Wednesday, May 11, 2016

Temperature Sensors for Process Measurement - Thermocouple, RTD, Thermistor

straight tube thermocouple, RTD or thermistor for temperature measurement
Simple RTD, thermocouple, thermistor
straight tube assembly
Courtesy Smart Sensors, Inc.
This post explains the basic operation of the three most common temperature sensing elements - thermocouples, RTD's and thermistors.

A thermocouple is a temperature sensor producing a micro-voltage from a phenomena called the Seebeck Effect. In simple terms, when the junction of two different (dissimilar) metals varies in temperature from a second junction (called the reference junction), a voltage is produced. When the reference junction temperature is known and constant, the voltage produced by the sensing junction can be measured and a corresponding temperature derived.

Thermocouples are widely used for industrial and commercial temperate control because they are inexpensive, exhibit appropriate accuracy for many applications, have a fairly linear temperature-to-voltage output curve, come in many “types” (different metal alloys) for many different temperature ranges, and are easily interchangeable. They require no external power to work and can be used in continuous temperature measurement applications from -185 Deg. Celsius (Type T) up to 1700 Deg. Celsius (Type B).

Common application for thermocouples are industrial processes, the plastics industry, kilns, boilers, steel making, power generation, gas turbine exhaust and diesel engines, They also have many consumer uses such as temperature sensors in thermostats and flame sensors, and for consumer cooking and heating equipment.
wire wound RTD
Coil wound RTD element
(image courtesy of Wikipedia)

RTD’s (resistance temperature detectors), are temperature sensors that produce a measurable change in resistance as the temperature of the RTD sensing element changes. They are normally designed as a fine wire coiled around a bobbin (made of glass or ceramic), and inserted into a protective sheath. They can also be manufactured as a thin-film element with the pure metal deposited on a ceramic base much like a circuit board.

thin film rtd
Thin-film RTD element
(image courtesy of Wikipedia)
The RTD wire is usually a pure metal such as platinum, nickel or copper because these metals have a predictable change in resistance as their temperature changes. RTDs offer considerably higher accuracy and repeatability than thermocouples and can be used up to 600 Deg. Celsius. They are most often used in biomedical applications, semiconductor processing and industrial applications where higher accuracy is important. Because they are made of pure metals, they tend to more costly than thermocouples. RTDs do need to be supplied an excitation voltage from the control circuitry.

The third most common temperature sensor is the thermistor. Thermistors work in a similar fashion to RTDs, in that they are a resistance based device, but instead of using pure metal, thermistors use a very inexpensive polymer or ceramic material as the element. The practical application difference between thermistors and RTD’s is the resistance response curve of thermistors. It is very non-linear, making thermistors useful over a narrower temperature range than RTDs.

thermistor
Thermistor bead with wires
(image courtesy of Wikipedia)
Thermistors however are very inexpensive and have a very fast response. They also come in two varieties, positive temperature coefficient (PTC - resistance increases with increasing temperature), and negative temperature coefficient (NTC - resistance decreases with increasing temperature). Thermistors are used widely in monitoring temperature of circuit boards, digital thermostats, food processing, and consumer appliances.

Temperature sensors are available in an almost infinite number of assemblies and configurations to accommodate every conceivable application. Share your application with a product specialist and take advantage of their application knowledge and experience.