Thermocouple and Resistance Thermometer Temperature Measurement Design Guide

These sensors are often being used close to their physical limits. The nature of the materials used in these sensors leads to more limited design shapes, larger sizes, thicker sections and limited material selection. There are three basic design forms for these sensors. The main process resisting component is the important choice.

Ceramic Sheaths

Ceramic materlas reach the highest temperatures that are often seen in industry. Detailed properties of each ceramic can be found by clicking its name.

Material	Max Temp	Notes
Recrystalised Alumina (RA)	1750 °C	A pure and tough ceramic exhibiting hardness and vacuum tightness. Excellent in oxidising and reducing atmospheres. Fair thermal shock resistance. Usually used with R, S and B Elements.
Aluminous Porcelain (AP or IAP)	1500 °C	Good thermal shock resistance. Usually used with J, K, N and E conductors.
Sillimanite 60	1600 °C	Very good thermal shock resistance. Porous.
Silicon Carbide (Clay Bonded)	1400 °C	Excellent thermal shock resistance. Tends to have thick wall to improve strength. A porous material that usually has a lining refractory for Platinum sensors. High thermal conductivity. Not suitable for highly oxidising atmospheres.
Silicon Carbide (Recrystalised)	1600 °C	Excellent thermal shock resistance. A porous material that usually has a lining refractory for Platinum sensors. High thermal conductivity.
Syalon 101	1000 °C	A strong and thermal shock resisting material for use in molten metals with little wetting or dross build up. Good in molten aluminium or salt baths. Fairly expensive.
Quartz	1500 °C	Low coefficient of expansion, High thermal shock resistance.

Ceramic Sheath Forms

The sheaths are sometimes used alone, but usually have either a metal shank attached, or have a shank and head. This provides a full enclosure for a sensor insert and to allow a process connection to be used without risk of damage to the ceramic.

Sheaths are available in the following forms:

Ceramic Protection Sheath
Ceramic Protection Sheath with Shank
Ceramic Protection Sheath with Shank and Head
Recrystalised Alumina with Platinum (& Rhodium alloy)- Half Thimble
Recrystalised Alumina with Platinum (& Rhodium alloy)- Full Thimble

The sensor inserts for these sheaths can be found on the relevant data sheets.

Tough Metal Sheaths for High Temperatures

Some applications are at temperatures where special alloys can offer a useful sensor life, and provide a sensor more robust than a ceramic sheath. The tube used will usually be seamless and have a welded disc at the cold end. The sensor diameter will be typically 20 to 30 mm. As the sheath is weldable, simple process connections can be easily fixed in place.

Material	Max Temp	Notes
310 SS	1100 °C	Commonly called 25/20. Good oxidation resistance and resistant to sulphur and reducing atmospheres. High temperature sheathing. Carbon dioxide to 900 °C. Continuous operation in air up to 1150°C, without temperature variation. Not recommended for use continuously between 550 °C and 850 °C.
446 SS	1100 °C	Very good resistance to sulphurous gases and salts. Good resistance to oxidation in air. Good resistance to oil ash corrosion, molten copper, lead, tin. In sulphurous reducing atmosphere 446 performs better than austenitic steels, although Nitrogen concentrations can result in early failure. Hot gases containing hydrocarbons and carbon monoxide can cause carburisation, if Oxygen is present for an oxide layer to form 446 performs well, otherwise failure occurs quickly.
253 MA	1150 °C	Very good resistance to oxidation, combustion gases and carburisation.
353 MA	1175 °C	Very good resistance to oxidation, combustion gases and carburisation. Good resistance to nitriding gases. Excellent in petrochemical furnaces including cracked ammonia.
Alloy 600 (Inconel)	1500 °C	A strong oxidation resistant material at high temperatures particularly with cycling. Good resistance to carburisation. Resists Sulphur compounds and carbon dioxide to moderate temperatures only. Excellent neutral and alkaline salt bath resistance Good in many acid salt baths. Good for nitriding atmospheres.
Sanicro 31 HT	1100 °C	Performs well in salt baths for heat treatment, cyanide or neutral salt baths.
Alloy 800 HT (Incoloy)	1100 °C	Resistance to high temperature corrosion. Excellent oxidation and carburisation resistance. Reasonable sulphur resistance at moderate temperatures Alloy 800 and Alloy 800 H are very similar but they have lower creep rupture strength at temperature.
Kanthal AF, APM	1400 °C	High resistance to attack by sulphur. Resists carburisation at temperature. Very little scaling reduces furnace contamination. Better heat transfer than ceramic.
Kanthal Super	1700 °C	Less brittle than ceramics. Excellent in corrosive furnaces. Suppresses electromagnetic oscillations. Very Expensive.

High Temperature Metal Sheath Forms

These sheath options are available in the following forms:

Tough Metal Sheaths for chemical environments

The materials used for these sensors must withstand the chemicals and the pressure associated with Chemical Plants. The potential for accidents is high, this demands material certification, pressure testing and a range of other contractual requirements. The starting materials can be commercially available alloys, special melts and forged grades. Very often sheaths and sensors are built to customers own drawings.

Material	Notes
321 SS	Commonly called 18/8. Excellent corrosion resistance and remains ductile. Used for MI sheathing for type J and Pt 100's. Withstands hot crude oil products, steam and combustion gases. Carbon dioxide to 650 °C. Air up to 900 °C without temperature variation.
316 SS	Excellent corrosion and pitting resistance. Similar to 321 SS but with better acid resistance. Usually used in chemical plants and many other industrial applications.
Monel 400	Resists sea water, steam, Salt and caustic solutions, Used in heat exchangers Other Monel materials available.
Hastelloy C276	Corrosion resistance in reducing and oxidising atmospheres. Severe environments, Maintains corrosion resistance after welding.
Hastelloy B2	Significant resistance to reducing environments, and many acids.