Thermocouple
Thermocouple
One-stop source for process measurement & control. Get a free gift.
Omega.com
Thermocouple from Iotech
Thermocouple and RTD measurement instrumentation from Iotech for portable, lab and distributed apps - from 8 to 1,000 channels.
www.iotech.com
Thermocouple Surplus or Repair
PlcCenter stocks hard to find thermocouple surplus parts.
www.PlcCenter.com
Thermocouple Wire by Multi/cable
Thermocouple wire or extension wire avialable in all calibrations, solid or stranded conductor, many insulations available.
www.multicable.com
RTD, IR, & Thermocouple Temp. Meters
Handheld Thermometers, Calibrators Transmitter, Datalogger, IR & more.
www.DASdistribution.com/products
Temperature Meters by Laurel Electronics
Digital panel meters & controllers for multiple T/C & RTD types.
www.laurels.com
Thermocouple Probe
Online Information Guide Find What You Need Quick & Easy.
www.Industrial101.com
Furnace Thermocouple
Find Furnace Thermocouples at Great Prices.
www.Pronto.com
Marathon Heater
Marathon Heater manufactures cartridge heaters, band heaters, nozzle heaters, thermocouples and temperature controllers. Large stock selection.
www.marathonheater.com
Thermocouple Search
Free Technical Search Engine Search Thousands of Catalogs Today.
www.globalspec.com
Warning: mkdir() [function.mkdir]: Permission denied in /home/webs/affiliatelib2/CacheManager.php on line 12
Warning: mkdir() [function.mkdir]: No such file or directory in /home/webs/affiliatelib2/CacheManager.php on line 12
Warning: fopen(/home/templatecore2cache//*cluesnet.com/dc/dc8a4b5e2c95a61f1ef44ec73e13beb5a716bdce.tc2cache) [function.fopen]: failed to open stream: No such file or directory in /home/webs/affiliatelib2/CacheManager.php on line 130
Warning: fwrite(): supplied argument is not a valid stream resource in /home/webs/affiliatelib2/CacheManager.php on line 131
Warning: fclose(): supplied argument is not a valid stream resource in /home/webs/affiliatelib2/CacheManager.php on line 132
In electronics, thermocouples are a widely used type of list of temperature sensors and can also be used as a means to convert thermal potential difference into electric potential difference. They are cheap and interchangeable, have standard connectors, and can measure a wide range of temperatures. The main limitation is precision; system errors of less than 1 °Celsius can be difficult to achieve.
Principle of operation In 1821, the Germany-Estonian physicist Thomas Johann Seebeck discovered that when any conductor (such as a metal) is subjected to a thermal gradient, it will generate a voltage. This is now known as the thermoelectric effect or Seebeck effect. Any attempt to measure this voltage necessarily involves connecting another conductor to the "hot" end. This additional conductor will then also experience the temperature gradient, and develop a voltage of its own which will oppose the original. Fortunately, the magnitude of the effect depends on the metal in use. Using a dissimilar metal to complete the circuit will have a different voltage generated, leaving a small difference voltage available for measurement, which increases with temperature. This difference can typically be between 1 and 70 microvolts per degree Celsius for the modern range of available metal combinations. Certain combinations have become popular as industry standards, driven by cost, availability, convenience, melting point, chemical properties, stability, and output.
It is important to note that thermocouples measure the temperature difference between two points, not absolute temperature. In traditional applications, one of the junctions — the cold junction — was maintained at a known (reference) temperature, while the other end was attached to a probe.
Thermocouples can be connected in series with each other to form a thermopile, where all the hot junctions are exposed to the higher temperature and all the cold junctions to a lower temperature. Thus, the voltages of the individual thermocouple add up, which allows for a larger voltage and increased power. With the radioactive decay of transuranic elements providing a heat source this arrangement has been used to power spacecraft on missions too far from the sun to utilize solar power.
Having available a known temperature cold junction, while useful for laboratory calibrations, is simply not convenient for most directly connected indicating and control instruments. They incorporate into their circuits an artificial cold junction using some other thermally sensitive device (such as a thermistor or diode) to measure the temperature of the input connections at the instrument, with special care being taken to minimize any temperature gradient between terminals. Hence, the voltage from a known cold junction can be simulated, and the appropriate correction applied. This is known as cold junction compensation.
