Temperature Uniformity By David Pye
Temperature uniformity within a furnace means; ‘a uniform temperature set to operate within a specific tolerance band to create conditions under which a final uniform resulting metallurgy will be accomplished in the treated component’.
Fig 1. A simple schematic sketch of the basic function of a thermocouple for heat treatment temperature measurement.
The writer now asks the question, ‘what does the thermocouple measure’? The immediate answer that comes to everyone’s mind, is ‘it measures the temperature of the furnace’. However, that is an incorrect answer!! The thermocouple can ONLY measure the temperature of the ‘Measuring Junction’ of the thermocouple and nothing else.
The basic thermocouple and temperature measuring device will require a minimum of two thermocouples, which are for Temperature Control and a safety thermocouple device for over temperature conditions. The over temperature is to protect the furnace work load from over shoot in temperature.
The temperature control that is desired. (in reality) is at the component itself/load. When conducting a temperature uniformity survey, the minimum requirement to establish the degree of temperature variance with the process chamber is 9 (nine).
Fig 3 Nine-point thermocouple temperature uniformity survey being prepared.
Once you have defined the area (within the process chamber), label the front door of the furnace with the dimension’s and location of the accurate temperature uniform zone within the process chamber. All work but then be processed within that defined envelop.
Conduct the temperature uniformity survey on the furnaces each week. Be sure to remember to conduct the temperature survey any timer that a repair has been completed on the furnace such (to name a few);
- Door seal replacement.
- Element repair (or replacement of a gas burner)
- Refractory repairs
- Thermocouple replacement
- Air circulation system repairs
Remember also the place the thermocouple as close to the work piece or load without the thermocouple interfering with the lading and unloading of the load or individual component.
Fig 4 Vacuum furnace temperature uniformity survey.
Remember also to be sure that you replace a thermocouple with the same type of thermocouple as the one that is being replaced!