Dangers of manual mode furnace operation…
Your high temperature lab furnace system was supplied with a programmable temperature controller which allows you to input numerous programs with various segments such as “ramp”, “soak”, and “repeat”. You can then, for example, select program #1 for product #1 and walk away, free of the need to monitor the furnace.
But that same temperature controller can also be operated in manual mode. In manual mode, the output power is adjusted by the user. The controller continues to receive a signal from the control thermocouple indicating the furnace temperature, but there is no temperature control. The PID controller is therefore not automatically adjusting the output power based on the differences between the set point temperature and the process variable. That is, it knows what temperature the furnace is at, but it is powerless to call for a change.
One reason why you might want to put the controller in manual mode is to test for a signal going to the power controller as part of your efforts to diagnose why your furnace isn’t heating. Another might be to maintain furnace operation while a broken control thermocouple is being replaced, thus avoiding shutting down the furnace during a production run.
But beware: manual mode operation can result in the destruction of your furnace!! Suppose you are calling for 100% power, and suppose that you forget to return the temperature controller to program mode once you have completed the desired task. The control system will continue to send 100% power to the furnace elements – regardless of the furnace temperature – because the automatic, PID based adjustment function has been disabled. The elements may then continue to function long after your refractory has melted and your product is destroyed… a bad day (or days) with a lost production run (or runs) and the downtime and cost of repairing your furnace.
Use Manual Mode only when necessary and only with caution!