Thermocouple selection guide
There are many sensors available for temperature measurement, and the thermocouple is probably the most commonly used in industry, as it offers good accuracy at low cost.
With so many variations, shapes and protective tubes to choose from, it's not always easy to decide which products to use.
There are several criteria to consider when choosing your thermocouple. These mainly relate to the environment, the accuracy required and, above all, the temperature of use.
There are many different types of thermocouple, but 8 are the most commonly used in our workshops.
| TYPE | Alloys used | Mounted under ceramic beads (rigid sheath) | Sleeved version with external sheath and MgO insulation (flexible sheath) |
| T | Copper (+) Constantan (-) |
Type T can be used in both oxidizing and reducing atmospheres. Low-temperature stability for greater precision Recommended for very low temperatures: cryogenics Recommended use of : -200 to 350°C |
Type T is used between -40 and 350°C and is very stable for low-temperature measurement applications and cryogenics. When used below 0°C, make sure that the outer sheath is compatible with the outer sheath. |
| K | Chromel (+) Alumel (-) |
Type K is used extensively at temperatures up to 1200°C, as it is reliable and precise. We recommend protecting this thermocouple with a metal tube or a ceramic/alumina tube, especially in a reducing atmosphere (presence of carbon dioxide). In an oxidizing atmosphere (e.g. electric furnace, presence of oxygen), protection is not necessary. Type K will generally last longer than type J, which will oxidize more quickly (especially at high temperatures). Recommended use of : -40 to 1200°C |
Type K is used between -40 and 1200°C. . For measuring temperatures between 315 and 600°C, we recommend type J or N, as instability in the structure of the internal alloys will cause a drift of around +2° after a few hours' use. Type K is also a good solution for nuclear environments. |
| N | Nicrosil (+) Nisil (-) |
Type N is mainly used at high temperatures up to 1250°C. It offers better resistance to high-temperature oxidation and longer service life in applications where sulfur is present. Like type K, it does not suffer from premature aging. Recommended use from : -40 to 1250°C |
Type N is used between 0 and 1250°C. It is very stable over time and does not drift at high temperatures. High-temperature applications require Pyrosil, Alloy TD or Nicrobel sheathing. Type N is particularly recommended for nuclear applications. |
| J | Iron (+) Constantan (-) |
Type J can be used, protected or unprotected, provided there is no oxygen present. Protection will prevent oxidation and extend life. As the positive conductor is made of iron, it will oxidize rapidly above 540°C. Simply using a larger-diameter conductor will extend its life. Recommended use from : -40 to 750°C. |
Type J is used between -40 and 750°C. Type J is stable between 0 and 538°C compared with types E and K (drift associated with ageing). This economical thermocouple is mainly available with stainless steel sheath. |
| E | Chromel (+) Constantan (-) |
Less commonly used, the E-type thermocouple is nevertheless a good choice for temperatures up to 900°C. Recommended for vacuum, inert gas, moderately oxidizing or reducing applications. For cryogenic applications, it is not subject to corrosion. Type E generates a higher voltage signal than all other thermocouple types. Recommended use from : -40 to 800°C. |
Type E is used between 0 and 900°C. For measurement temperatures between 315 and 600°C, type J or N is more suitable (drift associated with ageing). . |
| R | Pt 13% Rh (+) Pt (-) |
These thermocouples are designed for high temperatures. Recommended use of : Type S or R: 0 to 1600°C Type B: 0 to 1700°C. These thermocouples are easily contaminated, so they should not be used in a reducing atmosphere. These noble metal thermocouples should preferably be protected by a ceramic tube (alumina) or by an outer metal tube below 1200°C. |
Because of the high operating temperature of these thermocouples, the external metal sheath is often an obstacle to their use. Nevertheless, these products are available with Inconel or platinum sheaths (very costly), mainly for the S type. Recommended use of : Type S 0 to 1250°C. |
| S | Pt 10% Rh (+) Pt (-) |
||
| B | Pt 30% Rh (+) Pt 6% Rh (-) |
Precision class: what does the standard say?
| TYPE | Tolerance value | ||
| Class 1 | Class 2 | Class 3 | |
| T | ±0.5 or 0.004×T | ±1 or 0.075×T | ±1 or 0.015×T |
| -40...350°C | -40...350°C | -200...40°C | |
| E | ±1.5 or 0.004×T | ±2.5 or 0.075×T | ±2.5 or 0.015×T |
| -40...800°C | -40...900°C | -200...40°C | |
| J | ±1.5 or 0.004×T | ±2.5 or 0.075×T | ±2.5 or 0.015×T |
| -40...750°C | -40...750°C | ||
| K | ±1.5 or 0.004×T | ±2.5 or 0.075×T | ±2.5 or 0.015×T |
| -40...1000°C | -40...1200°C | -200...40°C | |
| N | ±1.5 or 0.004×T | ±2.5 or 0.075×T | ±2.5 or 0.015×T |
| -40...1000°C | -40...1200°C | -200...40°C | |
| R & S | ±1 for T<1100°C 1+0.003×(T-1100) for T>1100°C |
±1.5 or 0.0025×T | |
| 0...1600°C | 0...1600°C | ||
| B | ±1.5 or 0.0025×T | ±4 or 0.005×T | |
| 600 to 1700°C | 600 to 1700°C | ||
Temperature limits for sheaths and protective materials.
