I have calibrated both my deep fryer and oven with an industrial pt100 based system. Can highly recommend, both were quite far off. The oven about 20C too low, the fryer about 10 too high. Now the recipes actually work, this is highly important for someone who manages to burn water.
I tried the oven first with a thermocouple from my cheap multimeter, it claimed to be able to measure up to 500C or so, but the insulation of the thermocouple started to melt at about 250...