Protection Tube Type Thermocouples/Coated Thermocouples
The protection tube type thermocouple is a conventional thermocouple, whose structure is to protect the element of a thermocouple from the atmosphere of measuring targets. Currently, sheathed type thermocouples are used in most cases; however, for platinum based noble metal thermocouples, the protection tube type is used in most cases since, with the sheath type, manufacturing is difficult and there is a limitation on costs.
- Principle of Thermocouples
- Structure of Thermocouples
- Types of Thermocouples
Tolerance of Thermocouples and Standards Applied by
Material of Metal Protection Tube and Standard
Material of Non-metal Protection Tubes and
- Limit on Temperature Usable by Element Diameter
- Basic Model
Principle of Thermocouples
A thermocouple consists of two metal wires whose types differ from each other with both ends connected, and when a temperature difference occurs at the contacts of both ends, thermo-electromotive force occurs, and current flows within this closed circuit. The amount and the polarity of this thermo-electromotive force depend on the temperature of both ends and the combination of two metal wires, and are not influenced by the thickness and length of the metal wires. Therefore, if you know the thermo-electromotive force at each temperature of a specific thermocouple beforehand, you can measure the temperature.
As for thermocouples, appropriate elements must be selected according to the temperature measurement range, the state of the measuring place, necessary accuracy, etc., and thermocouples must always have the same performance even after long time continuous operation. We manufacture types B, R, S, N, K, E, J and T thermocouples of JIS standards and W-5Re/W-26Re thermocouples also. Please select thermocouples according to your purposes of the use for adoption.
Structure of Thermocouples
A thermocouple consists of a thermocouple element, protection tube, terminal box, insulator, necessary mounting bracket, etc. for practicality.
The following thermocouple element types exist. The tip of the element is made into a temperature measuring junction by welding.
The protection tube protects a thermocouple element and insulator from the ambient environment, and a mounting bracket and terminal box, etc. are attached to it. Very severe use conditions are applied to the protection tube depending on the places where temperature is measured. Therefore, for the protection tube, materials and forms suitable for use temperature, atmosphere, purposes, etc., need to be selected.
３．Terminal Boxes and Terminal Plates
A terminal box connects compensating cables to a thermocouple. We have drip-proof type EL terminal boxes (made of aluminum die-casting) and other types of materials and forms suitable for the purposes of use.
Insulators are used to provide insulation for thermocouple wires and protection tubes and to prevent short-circuits. We use JIS 2 type insulators.
A mounting bracket is attached to a protection tube to install the thermocouple onto a measuring place.
We have types of thermocouples which are used without protection tubes in addition to those above. Thermocouples optimum for simple measurement are available with only a heat contact attached to the tip of the coated thermocouple wires such as types EXE and EXS thermocouples. Since, especially, type EXS thermocouples are coated with Teflon resin such as FEP, an insulation type is available, formed with the heat contact part made of the same material.
Types of Thermocouples JIS C1602
They are the thermocouples in combination of a positive leg made of platinum (70%) rhodium (30%) alloy and a negative leg that is composed of platinum (94%) rhodium (6%) alloy. They have better heat-resistance and mechanical strength than type R thermocouples, and the heat-resistant temperature is up to 1,800℃.
Thermocouples in combination of a positive leg made of platinum (87%) rhodium (13%) alloy and a negative leg that is composed of pure platinum. They generally have good accuracy with excellent heat resistance and stability in an oxidizing atmosphere. They have a tendency to be weak in a reducing atmosphere or against metal vapor.
Thermocouples in combination of a positive leg made of platinum (90%) rhodium (10%) alloy and a negative leg that is composed of pure platinum.
They are called Nicrosil (positive leg) Nisil (negative leg), and their composition and characteristics are very similar to type K thermocouples. They are an improved version of type K thermocouples, and the added amount of Si is larger and the heat resistance is better.
They are the thermocouples in combination of a positive leg made of alloy of mainly nickel and chrome and a negative leg that is composed of an alloy of mainly nickel. They are used the most frequently for industrial purposes, and they show great resistance in an oxidizing atmosphere; however, they are weak in a reducing atmosphere and cannot be used in the atmospheres characterized by carbon monoxide, sulfurous acid gas, hydrogen sulfide, etc.
They are the thermocouples in combination of a positive leg made of type K thermocouples and a negative leg that is composed of type J thermocouples. They have extremely large thermo-electromotive force for temperature and are suitable to be used in an oxidizing atmosphere.
