Greisinger magnetic surface probe gmf 2x0 series greisinger gmf 250 the greisinger gmf 250 is a nicr ni type k magnetic surface probe for surface temperature measurements the gmf 250 is used exclusively for magnetic materials because of its self adhesive measuring sensor with cu plate the probe is not appropriate for use at induction furnaces etcthe thermocouple consists of two electric lines made of different materials nicr and ni the measuring principle of temperature measurement