d. The Chromel-alumel thermocouple has outstanding resistance to oxidation at
high temperatures and is particularly useful in the range from 1400 to 2200F. It
is peculiarly sensitive to contamination by reducing atmospheres, and its cost is
relatively high.
Extension lead wire of cheaper composition with similar
thermoelectric characteristics is available, but its use is not favored for precise
work.
e. Chromel-constantan has the highest thermoelectric power of any of the
common thermocouples.
It can be used up to about 2000F, and its stability and
reproducibility are excellent.
2. The EMF generated by a thermocouple does not depend on the diameter of the wire
as does its current handling ability.
Therefore, small, fine wire thermocouples
are ideally suited for the measurement of temperature in small spaces or when rapid
temperature changes require good dynamic characteristics. Time constraints of the
magnitude of 1 second are possible in gases at atmospheric pressure moving with a
moderate velocity. On many applications, however, an adequate couple life can be
achieved only with a heavier wire (for example, 14 gauge) and one or two protecting
tubes, both of which cause a reduction in speed of response.
3. At low temperatures (1000F and below), a single metallic tube is generally
adequate.
At high temperature, the thermocouple is often inserted in a primary
tube of porcelain or fused silica, and this is protected by a secondary tube of
metal, silicon carbide, or fire clay.
The construction is usually a compromise
between service life, dynamic characteristics, and errors due to conduction,
radiation, and other losses.
4. Thermocouple circuits have a reference junction (cold junction) from which
temperatures are measured.
Usually this junction is at 0C, or 32F; and in the
simplest circuits, the temperature is maintained by an ice and water bath. It is
more convenient, with instruments in continuous service, to maintain the reference
junction at a temperature slightly above the ambient temperature by a miniature
thermostatically controlled oven, and to zero the instrument, allowing for the
corresponding voltage. Other instruments allow the reference junction temperature
to vary with the ambient conditions within the voltage measuring instrument, and
provide a calibrated electrical or mechanical manual adjustment so the instrument
reading can be corrected for different ambient temperatures.
5. In recent years, most instruments that are used with only one type of
thermocouple have automatic reference-junction compensation, and their scale or
chart is calibrated directly in temperature units.
In the calibration program,
most instruments that use thermocouples are calibrated in electrical units. Figure
1-4 shows some typical circuit connections for thermocouples.
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