A thermocouple consists of two dissimilar metallic Wires joined at one end known as hot junction. When the hot junction is heated, an emf is produced which depends on the difference between the temperature of the hot junction and the temperature of free ends known as cold junction. This thermo emf is measured by Galvanometric/ Potentiometer Metric/ Digital Instruments, to display temperature.
- Most common, convenient and versatile devices used to measure temperature
- Sensing devices for measurement, control and recording of temperature from 200°C to +1800°C
- These are available in two basic designs:
Straight type & Angle Type: Angle type Thermocouples are used when it is not convenient to use Straight Type Thermocouples or Thermocouple connecting head is desired to be kept away from the place of insertion of Thermocouple so as to avoid attack of harmful fumes and vapors or high temperature
|Thermocouple Element||ANSI Type||Wire Dia. (mm)||Continuous Temp. Range °C||Max. Peak Temp. °C||Standard Limits of Error|
|± 2.2°C upto 293 °C|
± 0.75% above 293 °C
|Chromel/Alumel or NiCr/Ni|
Pt. 13% Rh./Pt.
|± 3°C/ 0.5%|
whichever is higher
|± 0.5% above 800°C|
Note:1) Limits of error for Type J,K & B are as per ANSI MC 96.1 and for S & R as per IS
2) Elements with special limits of error, other diameters and ANSI types e.g. T,E & N also available.
3) Calibration report for each thermocouple with traceability to NPL is optionally available to meet ISO 9000 requirements.
The connecting head is cast aluminum or cast iron with detachable cover and is fixed on open end of protecting sheath. Thermocouple wires are terminated on a ceramic terminal block with bass Terminals inside the head. From these terminals, compensating/ extension leads are taken for connections to the measuring or controlling instruments.Various type of heads are available such as standard (non-weatherproof), weatherproof (with threaded cap and cable entry (specify – e.g. 3/4” ET/1/2” BSP/NPT(F)) and gasket-IP65 protection – Fig.5), flameproof (CMRS approved for group 1, II A, II B & II C with flameproof double compression cable gland). In addition special heads with double cable entry, single/double compression cable glands, miniature type etc is also available.
Metallic and /or ceramic sheaths (tubes closed at one end) are put over the thermocouple elements for protection against corrosive atmosphere and harmful chemical action, which may be present at the place of use. When the corrosive atmosphere is too severe, an additional, inner gas tight ceramic sheath is provided for protecting the elements from corrosion, especially for the expensive rare metal thermocouples. In the case of angle type thermocouple, this ceramic tube is provided in the hot arm only. The measuring accuracy is, in no way, affected by protecting sheaths. The sheath diameter and wall thickness are selected to accommodate the elements and inner ceramic tube (if used) and the conditions of use/ tolerable life as a thin sheath will deteriorate faster than a thick one. Proper selection of the material of the protecting sheath is also important. General guidelines for protecting sheath for different applications are given in the table below:
|Protecting Sheath||Composition (Approx.)||Application Guidelines||Standard Sheath Dis. (Wall Thickness), mm|
|Mild Steel||Carbon Steel||For use upto 550°C in non-oxidizing environments main area of usage include non-corrosive liquids and gases. Melting point 1525°C.||21.3/22 (2)|
|Stainless Steel 304||18-20% Cr. 8-11% Ni., <2% Mn balance Iron||Upto 900°C in oxidizing conditions. Has good resistance to oxidation and corrosion. Generally used in process applications such as steam lines, oil refineries and chemical solutions. Resists nitric acids well, halogen acids poorly and the sulphur acids moderately. Subject to carbide precipitation in the 480-870°C range. Not recommended for use in reducing atmospheres. Melting Point 1400°C|| |
16 (1.6), 14 (1.6)
