Do you calibrate your themocouples? If so, how and how often? I'm not kevin or a chemist, but hey, this is the internet. I did do a reasonable amount of 'controls' work. A PID controller can easily do 0.1C, *if* the system it is controlling is correct. First off, maintaining 0.1C at 'steady state' with no changing outside parameters is pretty easy. Assuming the thermocouple that is responsive enough, and a heater that is powerful enough, "all" you need to do is characterize your system and plug in the P,I and D parameters. A controller might be able to learn this in a specific set-up. It is when you distrub the system, like dumping a thermal load suddenly in it, that it goes off. Now, to regain your footing, you'll have to pump heat into it. So, you will need a more (how much? hard to say) powerful heater. Of course, the heat is localized; to spread it around, you'll want to circulate that water; the more powerful the heater, the harder you will have to circulate. And your PID settings will have to be choosen to have the response you want to that. Of course, the typical water volume, thermal loss to the surroundings, and typical thermal load can all be guessed for most situations to within some kind of sane guess, so I'd bet it wouldn't be too hard to have a system that can not only maintain 0.1C, but recover to it quite quickly. Of course, it might cost ya. With a SousVideMagic and a stockpot (9L sideheater 400W) and a fountain pump, I have longtime stability within ±0.06°C, and disturbance with a 460g iceAKKU of -20°C causes a dip of about 0.5-1.0°C with a recovery time of 15-20min., depending on PID settings. When SousVideMagic is run in °C and not °F, PID-values from autotuning are quite acceptable.
