The timer would be checked against the electric clock nearby or somebody's timepiece. Don't laugh. The reasons given is that the time for baking and the varying temperatures are estimates arrived at through years of performing this process.
Can an Auditor question the methods proposed with such varying baking times and temperatures? The time would be +/- 2 minutes and the temperature would be +/- 5 deg.
Take it easy on me.
Here is a TRACEABLE, FREE alternative, and you can do it yourself! First, go to
and download the NIST standard for stopwatches. Then, ftp to time-b.nist.gov/pub/daytime/ and get nistime-32bit.exe and download this handy little app that will set your computer clock to the NIST UTC timeserver (make sure you do it a few times to reduce the effects of 'net delay). Start your timer, run it 3 hours, stop it, repeat. Make sure you run the NISTime app before the stop and start for full traceability. There you go, no counters, only a computer connected to the net. Your uncertainty will be about 0.2 sec, with a tolerance (at least) ten times greater, so you'll be golden.
For your temp, I wouldn't use a bimetal, but there are some very reasonably priced digital thermometers (<$80) with a k-type thermocouple included. It is cheaper to have a digital thermometer calibrated, and they are much more accurate, so you're better off all around.