Extension Wire (continued)
Noble 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 400F (204C). For Tungsten/Tungsten — 26% Rhenium (W/ W — 26% Re), Tungsten — 3% Rhenium/Tungsten — 25% Rhenium (W — 3% Re/W - 25% Re) it is 500F (260C). For W- 5% Re/W- 26% Re it is 1600F (871C). The reason for the temperature 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 AC line.
Limits of Error
Mention has been made to “limits of error” applicable to thermocouple calibrations. Limits of error pertain 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 calibrations 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 530F (277C) are 4F (2.2C) and for the extension wire below 400F are 4F. Combined standard limits of error are 8F (4.4C) and deviation from temperature-EMF tables for this combination within standard limits of error could be from 0 to Limits of error are only stated for common gauge sizes, and do not consider system errors.
Color Coding and Insulations
It is convenient to color code to identify the more common positive and negative legs of calibration. The negative thermoelement is identified by red, perhaps as a sleeving, paint, tag, or tracer in a layer of insulation.
Thermocouple and extension wire are available with either fibrous or plastic type insulation in “duplex construction.” The features of this construction are that the individual thermoele- ments are insulated and the pair of insulated thermoelements are combined under an outer layer of insulation. Color coding is often utilized with this method of insulating, and as follows:
Barber-Colman Company, Loves Park, IL.