A thermocouple consists of two dissimilar metallic wires joined at one end known as hot junction. When the hot junction is heated, an emf is produced which depends on the difference between the temperature of the hot junction and the temperature of free ends known as cold junction. This thermo emf is measured by Galvanometric/ Potentiometer Metric/ Digital Instruments, to display temperature.

Mineral Insulated Thermocouples & Resistance Temperature Detectors comprise of a metal sheath in which the thermo-electric elements/RTD element with conductors are embedded in highly compressed magnesium Oxide (MgO) insulation, thus providing the elements complete protection against oxidation and corrosion.

Thermocouples are available in type K, J, T, E, R, S, W, W3 and W5, simplex or duplex, in sheath diameters from 1mm to 19.05mm and length from a few cm to 200 meters or more. The hot junction is TIG welded and could be grounded (element hot junction welded to sheath tip giving faster response) or ungrounded (element insulated from sheath), one of which can be used as per control system requirement.

For temperature measurement in the range, -200°C to + 850°C Resistance Temperature Detector (RTD) is preferred to thermocouple and other sensors because of its higher accuracy, reliability, compact size and faster response. RTDs find use in almost all industries like plastic and rubber processing, food industries, pharmaceuticals, chemical & petrochemical plants, and power plants.

RTDs are available with single or double resistance elements and in 2, 3 or 4 wire circuits. Various types of protection sheaths for protection of the element and screw-in threaded bushes either fixed (welded) or adjustable by screws (for non-pressure application) or by compression fitting (for pressure application) for mounting are available.

RTD normally consists of spring-loaded insert, outer protecting sheath with mounting connections and cast aluminum terminal head. The insert assembly consists of Pt 100 element with silver/copper connection leads, insulated by ceramic multibore insulating tube/beads in a brass or stainless steel sheath with ceramic terminal block and brass terminals attached to a spring loaded arrangement.

Mineral Insulated Thermocouples & Resistance Temperature Detectors comprise of a metal sheath in which the thermo-electric elements/RTD element with conductors are embedded in highly compressed magnesium Oxide (MgO) insulation, thus providing the elements complete protection against oxidation and corrosion.

Thermocouples are available in type K, J, T, E, R, S, W, W3 and W5, simplex or duplex, in sheath diameters from 1mm to 19.05mm and length from a few cm to 200 meters or more. The hot junction is TIG welded and could be grounded (element hot junction welded to sheath tip giving faster response) or ungrounded (element insulated from sheath), one of which can be used as per control system requirement.

Thermowell is a closed-end reentrant tube designed for insertion of a temperature-sensing element, and provided with means for a pressure-tight attachment to a vessel.

In this instance, the ASTM definition of thermowell leaves a bit to be desired. Thermowells are typically constructed of solid drilled-out bar stock and are designed to protect a temperature sensor from flow, high pressure and harsh environments. Thermowells encase and protect temperature sensors from the harmful effects of the processes into which they are immersed without substantially insulating the temperature sensor (thermocouple, RTD, etc.) from the temperature of the process.

The most common types of thermowells are:

• Threaded : A threaded Thermowell is screwed into the process.
• Socket Weld : A weld in Thermowell is welded directly into the process
• Flanged Welded : A flanged Thermowell has a flange collar which is attached to a mating flange.

Portable digital Temperature indicator is used with sensor like Thermocouple or Resistance Thermometer for accurate and instantaneous measurement. Various temperature probes in different designs for special applications are also available.

When used with specially designed probes, IT63 Portable Indicator satisfies temperature measurement requirements in rubber and plastic processing, food processing & frozen foods, small ovens & heating appliances, metal working, electrical & other machinery, electronic parts & assemblies, air conditioning etc. Probes are also available for temperature measurement of curved, stationary or moving surfaces. Since this instrument operates on a 9V battery, its small size and light-weight, it can be transported any where for temperature measurement outdoors.

Automatic Cold Junction Compensation is built in ensuring accurate temperature measurement when used with thermocouples in the varying ambient temperatures between 0-50°C. One single 9V battery (Eveready No. 216or equivalent) is used to power the entire circuit and display.