Building an accurate tempering oven

[anthonyget]

Recently I have been having some really annoying problems during the hardening process of my blades. I achieve hardness after the quench, but then, following what I thought was careful tempering, the blades were coming out at around 45 to 50 Rockwell which is no good.
I finally checked with an oven thermometer and it seemed that although my oven reached 200C (for example) and held that temperature for a litle while, when I checked again after 15 or 20 minutes the temp at crept up to over 220C and sometimes even 230C which made the steel too soft. If I tried to dial the temp down by a tiny bit on the dial, it would go down as 175C. In other words, completely useful for accurate tempering.

I know I am far from being the first converting a mini oven for this purpose, but I hope a beginner might enjoy learning with me and maybe avoiding the mistakes I made.