First Measurement

Here's a plot of the first measurement I made, and it shows a lot. The power was set to $1/64$ of the possible output, so $P = 12.5~\mathrm{W}$:

The atmosphere in the oven takes some time to reach its final value. In this case I decided that the steady state was reached after about 100 minutes. This is far too long for soldering, but the oven doesn't need to have reached a steady state at all times (that would be impossible anyway). The overall shape of the response to the step input looks like that of a second order system. I'll have too see how well I can match it with a simulation.

The measured temperature shows small fluctuations at about $1600~\mathrm{s}$ and $4000~\mathrm{s}$. These are not caused by any thermodynamic process in the oven, but by the cold junction compensation. The DS1621 has a resolution of $0.5~\mathrm{K}$ and the output jumps back and forth while the PCB heats up a bit (the enclosure is heated up by the LCD's backlight).  As this jumping value is added to the thermocouple's output, it it shows up as jumps in the measured temperature. I'll need some filtering and a better sensor next time.

I've done some more measurements with higher power settings and will add them up in another plot. With about $1/3$ power the temperature rises to about $250~\mathrm{^\circ C}$, so there's plenty of room for quickly heating it up for the reflow phase.