The copper wire on our Kapton insulated thermocouples used in oven validation is being pushed out of the end of the thermocouple containing the junction. Is this a defect?
No, this is caused by differential expansion of the copper wire relative to the expansion of the Constantan wire. Because of the high temperatures found in ovens, the degree of differential expansion eventually forces the copper wire out the point of least resistance in the wire which is the exposed junction end. This can be stopped by installing a metal tip over the end of the junction and wire insulation and potting it in place. This produces enough resistance to stop the protrusion of the copper wire. It also slows the oxidation of the copper wire which eventually destroys the thermocouple.
October 20, 2009 at 2:27 pm | Uncategorized | No comment