|Stainless Steel 315||16-18% Cr, 10-14% Ni, 2-3% Mo, balance Iron||For use upto 900°C in oxidising conditions. Superior to SS304 in corrosion resistance. Resists pitting in phosphorous & acetic acids. Subject to carbide precipitation in the 430-815°C range. Not recommended for use in reducing atmospheres. Melting point 1400°C|
|Heat Resistance Steel 446||23-27% Cr, balance Iron||Upto 1100°C in oxidizing conditions. Excellent resistance to oxidizing & reducing flames containing sulphur. For sulphurous atmosphere salt baths & molten metal. Melting 1400°C.||21.3/22 (2)|
|Stainless Steel 310||24-26% Cr,19-22% Ni, <2% Mn., balance Iron||Upto 1100°C High resistance to corrosive & nitrogenous atmosphere. Provides goods resistance to both carburizing & reducing environments. Deficient in oxygen. Subject to carbide precipitation in the 490-870° range. Suitable for carburizing, annealing/ hardening furnaces, cyanide & salt bath. Melting point 1410°C.||21.3/22 (2)|
|Inconel 600||72%(min.) Ni, 14-17% Cr. balance Iron.||Upto 1150°C under oxidizing conditions, goods resistance in corrosive atmosphere at high temp. not recommended for use in sulphur or reducing atmospheres. Suitable for carburizing. Annealing and hardening furnaces, cyanide salt bath etc.||21.3/22 (2)|
|Incolloy 800||30-35% Ni,19-23% Cr balance Iron||Upto 650°C in sulphur corrosive & reducing atmosphere, 950°C in sulphur corrosive & oxidizing atmosphere & 1150°C in air, strong resistance to oxidation & carburizing at high temperature Resists sulphur attack, internal oxidation and scaling. Melting point 1370°C.||21.3/22 (2/3.77)|
|Wrought Iron||Technically Pure Iron||Upto 600°C in air. Can be used for metal melts especially HSS baths upto 1350°C when with exchangeable sleeve.||24 (2.5), 15 (2)|
|Impervious Ceramic||Alumina Silicate (sillimanite)||Upto 1500°C. Fair resistance to thermal stock and fair mechanical strength.||24 (2.5), 15 (2)|
|Sintered Alumina||Pure Alumina||Upto 1800°C. Nonporous with goods resistance to thermal & mechanical shock.||24 (2.5), 15 (2)|
|Silicon Carbide||90% Sic||Upto 1650°C. Can withstand flame impingement, has fair thermal stock resistance.||45 (10), 26 (8)|
Note: 1) Protective sheaths of other materials, such as Graphite, Cast Iron (for molten Aluminium), Hastelloy etc. other diameters and wall thickness are also available.
2) Operating temp. Varies, depending on the atmospheres.
3) These data are shown to serve only as a general suggestion and not as a guarantee. Final selection must be based on actual evaluation of the metal in the corrosive medium under study.
Metallic protective sheaths are also offered with coatings of Tungsten Carbide, Stellite, Teflon Ceramic, Glass etc. and sleeve of tantalum, titanium, hastelloy, nickle, zirconium etc. are also available for resistance to abrasion/chemical corrosion in special process applications.
Flange or screwed bush is used for mounting the Thermocouple at the place of its use. Flange in normally adjustable over the protecting sheath or holding tube of ceramic sheath. Apart from the standard cast aluminum oval flange, standard ANSI/BS flanges in different materials (CS, SS 304/SS 316) and pressure rating, either fixed (welded) or adjustable by screws (for non-pressure application) or by compression fitting (for pressure applications) are also available. Alternatively various types of standard screw-in threaded bushes (e.g. 3/4” /1” BSP/NPT or m33 x 2) either fixed (welded) or adjustable by screws (for non-pressure application) or by compression fitting (for pressure applications) are available for mounting. Counter flange (for flange mounting) and stub with female threads (for screw mounting) are also available.
They are used in industries, power plants and metal melting furnaces, salt baths, industrial process control, laboratories and numerous other applications as
- Salt Bath : Neutral/ Cyanide/ Borax/ HSS/ Chloride/ Other
- Molten Metal : Zinc/ Magnesium/ Aluminium/ Copper/ Brass/ Ferrous Alloys/ Other
- Process Furnaces : Glass melting/ Cracking/ Roasting/ Blast/ Calcining/ Hydrogenising/ High Pressure Stoves (specify process material)
- Ovens & Furnace with : Reducing/ Oxidising/ Sulphurous/ Carbonising gaseous atmosphere (Specify gas details)
- Plating : Galvanising/ Tinning