Additionally, cold junction compensation can be performed by software. Device voltages can be translated into temperatures by two methods. Values can either be found in look-up tables or approximated using polynomial coefficients.
Usually the thermocouple is attached to the indicating device by a special wire known as the compensating or extension cable. The terms are specific. Extension cable uses wires of nominally the same conductors as used at the thermocouple itself. These cables are less costly than thermocouple wire, although not cheap, and are usually produced in a convenient form for carrying over long distances - typically as flexible insulated wiring or multicore cables. They are usually specified for accuracy over a more restricted temperature range than the thermocouple wires. They are recommended for best accuracy.
Compensating cables on the other hand, are less precise, but cheaper. They use quite different, relatively low cost alloy conductor materials whose net thermoelectric coefficients are similar to those of the thermocouple in question (over a limited range of temperatures), but which do not match them quite as faithfully as extension cables. The combination develops similar outputs to those of the thermocouple, but the operating temperature range of the compensating cable is restricted to keep the mis-match errors acceptably small.
The extension cable or compensating cable must be selected to match the thermocouple. It generates a voltage proportional to the difference between the hot junction and cold junction, and is connected in the correct polarity so that the additional voltage is added to the thermocouple voltage, compensating for the temperature difference between the hot and cold junctions.
Voltage-Temperature Relationship The relationship between the temperature difference and the output voltage of a thermocouple is nonlinearity and is approximated by a polynomial interpolation.
T = \sum_{n = 0}^N a_n v^n
The coefficients an are given for n from 0 to between 5 and 9.
To achieve accurate measurements the equation is usually implemented in a digital controller or stored in a lookup table. Some older devices use analog filters.
Different types A variety of thermocouples are available, suitable for different measuring applications (industrial, scientific, food temperature, medical research, etc.). They are usually selected based on the temperature range and sensitivity needed. Thermocouples with low sensitivities (B, R, and S types) have correspondingly lower resolutions. Other selection criteria include the inertness of the thermocouple material, and whether or not it is magnetic. The thermocouple types are listed below with the positive electrode first, followed by the negative electrode.
Type K -- Chromel (Nickel-Chromium Alloy) / Alumel (Nickel-Aluminium Alloy): This is the most commonly used general purpose thermocouple. It is inexpensive and, owing to its popularity, available in a wide variety of probes. They are available in the −200 °C to +1200 °C range. The type K was specified at a time when metallurgy was less advanced than it is today and, consequently, characteristics vary considerably between examples. Another potential problem arises in some situations since one of the constituent metals is magnetic (Nickel). The characteristic of the thermocouple undergoes a step change when a magnetic material reaches its Curie point. This occurs for this thermocouple at 354°C. Sensitivity is approximately 41 µV/°C.
Type E -- Chromel / Constantan (Copper-Nickel Alloy): Type E has a high output (68 µV/°C) which makes it well suited to cryogenic use. Additionally, it is non-magnetic.
Type J -- Iron / Constantan: Limited range (−40 to +750 °C) makes type J less popular than type K. The main application is with old equipment that cannot accept modern thermocouples. J types cannot be used above 760 °C as an abrupt magnetic transformation causes permanent decalibration. The magnetic properties also prevent use in some applications. Type J's have a sensitivity of ~52 µV/°C.
Type N -- Nicrosil (Nickel-Chromium-Silicon Alloy) / Nisil (Nickel-Silicon Alloy): High stability and resistance to high temperature oxidation makes type N suitable for high temperature measurements without the cost of platinum (B, R, S) types. They can withstand temperatures above 1200 °C. Sensitivity is about 39 µV/°C at 900°C, slightly lower than a Type K. Designed to be an improved type K, it is becoming more popular.
Thermocouple types B, R, and S are all noble metal thermocouples and exhibit similar characteristics. They are the most stable of all thermocouples, but due to their low sensitivity (approximately 10 µV/°C) they are usually only used for high temperature measurement (>300 °C).
Type B -- Platinum 30% Rhodium / Platinum 6% Rhodium: Suited for high temperature measurements up to 1800 °C. Type B thermocouples (due to the shape of their temperature-voltage curve) give the same output at 0 °C and 42 °C. This makes them useless below 50 °C.
Type R -- Platinum 13% Rhodium / Platinum: Suited for high temperature measurements up to 1600 °C. Low sensitivity (10 µV/°C) and high cost makes them unsuitable for general purpose use.
