Unfortunately we can't do nothing with the voltage. This variable voltage is coming to the building from supplier.
Hi me356. Your setup and experimental runs have been impressive. As far as the voltage problem you have, it sounds like you need a current source. That is, a power supply that provides a constant current as opposed to a constant voltage. Another way to think of it is to consider a PID controller driving its output based on current (amperage) measurement and comparing it with a current setpoint (instead of measuring temperature and comparing it with temperature setpoint).