I need to measure the performance of some sensors with a strong temperature dependency. Later on, I will do some calibration for the entire temperature range, but for the moment I just want to do a test over 24 hours at a constant temperature.
So I have a styrofoam box and inside a heating element attached to a power supply. I set the power supply to a constant voltage, so that after a few hours it reaches a steady state about 55 degrees C.
Now, the problem is that even with the insulation and heating, there is an effect of the ambient temperature over the 24-hour cycle. It's only about 0.7 degrees C, but that's enough to screw up the results.
Of course, I could set up a PID controller for the temperature, but that introduces a few problems. First, it would be a pain to set up and tune, and second, I would be afraid of introducing oscillations which I was hoping to avoid by using a constant voltage power supply.
The other idea would be to try and improve the insulation or maybe introduce some thermal mass like rocks or those freezer packs with water inside. That would avoid the oscillations of an active PID controller, but would it be enough to suppress the daily temperature fluctuation?
It seems like a really stubborn problem for something so basic. Is there something I'm not seeing here? Is there some easy setup or principle I could exploit to get a decently constant temperature? I don't have to hit any specific target temperature range, just something that would settle at some temperature and not fluctuate over 24 hours.