They are the thermocouples in combination of a positive leg made of iron and a negative leg that is composed of an alloy of mainly copper and nickel. They are strong in a reducing atmosphere and robust for hydrogen and carbon. They cannot be used in an oxidizing atmosphere since iron si quickly oxidized. They are relatively cheap so that they are suitable for medium temperatures.
They are thermocouples in combination of a positive leg made of copper and a negative leg that is composed of an alloy of mainly copper and nickel. They show high accuracy at 300℃ or lower and are especially suitable for low temperatures of -200 to 100℃. They are also suitable for a weak oxidizing and reducing atmosphere.
List of Thermocouple Tolerance and Standards Applied by Each Country
|JIS C1602||IEC 60584-2||ASTM E230|
|B||600℃ or higher and lower than 1,700℃||2||±0.0025|t|||2||±0.0025|t|||870℃ or higher and lower than 1,700℃||STD.||±0.5％|
|600℃ or higher and lower than 800℃||3||±4||3||±4|
|800℃ or higher and lower than 1,700℃||±0.005|t|||±0.005|t||
|R&S||0℃ or higher and lower than 1,100℃||1||±1||1||±1||0℃ or higher and lower than 1,450℃||STD.||±1.5or
|0℃ or higher and lower than 600℃||2||±1.5||2||±1.5||SP.||±0.6or
|600℃ or higher and lower than 1,600℃||±0.0025|t|||±0.0025|t||
|N&K||-40℃ or higher and lower than +375℃||1||±1.5||1||±1.5||0℃ or higher and lower than +1,260℃||STD.||±2.2or
|+375℃ or higher and lower than +1,000℃||±0.004|t|||±0.004|t||
|-40℃ or higher and lower than +333℃||2||±2.5||2||±2.5||SP.||±1.1or
|+333℃ or higher and lower than +1,200℃||±0.0075|t|||±0.0075|t||
|-167℃ or higher and lower than +40℃||3||±2.5||3||±2.5||-200℃ or higher and lower than 0℃||STD.||±2.2or
|-200℃ or higher and lower than -167℃||±0.015|t|||±0.015|t||
|E||-40℃ or higher and lower than +375℃||1||±1.5||1||±1.5||0℃ or higher and lower than +870℃||STD.||±1.7or
|+375℃ or higher and lower than +800℃||±0.004|t|||±0.004|t||
|-40℃ or higher and lower than +333℃||2||±2.5||2||±2.5||SP.||±1or
|+333℃ or higher and lower than +900℃||±0.0075|t|||±0.0075|t||
|-167℃ or higher and lower than +40℃||3||±2.5||3||±2.5||-200℃ or higher and lower than 0℃||STD.||±1.7or
|-200℃ or higher and lower than -167℃||±0.015|t|||±0.015|t||
|J||-40℃ or higher and lower than +375℃||1||±1.5||1||±1.5||0℃ or higher and lower than +760℃||STD.||±2.2or
|+375℃ or higher and lower than +750℃||±0.004|t|||±0.004|t||
|-40℃ or higher and lower than +333℃||2||±2.5||2||±2.5||SP.||±1.1or
|+333℃ or higher and lower than +750℃||±0.0075|t|||±0.0075|t||
|T||-40℃ or higher and lower than +125℃||1||±0.5||1||±0.5||0℃ or higher and lower than +370℃||STD.||±1or
|+125℃ or higher and lower than +350℃||±0.004|t|||±0.004|t||
|-40℃ or higher and lower than +133℃||2||±1.0||2||±1.0||SP.||±0.5or
|+133℃ or higher and lower than +350℃||±0.0075|t|||±0.0075|t||
|-67℃ or higher and lower than +40℃||3||±1.0||3||±1.0||-200℃ or higher and lower than 0℃||STD.||±1or
|-200℃ or higher and lower than -67℃||±0.015|t|||±0.015|t||
The tolerance is the maximum allowable limit calculated by the value of the temperature obtained from the standard thermo-electromotive force table converted from thermo-electromotive force subtracted by the temperature at the temperature measuring junction.
The tolerance of ASTM is a larger value of either ℃ or the % of the measured temperature.
|t| is the measured temperature indicated with the temperature (℃) unrelated to + and - signs.
