Why use Calorised Emitter tubes?

Why do SPACE-RAY use Calorised Emitter tubes? The comparison tables below show the relative merits of emitter tube materials.

• Calorising gives protection to ferrous metal by providing a self-forming, self-healing coating of practical infusible material (alumina) which is impervious to oxygen, carbon, sulphur and the products of combustion of all common fuels.
• Various grades of the process are available according to the article and the use for which it is intended up to maximum metal temperature of 950 degrees centigrade for calorised aluminised mild steel or cast steel.
• The process is accomplished by fusing aluminium into the surface of the base metal with which it forms a homogenous alloy.
• The protective action is due to the aluminium oxide formed on the surface by the action of heat.
• Under working conditions the exposed surface of the alloy is converted to alumina which has a fusion point in excess of 1,000 degrees centigrade.
• Due to the protection given to the base metal, calorised aluminised mild steel has improved resistance to deformation and scaling at elevated temperatures.
• It is particularly useful for parts subjected in service, at elevated temperature, to sulphurous gases and for case-hardening boxes and pots, cyanide, salt and lead baths. Also for steel protection tubes for thermo-couples.