The combination of CarTech NICROSIL/NISIL Alloys has been designated Type N by ASTM and ISA.

There are three major deficiencies with the Type K thermocouple used in oxidizing atmospheres, those being:

• n the temperature region of 500°C (932 °F) the positive thermoelement of the Type K couple is subject to short-range ordering causing a positive rapid change in the EMF.

• ue to the oxidation of alloying constituents, EMF changes can occur at high temperatures in oxidizing atmospheres.

• ailure (open circuit) of the thermoelements, normally the negative thermoelement, due to the oxidation of the wire at high temperatures.

Originally, the CarTech NICROSIL/NISIL alloys were developed to improve oxidation resistance; however, the current CarTech NICROSIL alloy with chromium content 14.5% eliminates the drift in the EMF found in the positive Type K thermoelements operating at approximately 500°C (932°F).

The increase of silicon plus the complete elimination of aluminum in the negative thermoelement CarTech NISIL, along with the addition of magnesium in both legs, reduces the drift in the EMF caused by preferential oxidation, and produces a better oxide film which slows down physical failure of the thermoelement. The magnesium addition enhances the formation of an oxide film which is resistant to spalling and progressive and/or selective oxidation.

Tests indicate longer life, better accuracy for longer time periods and/or faster temperature response because finer sizes of CarTech NICROSIL/NISIL alloys can be used for the Type N compared to Type K thermocouples.