Compare your cheap ones against a mercury / alcohol thermometer at mash temp. Whichever one is closest to the mercury / alcohol thermometer is the one I'd trust.
I bought a decent mercury thermometer - tested it in boiling and ice water and got the right temps. This is the thermometer I use to calibrate my other ones.
If you don't want to buy another thermometer, test your ones like I said above. Choose the one that's closest to being true. You can then take note of how much it is out and adjust your readings.
Permalink Reply by MrC on January 17, 2010 at 3:38pm
I use a cheap cappuccino thermometer that I got from Moore Wilsons. I calibrated it using boiling water and it works fine. To calibrate them you just hold the nut on the back of the thermometer head with a spanner and turn the head until it reads what it should.
no nuts on the back of mine! boiled water 2 read 95 and 1 read 99 however the 2 that read 95 read 22 and 27 respectively once the water had cooled and the 3rd only starts at 50c as its what the missus uses at work for making caramel,she reckons its crap and doesn't use it anymore but it was the only 1 accurate in boiling water!(i'm at 400 metres so water boils at 99)
i'll either buy or borrow a mercury thermometer if i can find one on trade me and check them in the 60-70 range and compensate accordingly.