Special Purpose Thermocouples & Thermowells, Manufacturer of Thermocouple, Thermocouple Type, Thermocouple Assemblies, Resistance Temperature Detectors, India

Nutech Engineers
143-D Government Industrial Estate, Kandivli West, Mumbai - 400 067

Phone : 91-22-28683549 / 28684580,  Fax : 91-22-28674580
email : nutechen@vsnl.com , nutechen@bol.net.in,

Temperature Sensing Devices

Technical Information

Protection Tube Selection

Selection Information
A Thermocouple should have as little protection as possible, consistent with the length of service consideration.  Sensitivity will be reduced and slower response to temperature changes results from very heavy tubes.

To insure longer life with unimpaired accuracy, thermocouples must be adequately protected.  Corrosion, contamination, and abrasion all shorten the life of bare thermocouples.  Protecting tubes are capable of withstanding these actions.

For continuous duty the temperatures shown are the maximum recommended.  For intermittent duty tubes may be worked at slightly higher temperatures than shown, but total length of life may be less.

Thermocouple protecting tubes are available in a wide range  of  types  and  sizes.  The  tubes  listed  here  are

those which have been in greater demand and many of these are stocked for last delivery.  Other tubes, for special purposes, are available.

Note: The following comments, temperature ratings, and application information are offered as a general guide only.  This information is not to imply a guarantee of adequate or successful use of any of the listed materials in any specific application.  Numerous variables such as impurities, concentrations, vibration, shock, temperature cycling, abrasion, etc., greatly influence the satisfactory life experienced from a protection tube material.

The length of service life considered satisfactory in one application may be unsatisfactory in another.  Testing of selected materials should be performed In the application for final determination of a suitable material.

Tube Specifications and Applications

Typical Applications Tube Description Composition or
General Comments
Food Preparation
Petroleum Industry
Chemical Processes
Mixed Acids
Lactic Acid
Dyeing Tanks
304 Stainless Steel 10% Nickel
19% Chromium
0.08 Carbon Max
2% Manganese max.
1% Silicon Max., traces of Phosphorous & Sulfur, balance Iron
871oC Oxidizing
1260oC Non Oxidizing
Good resistance to corrosion.  For wet process applications such as steam, oil and many chemical solutions.
Petroleum Industry,
Chemical Process
316 Stainless Steel 12% Nickel
17% Chromium
2-1/2% Molybdenum,
2% Manganese max.,
0.08% Carbon max.,
1% Silicon, max., traces of Phosphorous & Sulfur, balance Iron
871oC Oxidizing Good resistance to corrosion. Resists pitting corrosion.  More resistant to acids than 304 SS.
High Temperature Hardening
Salt Baths
Vitreous Enameling
Non-ferrous Metals such as Tin, Lead, or Babbit Metal Smelting
Low Temperature Blast Furnaces
446 Stainless Steel 27% Chromium
0.25% Nitrogen max.,
0.20% Carbon max.,
1.50% Manganese max.,
1.00% Silicon max., traces of Phosphorous & Sulfur, balance Iron
1094oC Oxidizing
1260o Non Oxidizing
Good resistance to corrosion at high temperatures.  Impervious to sulphurous atmospheres, salt bath or low temperature molten metals.
High Temperature Heat Treating
Salt Baths
Blast Furnace Operations
Gas Generators
Ceramic Kilns.
Inconel* 601 61% Nickel
23% Chromium,
14% Iron
1.35% Aluminium
1260oC Oxidizing Excellent resistance to corrosion and oxidation at high temperature.  Good mechanical strength.  More resistant to sulphur than Inconel 600.  Hydrogen causes embrittlement.
Potassium Cyanide Salt Baths 1094oC
Caustics and Brines
Pure Nickel Drawn or Drilled
99.5% Nickel
1204oC Oxidizing 538cC Reducing 1316oC Neutral For High temperature applications. will withstand many chemical actions, but must not be placed in the presense of sulphur.  Frequently placed in caustic and molten salt baths.  Drilled tube recommended for hydrogen atmospheres.
Neutral Salt Baths Chrome Plated Steel Chrome Plating over Steel 848oC Can be used in Cyanide Salt Baths and in presence of Sulphur.
Molten Non-ferrous Metals
(Babbit, magnesium, tin, etc.)
Water Lines
Petroleum Applications
SAE 1020 or
SAE 1026
(Black Steel)
0.17% Carbon
0.75% Manganese
0.035 Phosphorous Max.
0.045 Sulfur, Max.,
balance Iron
537oC Oxidizing
1093oC Non Oxidizing
Economical for low temperature applications.  Scales easily at higher temperature.  For non corrosive atmospheres.
Glass Lehrs

Power Plant Pre heaters
Food Baking Ovens
Asphalt Mixers
Low Alloy Steel 0.14% Carbon
0.48% Manganese
0.16% Silicon
0.98% Copper,

traces of Phosphorous & Sulfur, balance Iron

677oC For non-corrosive atmospheres and in low temperature molten metals

This is a replacement for wrought iron.

Similar applications as Inconel, but less well suited to nitriding and molten caustics Incoloy 800 33% Nickel
21% Chromium
46% Iron
982oC Oxidizing 649oC Reducing.
1204oC Neutral
Oxidation resistance similar to Inconel, but has superior resistance to sulphur attach.  Incoloy is a trade name of International Nickel Company.
Chemical Industry
Molten Aluminium
Die Cast Metals
Cast Iron Cast 704oC Oxidizing
1093oC Non Oxidizing
To 871oC in reducing atmospheres.  Will withstand sulphuric acid and caustic solutions.  For extra long life, process coated tubes are available.  Cast Iron tubes should be painted daily with shiting when measuring aluminum or die cast metal temperatures .01 tubes have a special coating to extend tube life in some operating conditions.
High temperature heat treating:
Molten copper base alloys to 1150oC, Blast furnace and stack gases to 1316oC,
Sulphur burners to 1093oC
Cement kilns to 1204oC
Chemical process reactors to 1371oC
Metal-Ceramic Tubes (Slip cast composite of Chromium and Aluminum Oxide)
77% Chromium
23% Aluminum Oxide
1371oC Superior oxidation resistance to 1371oC.  Thermal conductivity equal to that of stainless steel.  Good resistance to most molten metals to 1149oC.  Not suitable in molten aluminum.

With nobel metal element, a ceramic primary tube is required.

Ceramic Industry
Bright Annealing

Forging Furnaces
Glass Making
High Speed Salt Baths

High Temperature
63% Al2O3
1510oC Impervious to gases at high temperature.  Possesses good thermal shock but poor mechanical shock.  Often necessary to provide secondary tube protection. Should be mounted vertically.  Usable in Barium Chloride salt baths to 1288oC
Induction meting up to 1760oC
Applications for metal and ceramic
industry requiring extreme temperatures.
Aluminum-Oxide Molded, 99-1/2%
1760oC Fair resistance to thermal and mechanical shock.  For very high temperature processes.  impervious to gases up to 1760oC
Brick and Ceramic Kilns
Steel Soaking Pits
Applications requiring resistance
to cutting action of flames and gases
Silicon Carbide 90% Silicon Carbide
9% Silicon Dioxide,
balance Aluminum
Oxide & Ferric Oxide
1649oC For molten non-ferrous metals. Also is a secondary protecting tube for resistance to thermal shock.
Silicon Carbide
Reinforced with Steel Pipe
Silicon Carbide
Outer Tube Low
Carbon Steel
Reinforcing Pipe
843oC High Heat Transfer.  Superior resistance to wetting by molten aluminum.

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