December 4, 2008 - Hot zone temperature measurement in a vacuum furnace typically is done using thermocouples located in the vicinity of the heating elements. Normally a minimum of two are used, one for furnace control and the other connected to an independent hot-zone power supply shutdown and alarm, for over-temperature control. In practice, the size of the hot zone may dictate multiple zone control, with multiple thermocouples for each function. An optical pyrometer may be used where process temperatures exceed normal thermocouple ranges, or in special situations such as noncontacting applications in a semicontinuous vacuum furnace. Hot zone control thermocouple signals in the millivolt range are generally transmitted to a process temperature controller-programmer, often connected in parallel with temperature recorders, data logging instruments, or computers.
Source : ASM International (http://asmcommunity.asminternational.org)
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