Temperature Sensing Devices
SECTION - 7
A thermocouple is
a temperature sensor. In its most common form it consists of
two wires of different composition. The two wires are joined
together at two points which have different temperatures.
One of points is at a known temperature. This point is the reference junction. The reference junction is also often, but less preferably, called the "cold" junction. The temperature of the reference junction is held constant, or its variation is electrically compensated for in the associated measuring instrumentation.
The second junction is the measuring junction. the measuring junction is also often, but less preferably, called the "hot" junction. The measuring junction is often at an unknown temperature at which control is required.
A thermocouple is useful for temperature sensing because a measurable electrical signal is produced. the signal is a function of the difference in temperature between the measuring and difference in temperature between the measuring and reference junctions. Numerous combinations of dissimilar metals are used as thermocouples. Some of these combinations have become relatively standard and widely accepted for a large segment of industrial temperature measurements. A specific combination is generally referred to as a type, or calibration. Most of the common calibrations have American National Standards Institute (ANSI) letter codes. These letter codes were
originally established by the Instrument Society of America.
The common calibrations are identified in Table 1 with application data for each.
The recommended temperature range is that or which limits of error are established. No guarantee is made, or implied, regarding the successful use of any of the above calibrations in their recommended range.
Use of a thermocouple outside its recommended temperature range may adversely affect its reliability over its recommended range.
Numerous factors combine to determine the successful application of a particular thermocouple. Some of these factors are
temperature, cycling, chemical exposure, degree of protection provided, and mechanical abuse given to the thermocouple.
The above mentioned thermocouple calibrations are maintained by proper manufacturing control of each of the thermo elements. Elemental constituents are controlled to a high degree. Homogeneity must be maintained, and all wire must be properly annealed.
To reduce costs when long thermocouple lengths are required, especially with the noble metal calibrations, extension lead wire extends the reference junction of the thermocouple to the instrument. For the base metal calibrations the extension wire is nominally of the same composition as the thermocouple grade material. Control in manufacturing is not to to the same degree as thermocouple grade wire. With lessening rigidity of manufacturing control considerable expense can be saved. There is a limitation on the maximum temperature to which the junction of extension wire and thermocouple wire should be exposed. For the base metal calibration except type T the maximum temperature is 204oC. For type T it is 93oC.
Nobel metal types R, S, B, Platinel, the tungsten-rhenium calibrations are used with "compensating alternate" extension wire, which means the extension wire is made of material differing in composition from the thermocouple wire, but at temperatures encountered at the thermocouple extension junction, has corresponding Temperature - EMF characteristics. The maximum temperature limitations for the thermocouple extension junction for calibration types.R, S, B, and Platinel is 204oC. For Tungsten/Tungsten-26% Rhenium (W/W-26% Re), Tungsten - 3% Rhenium/Tungsten-25% Rhenium (W-3% Re/W-25% Re) IT 260Oc. For W-5% Re/W-26% Re IT 871Oc. The reason for the limitation is that the thermocouple and extension wire junction is one of the materials of differing composition, and hence another thermocouple.
Whenever extension wire is used, precautions should be taken to insure a uniform temperature exists across both thermocouple and extension wire junctions. If there is sufficient temperature gradient between the temperature and extension wire junctions and the terminals at the instrument when copper extension wire is used. appreciable error may be produced.
Thermocouple extension wires should be installed in conduit whenever possible and the conduit should be well grounded. Never run other electrical wires in the same conduit with extension wires. Keep the extension wires at least a foot away from any a-c line.
Mention has been made to "limits of error" applicable to thermocouple calibrations. The limits of error pertains to the temperature deviation tolerance acceptable for the calibration. Limits of error are stated either in degrees or as a percentage of the temperature measured. Limits of error of the extension wire for the Tungsten-Rhenium calibration are presently given in millivolts. Two levels of limits of error are published for the common calibrations; Standard and Special. Special Limits of Error are generally one-half the magnitude of Standard Limits of Error except for calibration type E. Limits of error are additive. For example, when a thermocouple-extension wire junction for type J calibration exists the standard limits of error for the thermocouple wire below 277oC are ±2.2oC and for the extension wire below 205oC are ± 2.2oC. Combined standard limits of error are 4.4oC and deviation from NBS Temperature-EMF tables for this combination within standard limits of error could be from 0 to 4.4oC. Limits of error are only stated for common gauge sizes, and do not consider system errors.
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