Since the open-circuit (Seebeck) voltage is a monotonic function of the temperature, thermocouples are an obvious choice as thermometers. When carefully calibrated, they will function with surprising accuracy over the temperature range from some 20 K to over 1700 K. However, to cover this full range, different units are required.
Because the Seebeck voltage is not a linear function of temperature. this voltage must be translated to temperature by means of look-up tables.
A very large number of combinations of materials have been considered for thermometry, but only a few have been standardized. Nine of these combinations are designated by identifying letters as shown in Table 5.1.
The first material in each pair in the table is the positive leg; the second is the negative leg. Composition is by weight.
Many of the alloys are better known by their commercial name:
55%Cu+45%Ni: Constantan, Cupron, Advance, ThermoKanthal JN.
90%Ni+10%Cr: Chromel, Tophel, ThermoKanthal KP, T-1. 99.5%Fe: ThermoKanthal JP.
95%Ni+2%Al+2%Mn+1%Si: Alumel, Nial, ThermoKanthal KN, T-2.
84%Ni+14%Cr+1.5%Si: Nicrosil. 95%Ni+4.5%Si+0.1 Mg: Nisil.
To identify a single-leg thermoelement, a suffix (P or N) is attached to the type letter to indicate the polarity of the material. Thus, for instance, EN—usually constantan—is the negative leg of Type E thermocouples. Materials must be selected taking into account a number of characteristics, including:
1. Stability. The properties of the material should not change significantly with their use. The materials must be chemically stable in the environment in which they are to operate. This is particularly true for high-temperature devices operating in an oxidizing atmosphere and is the reason for using noble metals instead of base ones.
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