Thermocouples come in 3 main forms:
- Bare wires with rigid outer tube protection
- Sleeved version: in this case, the thermocouple wires are positioned in a flexible metal tube filled with a mineral insulator (magnesia, aluminum oxide....).
- Wired version: thermocouple wires are insulated with PVC, silicone, Teflon, glass silk or ceramic fiber.
Flexible wired versions with insulation :
- These versions are used to make single thermocouples or to make compensating or extension cables.
The cable used to build a wired sensor guarantees all the thermocouple's thermoelectric characteristics over the entire temperature range. - Extension cable: used to connect the thermocouple to the measuring instrument; its conductors have the same thermoelectric characteristics and properties as their respective thermocouples.
- Compensating cable: used to connect the thermocouple to the measuring instrument; its conductors have different characteristics from those of the thermocouples to which they are connected, although they retain the same thermoelectric properties.
| Temp. max. °C |
INSULATION | Use | |
| -20...105 | PVC | Good mechanical and electrical characteristics | |
| -40...200 | SILICONE | Excellent flexibility even at low temperatures | |
| -200...250 | TEFLON | Chemical resistance and excellent mechanical properties | |
| -200...400 | KAPTON | Excellent dielectric and chemical properties | |
| 400 | GLASS SILK | Good resistance to high temperatures - non-combustible but porous | |
| 1200 | CERAMIC FIBER | Very good resistance to high temperatures - non-combustible but porous |
Flexible jacketed versions with mineral insulation :
This version consists of a metal sheath containing conductors insulated from each other by highly pure, highly compressed metal oxides. This product offers many advantages: robustness, short reaction time, good insulation, high temperature.
| Type | Temp. max. °C |
Application | |
| 304SS | 800 | General-purpose stainless steel. Good corrosion resistance. |
|
| 316SS | 1050 | General-purpose stainless steel. Superior corrosion resistance. |
|
| 310SS | 1050 | Superior corrosion resistance. Highly resistant to high temperatures. |
|
| 446SS | 1100 | Used in sulfurous atmospheres. | |
| Inconel 600 | 1150 | Excellent resistance to oxidation and high-temperature corrosion. | |
| Pyrosil D | 1250 | Greater resistance to oxidation than 310SS & INC 600. More resistant to high temperatures than 310SS & INC 600. Coefficient of thermal elasticity compatible with nickel alloys (thermocouple type K and N) |
For this type of product, a minimum external diameter must also be respected, depending on the temperature of use:
| Type | Ø0.5 | Ø1 | Ø1.5 | Ø2 | Ø3 | Ø4.5 | Ø6 | Ø8 | ||||||
| 304SS | 700°C | 700°C | 800°C | 800°C | 800°C | 800°C | 800°C | 800°C | ||||||
| 310SS | 700°C | 700°C | 920°C | 920°C | 1050°C | 1050°C | 1050°C | 1050°C | ||||||
| 446SS | 700°C | 700°C | 920°C | 920°C | 1070°C | 1100°C | 1100°C | 1100°C | ||||||
| Inconel 600 | 700°C | 700°C | 920°C | 920°C | 1070°C | 1150°C | 1150°C | 1150°C | ||||||
Rigid versions with external protection tube :
These versions are particularly suitable for very high temperatures. The thermocouple is mounted under a ceramic bead with one or two protection tubes. A DIN head is often used for the electrical connection.
| Type | Temp. max. °C |
Application |
| 446 SS | 1100 | Excellent resistance in salt bath applications (heat treatment). Fairly resistant to aluminum electrolytic baths. |
| Cast iron | 870 | Inexpensive material used in liquid aluminum. The tube has a relatively short life due to oxidation, but other materials also have their drawbacks in this application. Fragile metal. |
| HR-1600 | 1204 | Exceptional resistance to various forms of corrosive attack at high temperatures. |
| Inconel 6011 | 1260 | Highly resistant to oxidation for processes with high to moderate oscillating temperatures. Resistant to sulfur compounds and carbon dioxides at moderate temperatures only. |
| Alumina (99%)1 | 1900 | Used in the glass and non-ferrous metal industries. Highly gas-tight, it provides excellent protection for noble metal thermocouples. |
| Hexoloy SA2 | 1650 | Incinerator, liquid aluminum and non-ferrous metals, hydrofluoric and sulfuric acids, bauxite calcination. |
| Mullite | 1700 | Used in the non-ferrous metals industry and in various industrial furnaces. Highly gas-tight, not recommended for noble metal thermocouples as it contains silica. |
| Syalon3 | 1150 | A strong material, highly resistant to thermal shock. Widely used in the aluminum industry, especially for scuppers. Rather expensive material. |
| Silicon carbide | 1650 | Excellent thermal shock resistance. High thermal conductivity. A porous material which, combined with an internal ceramic tube, provides good protection for noble metal thermocouples. |
Summary : How to select a thermocouple: standards, performance, accuracy and recommendations
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https://mesurex.fr/wp-content/uploads/2020/10/Guide-de-sélection-des-thermocoulpes-2.pdf
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