Thermocouples

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. Thermocouple instruments 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, high temperature 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 the reach. Alternatives to thermocouples include resistance temperature detectors, or RTDs, and thermistors which are used for high accuracy applications.

Depending on the required temperature range and the intended environment for a thermocouple, it can be constructed with different combinations of metals and calibrations. The diameter of the thermocouple wire is typically used to determine the maximum temperature the thermocouple works with, although the calibration can also determine the full range. A thermocouple with a very thin wire will not have as broad a temperature range capacity as that with a thicker wire. When determining the best option for a type of thermocouple, there are a number of factors to consider. Thermocouples can vary widely in terms of temperature handling capacity, and so the desired or intended temperature range for a thermocouple should be decided upon. The various materials respond differently to temperature and other factors such as chemical exposure, abrasions or mechanical vibrations and therefore these factors should also be considered in material choice. Thermocouples are also often installed into already existing systems and therefore need to be designed or adjusted for compatibility and to ensure the most effective and accurate results are given to the operator.

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 Celsius. Thermistors work similarly to RTDs, but are made from metal oxides which have an inverse resistance to increasing temperature. As temperatures rise, the resistance of a thermistor 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 to those which do not require higher temperatures. 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, that is covered with a protective tubing and insulator of sorts. There are three possible thermocouple 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 to a change in temperature, and provides a 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 main types of thermocouples offer a broad range of application options which are not always met by other types of temperature sensors.

Images Provided by Thermo Sensors Corporation

Image Provided by Thermal Devices

Types of Thermocouples

• Clamping ring thermocouples are used for the temperature measurement of piping and other cylinder shaped entities.
  
• Infrared thermocouples sense heat which has wavelengths longer than red visible light and shorter than microwaves.
  
• Resistance temperature detectors register changes in resistance as changes in temperature.
• RTD Probes are wires made of a different homogeneous metal or alloy, joined at one end to form a measuring junction.
  
• RTD sensors gauge temperatures with a bridge measurement.
  
• RTD temperature transducers gather information on the change in resistance of a metal wire element.
  
• RTD thermocouple 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.
  
• Temperature sensors measure the temperature of a material or substance.
• Thermistors are semiconductors with varying resistance that relates to temperature.
  
• Thermocouple assemblies are a collection of tubes, wires and insulators. Often included to in classification with heating elements. ( http://www.iqsdirectory.com/heating-elements/ )
• Thermowells are used to protect temperature sensors.

Thermocouples and Thermocouples Manufacturers Terms

Ambient Temperature - The temperature of the air surrounding the equipment.
 
Base metal - Any metal other than precious metal, such as copper, aluminum, lead, nickel and tin.
 
Btu -- 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.
 
Calibration - Adjusting the equipment so readings and accepted measurements are correlated so the value is accurate.
 
Celsius (centigrade) - A temperature scale defined by 0 °C at the ice point and 100 °C at boiling point of water at sea level.
 
Color Code - Established by ANSI to distinguish wires for thermocouples.
 
Compensating Alloys - Alloys with similar thermoelectric properties to the alloys in the thermocouple, used to connect the thermocouple to the instrument.
 
Deviation - The difference between the value of the controlled variable and the value at which it is being controlled.
 
Fahrenheit -The temperature scale defined by 32° at the ice point and 212° at the boiling point of water at sea level.
  
Joule - Unit of thermal energy.
 
Junction - In a thermocouple where two different metals are joined.
 
Latent Heat - Expressed in BTU per pound. The amount of heat needed (absorbed) to convert a pound of boiling water to a pound of steam.
 
Noble metal - A metal with high resistance to chemical effect, especially corrosion and solution by organic acids; occasionally called precious metal.
 
Probe - A generic term that is used to describe many types of temperature sensors .
 
Refractory metal - Metal containing a coating consisting of material with a high melting point. Used in high temperature capacity thermocouple devices.
 
Radiationange --The transmission of energy by electromagnetic waves and may become thermal energy when absorbed and increase in the temperature of the absorbing body.
 
RTD - Stands for Resistance Temperature Detectors.
 
Sensitivity - The minimum change in a physical variable to which an instrument can respond.
 
Sterling Cycle - Thermodynamic cycle commonly used to cool thermo graphic detectors.
 
Therm - A measurement of heat equal to 100,000 btu.             
 
Thermocouple - Measures the difference in potential created at the junction of two different metal wires which feed from the measuring instrument.
 
Thermopile - Many Thermocouples grouped together in a series to increase the thermoelectric output.