Type S -- Platinum 10% Rhodium / Platinum: Suited for high temperature measurements up to 1600 °C. Low sensitivity (10 µV/°C) and high cost makes them unsuitable for general purpose use. Due to its high stability, type S is used as the standard of calibration for the melting point of gold (1064.43 °C).
Type T -- Copper / Constantan: Suited for measurements in the −200 to 350 °C range. Often used as a differential measurement since only copper wire touches the probes. As both conductors are non-magnetic, type T thermocouples are a popular choice for applications such as electrical generators which contain strong magnetic fields. Type T thermocouples have a sensitivity of ~43 µV/°C.
Type C -- Tungsten 5% Rhenium / Tungsten 26% Rhenium: Suited for measurements in the 32 to 4208°F ((0 to 2320°C). This thermocouple is well-suited for vacuum furnaces at extremely high temperatures and must never be used in the presence of oxygen at temperatures above 500°F.
Type M -- Nickel Alloy 19 / Nickel-Molybdenum Alloy 20: This type is used in the vacuum furnaces as well for the same reasons as with type C above. Upper temperature is limited to 2500°F (~1400°C). Though it is a less common type of thermocouple, look-up tables to correlate temperature to EMF (milli-volt output) are available.
Identification Thermocouple types can be identified based on wire insulation color.
{| class="wikitable"|-! Type! Temperature range °c (continuous)! Temperature range °c (short term)! Tolerance class 1 (°c)! Tolerance class 2 (°c)! IEC Colour code! BS Colour code! ANSI Colour code|-| K| 0 to +1100| -180 to +1300| -40 to +375 ± 1.5 °c, 375 to 1000 ± 0.004*°c| -40 to +333 ± 2.5 °c, 333 to 1200 ± 0.0075*°c| | | |-| J| 0 to +700| -180 to +800| -40 to +375 ± 1.5 °c, 375 to 750 ± 0.004*°c| -40 to +333 ± 2.5 °c, 333 to 750 ± 0.0075*°c| | | |-| N| 0 to +1100| -270 to +1300| -40 to +375 ± 1.5 °c, 375 to 1000 ± 0.004*°c| -40 to +333 ± 2.5°c, 333 to 1200 ± 0.0075*°c| | | |-| R| 0 to +1600| -50 to +1700| 0 to +1100 ± 1.0°c, 1100 to 1600 ± (1+0.003 (t-1100))*°c| 0 to +600 ± 1.5 °c, 600 to 1600 ± 0.0025*°c| | | Not defined.|-| S| 0 to 1600| -50 to +1750| 0 to +1100 ± 1.0 °c, 1100 to 1600 ± (1+0.003(t-1100))*°c| 0 to +600 ± 1.5°c, 600 to 1600 ± 0.0025*°c| | | Not defined.|-| B| +200 to +1700| 0 to +1820| Not Available| 600 to 1700 ± 0.0025*°c| No standard use copper wire| No standard use copper wire| Not defined.|-| T| -185 to +300| -250 to +400| -40 to +125 ± 0.5°c, 125 to 350 ± 0.004*°c| -40 to +133 ± 1.0°c,133 to 350 ± 0.0075*°c| | | |-| E| 0 to +800| -40 to +900| -40 to + 375 ± 1.5°c, 375 to 800 ± 0.004*°c| -40 to +333 ± 2.5°c, 333 to 900 ± 0.0075*°c| | | |}
Applications Thermocouples are most suitable for measuring over a large temperature range, up to 1800 K. They are less suitable for applications where smaller temperature differences need to be measured with high accuracy, for example the range 0–100 °C with 0.1 °C accuracy. For such applications, thermistors and Resistance temperature detectors are more suitable.
Steel Industry Type B, S, R and K thermocouples are used extensively in the steel and iron industry to monitor temperatures and chemistry throughout the steel making process. Disposable, immersible, Type S thermocouples are regularly used in the electric arc furnace process to accurately measure the steel's temperature before tapping. The cooling curve of a small steel sample can be analyzed and used to estimate the carbon content of molten steel.