Classes 1, 2, and 3 correspond to Classes 0.4, 0.75, and 1.5 of the old JIS.
JIS, BS, and DIN Standards are the same as the IEC Standard.
ASTM Standard is the old ANSI Standard.
The standard year is applied to the latest version.
Materials and Standard Dimensions of Metal Protection Tubes
JIS code is in ( ).
|Dimension (φ/mm)||Max. length
|Outer diameter||Inner diameter|
|A||10||7||2000||900||Excellent at heat/acid/alkali resistance. Weak against sulfur and reducing gas.|
|C||10||7||2000||900||Heat/acid/alkali resistance is the same as the resistance of 304SS; however, corrosion resistance is excellent at high temperatures.|
|CL||10||7||2000||900||The amount of C is less than that in 316SS, and the material has grain-boundary corrosion resistance.|
|D||15||11||3950||1000||It is heat-resistant steel with a high content rate of Ni-Cr and great at oxidizing atmospheres at high temperature.|
|SUH446||P||21.7||16.1||3950||1000||It is heat-resistant with 27Cr steel and strong against reducing flame and sulfur gas.|
|UMCo50||U||22||16||3950||1150||It is a cobalt-base alloy, which has strong heat-resistance/wear-resistance and is also strong for sulfur gas.|
|B||22||16||3950||1050||It is strong in both a high temperature oxidizing and reducing atmosphere.|
|TITANIUM||T||15||11||3950||250||Corrosion resistance is extremely excellent at low temperatures; however, the material becomes fragile with oxidation at high temperatures.|
Material and Standard Dimensions of Non-metal Protection Tubes
|Types||Material code||Dimension（φmm）||Max. length
|Common use temperature
|Alumina quality||P1||8||5||1000||1500||It is excellently stable in oxidizing/reducing atmospheres at a high temperature equivalent to JIS R1401 PT1 and excellent in corrosion resistance at a high temperature. It is suitable for measuring temperature within a furnace of 1,600℃ or higher and molten metal.|
|High-alumina quality||P0||8||5||1000||1600||It has excellent heat resistance/corrosion resistance/electric insulation/mechanical strength equivalent to JIS R1401 PT0. It is excellent in wear resistance because of high hardness.|
|Silicon carbide Refractory||GK||20||12||1000||1600||It has high refactoriness and a large heat conduction rate. It is not easily affected by zinc, aluminum, lead, acids and alkalis. It is robust for rapid heating/cooling and excellent in spalling resistance. It is suitable for outer tubes for doubled protection tubes.|
※1The common use temperature limits were described referring to JIS and manufacturer’s catalog; therefore, they are not guaranteed values.
Use Temperature Limit for Element Diameter
|Thermocouple types||Element Diameters||Use Temperature Limit（℃）||Applied Protection Tube Dimension|
|Symbols||Outer Diameter||Normal Operating Temperature Limit||Overheated Operating Temperature Limit||Metal Protection Tubes（Φmm）||Non-metal Protection Tubes（Φmm）|
EXE: Coated Thermocouples
They are glass coated thermocouples suitable for simplest measurement. We have ones with the element diameters of φ0.32 and 0.65 in our standard stock.
EXS: Coated Thermocouples
They are FEP (Perfluoro ethylene propylene copolymer) coated thermocouples suitable for simple measurement. We have ones with the element diameters of φ0.32 and 0.65 in our standard stock. Also, we have thermocouples with an extremely thin element diameter of φ0.06 and φ0.1 coated with PFA insulator. Furthermore, we can manufacture an insulation type with a tip molded with the same material.
TC: Thermocouples with Insulators
They are the most basic model which can be used at a high temperature because of insulation with a (ceramic) insulator. They are limited to indoor usage. It is used for simple measurement or as an element for replacement for TCW and TCG.
TCW: Thermocouples with Outdoor Type Metal Protection Tubes
This type has a drip-proof type terminal box attached to a metal protection tube, suitable for usage in an outdoor general environment. A thermocouple with an insulator is combined.
TCP: Thermocouple with Outdoor Type Non-metal Protection Tubes
This type has a drip-proof type terminal box attached to a non-metal protection tube, suitable for usage in an outdoor general environment. It is used with types R and S for high temperature.
TCG: Thermocouples with Explosion/Flame Proof Type Metal ProtectionTubes
This type is suitable for usage in dangerous places where explosive gas may occur such as petroleum refining/petrochemical plants.