Thermocouple Protection Tube

What sheath material is the best for my application?

In mild corrosive environments and general purpose applications, 304 SS and 316 SS are ussually the best choice when considering cost vs. Performance. Choose Alloy 600 over 304 SS or 316 SS when temperatures exceed 1650°C (899°C). When using acids at temperatures below 482°F (250°C) PFA coatings should be used.

Used the chart provided as aquick reference for determining the best sheath material for your environmental condition.

Common Metalic Sheath & Protection Tube

Material Operating Temp (C) Maximum Temp (C) Characteristic & Features
304 S. S. 900 1.000 Widely used as common protection tube against heat and corrosion but not recommendable for use in the presense of sulphus or reducing flame.
316 S. S 900 1.000 Contains Mo and excellent resistance to heat, acid and alkalis.
316L. S. S 900 1.000 Less carbon content than 316S.S. and has better resistance to grain boundary corrosion.
310S S. S 950 1.050 High Ni-Cr content and food high temperature strenght with resistance to oxidation at high temperatures.
Sandvik 253MA 1.050 1.150
  • High creept strength
  • Very good resistance to isothermal and above all, cyclic oxidation
  • Very good resistance to combustion gases
  • Very good resistance to carburisation
  • Good structural stability at high temperatures
  • Good weldability
Inconel 600 1.180 1.250 Exellent resistance to axidizing and reducing atmospheres at high temperatures. But sulphurous atmosphere should be avoided.
Hastellay B 800 1.100 Excellent resistance to heat and corrosion, especially to HCl and H2SO4

Oxl. 250
Red. 1.000

Superior corrosion resistance in cyrogenyc temperatures but at high temperatures easily oxidized and becomes brittle.
Coating SS316 + FEP 250 Suitable in concentrated 2HCl, H2SO4 and HNO3 and most of chemicals but depending on temperatures conditions


Non - Metallic Sheath & Protection Tube
Mullite (Phytagoras) PT2 1.400 1.500
  • High alumina ceramic.
  • Good resistance to thermal shock.
  • Recommendable for used in cal or oil burning and electric furnaces.
PT1 1.500 1.600
  • 60% Alumina - 40% Silica
  • Sintered alumina. Better than PT 2 slightly less thermal shock resistance.
  • Recommendable for use in heating furnace and regenerator, impervious.
Recrystallized Alumina (Alsin) PT0 1.600 1.800
  • 99.7% Alumina
  • Superior chemical stability and bettter that PT 1
  • Recommendable for use in molten steel, slag, impervious
Recrystallized Silicon Carbide Y 1 1.650 -
  • 99% Sic
  • Good resistance to acids and alkalis
  • Recommendable for the use in neutral atmosphere up to 1.500 and also in molen zine, aluminium, lead, copper, etc.
  • Porous
Nitride Bonded Silicon Carbide Y 4 1.550 1.750
  • 78% SiC + 3% SiO2 + 18% Si3N4 (Si2)N2)
  • Excellent perfomance similar to Y3 Sic but contains Si3N4
  • Most suitable for use in molten aluminium, reheating.
Silicon Nitride SNT (Si3N4)   1.350 -
  • Excellent thermals shoc resistance.
  • Less corrosion to acids and alkalis.
  • High hardness
  • Fairly good resistance against most of molten metals.
Hitachi sialon Sil 1.250 -
  • Good oxidation and thermal shock resistance.
  • Better corrosion resistance to molten metals, especially good for molten Aluminium bath than Silicon-Nitride. Durable to iron and steel up to 1.600°C