IQS Newsroom Articles on Thermocouples
About Thermocouples and Thermocouple Assemblies Including: Resistance
Temperature Detectors, Temperature
Sensors, Thermistors & Thermowells.
Thermocouples are heat sensors used to measure temperature in a wide range of industrial, commercial and residential applications. Thermocouples, often called temperature probes or temperature sensors, consist of two dissimilar metals connected at two points. According to Seebeck's principle, a voltage is always created between two dissimilar metals, and the voltage changes in proportion to exterior temperature changes; thermocouples harness this reaction to measure temperature changes. Thermocouples have a basic construction, consisting of two metal wires connected at the base and with a bead at the tip. Type K thermocouples, the most universal type of thermocouple, are constructed of chromel (chromium and nickel alloy) and alumel (aluminum and nickel alloy) wires; type K thermocouples have relatively high temperature resistance and can be found in many everyday devices. For industrial purposes, thermocouple assemblies may be used in kilns, ovens, plastic extrusion machines, pressure chambers, water tanks, heat exchangers, parts washers and many other processors as water heater thermocouples, furnace thermocouples and in most temperature sensitive applications. Residential and commercial thermostats and temperature switches commonly use thermocouples as well. Thermowells and thermocouple wires are thermocouple accessories used to isolate the thermocouple device from damaging heat sources and to extend a thermocouple's reach.
Alternatives to thermocouples include resistance temperature detectors, or RTDs, and thermistors. Both RTDs and thermistors use principles of certain metals' electrical resistance which varies with temperature. RTDs are by far the most accurate type of temperature sensor, offering accuracy of +0.5 percent; platinum resistance thermometers, the most common RTD material, can accurately measure temperatures between -200 and 800 degrees celcius. Thermistors work similarly to RTDs, but are made from metal oxides which have an inverse resistance to increasing temperature. As temperatures rise, thermistors resistance falls, giving rise to the pseudonym "negative temperature coefficient", or NTC sensors. Unlike RTDs, thermistors may only accurately measure up to 200 degrees Celcius, limiting their applications. Thermistors are simpler and more cost effective than most RTDs or thermocouples with fast response times. In mid to low temperature applications, RTDs, thermocouples and thermistors may often be used interchangeably.
Each of the metals used in thermocouples have specific capacities for measuring certain temperature ranges. There are three functional classes of thermocouple assemblies based on the metal used. Base metal thermocouples, or type T & J thermocouples, are good for measuring temperatures under 1000 degrees Celsius. Noble metal thermocouples, including types K, N, R and S thermocouples, measure up to about 2000 degrees. Lastly, type C refractory metal thermocouples can handle upward of 2600 degrees Celsius. Most thermocouple assemblies are sheathed and are one of three junction types: grounded, ungrounded or exposed. In an exposed thermocouple, the tip of it protrudes out beyond the sheath, exposing it directly to the surrounding environment. This provides fast response and reading of the temperature, but this type of reading is limited to non-corrosive and non- pressurized situations. Thermocouples often work together as part of a larger measurement or other data acquisition system. These systems are often computerized and have automated capabilities. They gather information from one or more signal inputs or sensor sources such as thermocouples and convert this information into a digital form for further analysis.
While there are alternatives to thermocouples, they are the most popular temperature measuring device due to their low cost, simple construction and ease of installation. Most thermocouples have a wide temperature range, good repeatability and short response time; RTDs tend to measure with tighter accuracy than thermocouples, but they do not have nearly as high heat capacity and are more costly. Thermocouples are generally not sensitive enough for certain precision laboratory and testing applications, in which case thermistors and RTDs are used instead. The 10 main types of thermocouples offer a broad range of application options which are not always met by other types of temperature sensors. Thermocouples, particularly type K thermocouples, may be joined with digital devices for a fast, easy readout.
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Thermocouples and Thermocouple
Assemblies Image Provided by Thermal
Devices |
Types of Thermocouples
- are used for the temperature
measurement of piping and other cylinder shaped entities.
- sense heat which has wavelengths longer than
red visible light and shorter than microwaves.
- register changes in resistance as changes in temperature.
- are wires made of a different homogeneous metal or alloy,
joined at one end to form a measuring junction.
- gauge temperatures with a bridge measurement.
- gather information on the change in resistance
of a metal wire element.
- is a resistance temperature detector with a pair
of different metals joined at two points so a voltage results
when the
junctions are at different temperatures.
- measure the temperature of a material or substance.
- are semiconductors with varying resistance that relates
to temperature.
- are a collection of tubes,
wires and insulators. Often included to in classification with heating
elements.
( heating-elements.com )
- are used to protect temperature sensors.
Thermocouples and Thermocouples Manufacturers Terms
- The temperature of the
air surrounding the equipment.
- Any metal other than precious metal, such as copper,
aluminum, lead, nickel and tin.
-- British thermal unit; a unit for measuring a quantity of heat.
1 btu is the amount of heat required to raise the temperature of 1 pound
of water 1°F.
- Adjusting the equipment so readings and accepted
measurements are correlated so the value is accurate.
(centigrade) - A temperature scale defined by 0 °C at the
ice point and 100 °C at boiling point of water at sea level.
- Established by ANSI to distinguish wires for thermocouples.
- Alloys with similar thermoelectric properties
to the alloys in the thermocouple, used to connect the thermocouple to
the instrument.
- The difference between the value of the controlled variable
and the value at which it is being controlled.
-The temperature scale defined by 32° at the ice point
and 212° at the boiling point of water at sea level.
- Unit of thermal energy.
- In a thermocouple where two different metals are joined.
- Expressed in BTU per pound. The amount of heat needed (absorbed)
to convert a pound of boiling water to a pound of steam.
- A metal with high resistance to chemical effect,
especially corrosion and solution by organic acids; occasionally called
precious metal.
- A generic term that is used to describe many types of temperature
sensors .
- Metal containing a coating consisting of material
with a high melting point. Used in high temperature capacity thermocouple
devices.
--The transmission of energy by electromagnetic waves and
may become thermal energy when absorbed and increase in the temperature
of the absorbing body.
- Stands for Resistance Temperature Detectors.
- The minimum change in a physical variable to which an instrument
can respond.
- Thermodynamic cycle commonly used to cool thermo graphic
detectors.
- A measurement of heat equal to 100,000 btu.
- Measures the
difference in potential created at the junction of two different metal
wires which feed from the measuring instrument.
- Many Thermocouples grouped together in a series to
increase the thermoelectric output.