When the conductors and the thermocouple's wires are of the same nature, it is called extension cable, and when the conductors are of a different nature to that of the thermocouple's wires, a compensation cable.
The pyrometer is based on two effects:
- PELTIER effect, if we establish an electrical contact between two wires of dissimilar metals, an electromotive force (EMF) is created at their junction point.
- THOMSON effect, if there is a different temperature at each of the two ends of a conductor wire of uniform composition, an EMF is created.
The ideal thing would be for the cable concerned to be of the same material as the thermocouple (extension cable) but since the thermocouple is usually made of costly materials, another law of thermoelectricity is used to allow incorporation of a third metal into the circuit without causing the EMF to vary, as long as such a metal maintains the same temperature throughout its length.
Other cables are used with the particularity of giving in the same temperature zone of use, the same temperature-EMF (compensation cables) as the thermocouple.
Compensation cables should have the following qualities:
- Homogeneous conductors.
- High isolation resistance between the conductors and between them and the screen, if there is one.
- Best watertightness possible.
- Protection that best responds to conditions of use such as temperature, chemical action, mechanic resistance, etc.
- Rapid response speed.
Here below is a list of the mistakes that would cause a variation of tenths of grades and would inutilise the pyrometer:
- Substitution of the compensation cable for an ordinary copper one.
- Use of a type of compensation cable destinated to a different couple - Inversion of polarity.
- Inversión de polaridad.
- It is necessary to know and maintain constant the temperature of the comparison point in order to determine the temperature of the measurement point.
When you order a compensation cable you must remember the following:
- Thermocouple class.
- Maximum and minimum ambient temperature.
- Working conditions (humidity, carbohydrates, etc.)
- Cable's working conditions (traction, abrasion, mobile equipment, etc.)
- The scale of temperature needs to be measured to know if they correspond to the EMF- temperature lineality.