the temperature of battery packs while charging

• To monitor temperature of hot ends of 3D printers

• To maintain correct temperature in the food Handling and processing industry equipments

• To control the operations of consumer appliances such as toasters, coffee makers, refrigerators, freezers, hair dryers, etc.

Thermocouple

Thermocouple works on the fact that when a junction of dissimilar metals heated, it produces an electric potential related to temperature. As per Thomas Seebeck (1821), when two wires composed of dissimilar metals are joined at both ends and one of the ends is heated, then there is a continuous current which flows in the thermoelectric circuit. Figure shows the schematic of thermocouple circuit.

The net open circuit voltage (the Seebeck voltage) is a function of junction temperature and composition of two metals. It is given by, ∆ VAB = α∆T

Where α, the Seebeck coefficient, is the constant of proportionality.

Generally, Chromel (90% nickel and 10% chromium)‒Alumel (95% nickel, 2% manganese, 2% aluminium and 1% silicon) are used in the manufacture of a thermocouple.

Applications of Thermocouples

• To monitor temperatures and chemistry throughout the steel making process

• Testing temperatures associated with process plants e.g. chemical production and petroleum refineries

• Testing of heating appliance safety

• Temperature profiling in ovens, furnaces and kilns

• Temperature measurement of gas turbine and engine exhausts

• Monitoring of temperatures throughout the production and smelting process in the steel, iron and aluminum industry