Single or multi-strand?
The choice is mainly determined by the application (e.g.. termination considerations and internal diameter of associated sheath). Generally, single strand wires are used for hot junctions, and multi-strand or thicker single strand for extensions of the thermocouple. The greater the effective conductor diameter, the lower the value of thermocouple loop resistance, an important consideration with long cable runs.
Performance Considerations When Connecting Thermocouples
Length of cable runs and loop resistance.
The resistivity of extension and compensating cables varies according to the different conductor metals; the limit to cable lengths which can be accommodated by measuring instruments therefore depends on both the thermocouple type and instrument specifications. A general rule for electronic instruments is that up to 100 Ohms loop cable resistance (i.e. total of both legs) will not result in measurement errors.
Unlike thermocouples, resistance thermometers do not require special cable and standard electrical wires with copper conductors should be used. The heavier the gauge of the conductors, the less the impact is on errors due to lead resistance effects as described. Typically 7/0.2mm or 14/0.2mm conductors are specified with insulation chosen to suit a particular application. With long cable runs, the cables may need to be screened and earthed at one end (at the instrument) to minimise noise pick-up (interference) on the measuring circuit.
|Which insulation Material
|Usable temperature range
|-10C to 105C
|Good general-purpose insulation for "light" environments. Waterproof and very flexible.
|-75C to 250/300C
|Resistant to oils, acids, other adverse agents and fluids. Good mechanical strength and flexibility.
|-75 to +200
|Excellent dielectric properties, chemically inert, tough at low temp.
|-75 to +260
|Excellent chemical stability, mechanical strength, and electrical properties.
|-50 to +200
|Offers excellent dielectric strength and flexibility. Operation over a wide temperature range and ease of silastic bonding are other outstanding characteristics of silicone rubber cable.
|Glass fiber (Varnished)
|-60C to 350/400C
|Good temperature range but will not prevent ingress of fluids. Fairly flexible but does not provide good mechanical protection.
|High-temperature glass fiber
|-60C to 700C
|Will withstand temperature up to 700C but will not prevent ingress of fluids. Fairly flexible, not good protection against physical disturbance.
|0C to 1000C
|Will withstand high temperature, up to 1000°C. Will not protect against fluids or physical disturbance.
|Glass fiber (Varnished) Stainless steel over braid
|-60C to 350/400C
|Good resistance to physical disturbance and high temperature (up to 400C). Will not prevent ingress of fluids.