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See the WatlowTemperature SensingSolutions catalog forinformation on soft wire constructions, insulations,physical properties andtemperature ratings; information onmetallic overbraids; andinformation on UL Soft Wire and Cable Soft wire and cable use one or moreflexible dielectric materials to insulate each conductor and produce constructions by:•Duplexing with an additionalinsulation covering•Duplex extruding (as with lamp cord) •Twisted to keep the conductorstogether and add resistance to electromagnetic interferenceMany different dielectric materials areused as insulation, from lacquers to elastomer plastics to high temperature fibers. Each material or combinationof materials is used to achieve theobjective of preventing shunts andshort circuits for a given application.Constructions are made to withstand ambient operating temperature and electrical noise conditions; with physi- cal properties to withstand moisture, chemicals and abrasion.In addition to dielectric insulations, softwire can be supplied with a number ofmetallic overbraids to increase abra-sion resistance or provide additional electrical shielding.

W A T L O W

Application Guide catalog - 7347 Application Guide

The Art of TemperatureSensing

Wire Insulations

Due to a temperature sensor’s elec-trical nature it’s important to provide conductors with sufficient insulation to avoid shunts or short circuits. To accomplish this, thermocouple wire,thermocouple extension wire and RTDlead wire come in a variety of insula-tions and configurations. The two primary constructions are:•“Soft” wire and cable •Mineral insulated, metal-sheathed cableWire is generally understood to meana one-or two-conductor soft insulated construction. Cable can be a soft con-struction with more than two conduc-tors, excluding drain wires, or conduc-tors encased in a metal sheath.
Temperature Sensors See Watlow TemperatureSensing Solutions catalog for: • MI cable calibrations• MI cable sheath materials• MI cable insulations• Grounded and ungrounded junction MI cable sensors• Fabricating sensors with MI cable
® listed PLTCconstructions. Mineral Insulated, Metal-Sheathed Cable Commonly referred to as MI cable, thisconstruction is well suited for demanding applications where elevated tempera-tures or hostile environments makesoft insulated wire unsuitable.MI cable is most often made by placingcrushable mineral oxide insulatorsaround conductors and inserting theminto a metal tube. This tube is then drawn down to the desired outsidediameter and conductor gauge. Some materials that do not lend them-selves to being drawn, such as molyb-denum, are used in undrawn or “uncompacted” constructions.MI cable construction provides aprotective sheath around the conduc-tors, with the added advantage of acompacted insulation that improves thermal conductivity from the sheathto the conductors. Sheaths can bemade from a wide variety of malleable metals, so matching sheath materialto temperature range and chemical environment is relatively easy. Conductors are insulated from thesheath using a variety of mineral oxidedielectric materials. The most common material is magnesium oxide (MgO).Insulation is selected according to itsdielectric strength for the intended operating temperature and matched up with a sheath material selected to withstand the same temperature.MI cable has the advantage of beingfabricated into a complete sensorassembly, combining sensing ele-ment and protective sheath in one.
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