Page 144 - Mechanical Engineers' Handbook (Volume 2)

2 Thermocouples 133

Table 1 Thermoelectric Polarity Order of Metallic Materials
100 C 500 C 900 C
Antimony Chromel Chromel
Chromel Nichrome Nichrome
Iron Copper Silver
Nichrome Silver Gold
Copper Gold Iron
Silver Iron Pt 90 Rh 10
Platinum
Pt 90 Rh 10 Pt 90 Rh 10
Platinum Platinum Cobalt
Palladium Cobalt Alumel
Cobalt Palladium Nickel
Alumel Alumel Palladium
Nickel Nickel Constantan
Constantan Constantan
Copel Copel
Bismuth
Source: Reference 5.

each material in the listing is positive with respect to all beneath it. In an iron–palladium
thermocouple, for example, the cold end of the iron wire will be positive with respect to the
cold end of the palladium.
In some instances the operating temperatures of machinery elements have been measured
using the machine structure as part of the thermoelectric circuit (cutting-tool tip temperatures,
cam shaft/rocker arm contact temperatures, etc.). In such cases each material in the circuit
must be calibrated, and all intermediate temperatures must be measured in order to interpret
the signal.
The alloys usually used for thermoelectric temperature measurement are listed in Table
2. These have been developed over the years for the linearity, stability, and reproducibility
of their electromotive force (emf)–temperature characteristics and for their high-temperature
capability. Tables of thermocouple emf versus temperature [referenced to the international
1
practical temperature scale of 1990 (IPTS9O)] are available as DOS files on a disk from
the National Institute of Standards and Technology (NIST). The standardized letter-
designated thermocouple pairs are treated: B, F, J, K, N, R, S, and T. This is the primary
source for reliable thermocouple data.
The noble metal and refractory metal thermocouples are used generally with extension
wires of substitute materials, which are cheaper and easier to handle (more ductile). The
extension wires used are described in Table 3. Except for the substitute alloys, thermocouple
extension wire is of the same nominal composition as thermocouple wire and differs from
it mainly in the accuracy of its calibration and the type of insulation used. Extension wire
is not calibrated as accurately as thermocouple-grade wire.
Thermocouple material can be purchased as individual bare wires, as flexible, insulated
pairs of wires, or as mineral-insulated pairs swaged into stainless steel tubes for high-
temperature service. Prices range from a few cents to several dollars per foot, depending on
the wire and the insulation. There are many suppliers.

2.2 Peripheral Equipment
Any instrument capable of reading low-dc voltages (on the order of millivolts) with 5–10
V resolution will suffice for temperature measurements; the accuracy depends upon the