Heating appliance safety Many Natural gas-fed heating appliances like ovens and water heaters make use of a pilot light to ignite the main gas burner as required. If the pilot light becomes extinguished for any reason, there is the potential for un-combusted gas to be released into the surrounding area, thereby creating both risk of fire and a health hazard. To prevent such a danger, some appliances use a thermocouple as a fail-safe control to sense when the pilot light is burning. The tip of the thermocouple is placed in the pilot flame. The resultant voltage, typically around 20 mV, operates the gas supply valve responsible for feeding the pilot. So long as the pilot flame remains lit, the thermocouple remains hot and holds the pilot gas valve open. If the pilot light goes out, the temperature will fall along with a corresponding drop in voltage across the thermocouple leads, removing power from the valve. The valve closes, shutting off the gas and halting this unsafe condition.
Many systems (Millivolt control systems) extend this concept to the main gas valve as well. Not only does the voltage created by the pilot thermocouple activate the pilot gas valve, it is also routed through a thermostat to power the main gas valve as well. Here, a larger voltage is needed than in a pilot flame safety system described above, and a thermopile is used rather than a single thermocouple. Such a system requires no external source of electricity for its operation and so can operate during a power failure, provided all the related system components allow for this. Note that this excludes common forced air furnaces because external power is required to operate the blower motor, but this feature is especially useful for un-powered convection heaters.
A similar gas shut-off safety mechanism using a thermocouple is sometimes employed to ensure that the main burner ignites within a certain time period, shutting off the main burner gas supply valve should that not happen.
Out of concern for wasted energy, many newer appliances have switched to an electronically controlled pilot-less ignition, also called intermittent ignition. This eliminates the need for a standing pilot flame but loses the benefit of any operation without a continuous source of electricity.
Thermopile radiation sensors Thermopiles are used for measuring the intensity of incident radiation, typically visible or infrared light, which heats the hot junctions, while the cold junctions are on a heat sink. It is possible to measure radiative intensity of only a few μW/cm2 with commercially available thermopile sensors. For example, laser power (physics) meters are based on such sensors.
Radioisotope thermoelectric generators (RTGs) Thermopiles can also be applied to generate electricity in radioisotope thermoelectric generators.
See also
- Bolometer
- Resistance thermometer
- Thermistor
- Thermopile
- Thermostat
External links
- NIST ITS-90 Thermocouple Database
- Thermocouple design guide
- Notes from a cold junction
- Mineral-Insulated Thermocouple Know-How
- Thermocouple Color Code Chart and Specifications
Welcome to Thermocouple Instruments
Temperature and Flow Measurement Specialists. Thermocouples, RTD's, Temperature Transmitters, Thermowells, Orifice Plates, Venturis, Nozzles, Pitots
Thermocouple Instruments - Contact Details
Main Switchboard +44 (0) 1656 740 551: Main Fax Number +44 (0) 1656 745 915 : Main E-mail Address: enquiries@thermocouple.co.uk
Thermocouple - Wikipedia, the free encyclopedia
In electrical engineering and industry, thermocouples are a widely used type of temperature sensor [1] and can also be used as a means to convert thermal potential difference into ...
Thermocouple application note from Pico Technology
Thermocouple application note. How they work, choosing the right thermocouple for your application. Hints, tips and pitfalls to avoid. Includes information on thermocouple data ...
TC-08 thermocouple data logger
Measure and record temperature with the USB TC-08 thermocouple data logger from Pico ... The TC-08 thermocouple data logger offers industry-leading performance and an inexpensive ...
Thermocouple - a reference guide from Engineeringtalk
Looking for the latest thermocouple-related news? Find it here on the Engineeringtalk website with Dave Wilson, Editor ... Dave Wilson, Editor, writes: We see from your search that ...
Thermocouples, Thermocouple Probes and Sensors
Your single source for process measurement and control. Everything from thermocouples to chart recorders and beyond. Temperature, flow and level, data acquisition, recorders and ...
Thermocouple Cable and Thermocouple Wire
Your single source for process measurement and control. Everything from thermocouples to chart recorders and beyond. Temperature, flow and level, data acquisition, recorders and ...
Thermocouples, Thermocouple Cables, Pt100 Sensors & Temperature ...
Thermocouples, Thermocouple Cables, Thermocouple Sensors, Thermocouple Connectors, Resistance Thermometers, Temperature Controllers and accessories as well as their FREE 'how to ...
Thermocouple - a reference guide from Electronicstalk
Looking for the latest thermocouple-related news? Find it here on the Electronicstalk website with Laurence Marchini, Editor ... Laurence Marchini, Editor, writes: We see from your ...