Specification for Type N Thermocouple Wire (Withdrawn 2000)
1.1 This specification covers the requirements for bare, solid conductor, ISA Type N, base metal thermocouple wire, NiCrSi-NiSi. Nominal composition is 14.2% Cr, 1.4% Si, balance Ni for the positive leg (ISA Type NP) and 4.4 % Si, 0.1% Mg, balance Ni for the negative leg (ISA Type NN). These thermoelements, when fabricated and applied properly, are suitable for use either as bare wire thermocouples in oxidizing or inert atmosphere, or with proper care in metal sheathed, compacted-ceramic-insulated thermocouples.
1.2 The Type N alloys were developed to specifically overcome some of the drawbacks of the Type K thermoelements and are intended for generally the same applications. The particular alloy compositions selected are intended to greatly reduce the gradual and generally cumulative drift in thermal emf resulting from long term exposure in air, at high temperature (1000°C), to lengthen the wire oxidation life in air, and to reduce the reversible change in thermal emf on heating to 250 to 600°C related to atomic ordering of the positive leg of Type K thermocouple. These improvements are demonstrated by the ability of Type N thermoelements to meet the requirements fo 220.127.116.11, 18.104.22.168, and 22.214.171.124. No effort was made to eliminate other application problems such as sulfur attack, cold work effects on emf, selective oxidation, etc. The emf-temperature curve is different from that for Type K and requires its own extension wire, scales, chart paper, or instruments for proper use. For greater detail, see references in Section 2.
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