A thermocouple is used to measure temperature. The principal operation of a thermocouple is electric current produced when two dissimilar metal materials are combined at one end and heat (or cold) is applied. This principal was discovered in 1820’s by Thomas Johann Seebeck, coined the “Seebeck Effect.” Thomas Seebeck discovered that current flows when the two junction’s temperatures were different. Different materials when heated caused different direction and degree of current to flow which could be calibrated to provide an accurate reading of temperature.

Thermocouples are essentially simple, inexpensive temperature sensing devices used to measure temperature for a variety of purposes, typically, industrial uses. In addition to its functional use and low cost, another advantage of a thermocouple is it does not require external power to operate. An additional feature of thermocouples is their ability to measure a wide range of temperatures. This makes them popular with a variety of industries and very useful for a large range of applications.

RTDs (“Resistance Temperature Detectors”), also called Resistance Thermometers, are another type of sensor used to measure temperature. The basic element of a RTD consists of fine coiled wire wrapped around a glass or ceramic core. This makes them a bit more fragile. The element requires protection which is often housed in a metal sheath.  The element material is made from common metals (platinum, cooper, nickel, etc.) which have unique, but known resistance properties that are used to derive a temperature vs. resistance relationship.

RTDs, like thermocouples, are simple and inexpensive. One of the primary advantages of RTDs is their accuracy. They offer a tighter tolerance and better resolution providing more precise measurements.  They also offer the user more stable readings (less drift) over a long period of time. A key feature of an RTD is their immunity to electrical noise and ideal for applications around motors, generators and high voltage equipment. The primary disadvantage of an RTD is the narrow temperature range (-200 to +650°C) and can their response time can be slow (1 to 50 seconds).

A thermowell is a protective casing used to enclose a temperature sensor (thermocouple, RTDs, etc.) to protect it from harsh environments. The basic construction of a thermowell consists of a metal tube (a machined piece of bar stock) with an opening at one end and closed at the other end. The predominant uses of thermowells are for applications involving exposure to high pressure, contact with fluids or hazardous chemicals. The thermowell provides a barrier for the sensor from interaction with hostile elements.

The disadvantage of using a thermowell is the effect on response time. Depending on the material and its thickness, the transmission of the reading can be slowed. One additional consideration for thermowell applications are proper fit and sealing which are required to prevent the leaking of gas or liquids.