Temperature uniformity within a furnace can be defined as: “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.”
Your eyes can be a valuable resource in estimating the furnace temperature. Practice observing the color of the furnace in relation to the generated signal/reading on the control instrument.
With practice, you can become very competent at estimating furnace temperature. It is not by the stretch of anyone’s imagination entirely accurate, but it can be simply used as a guide to come within 40-50°F (4.5-10°C) of the setpoint temperature.
I would now like to ask, “What does the thermocouple measure?” The immediate answer that comes to everyone’s mind is that it measures the temperature of the furnace. However, that is incorrect! The thermocouple can ONLY measure the temperature of the “measuring junction” of the thermocouple and nothing else.
Fig. 1. A simple schematic of the basic function of a thermocouple for heat-treatment temperature measurement
For those heat treaters who are not familiar with the requirements necessary for aerospace, I recommend starting with ARP 1962 and AMS 2750. The basic thermocouple and temperature-measuring device requirements will require a minimum of two thermocouples: one for temperature control and one for over-temperature control. The over-temperature control is simply to protect the furnace workload from an overshoot in temperature.
The temperature control that is desired, in reality, is at the component itself. When conducting a temperature uniformity survey, the minimum requirement to establish the degree of temperature variance within the process chamber is nine thermocouples.
Once you have defined the area, label the front door of the furnace with the dimensions and the locations of the accurate temperature uniform zone within the process chamber. All work then must be processed within that defined envelope.