Brian

Just a quick paragraph or two from a book by George E Todd:

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The digital thermometer seen here (fancy one with coiled wire and probe sitting in sollution) is one of five temperature measuring devices in my darkroom. One is a murcery thermometer calibrated to 0.1* accuracy, while the electronic model differs from the latter by 0.4*C. Between the lowest and highest of all five is a range of 1.6 degrees. This reminds me of a saying attributed to Confucious: Man with one clock knows time; with two clocks not. My point is that, within reason, it doesn't matter, as long as you always use the same yardstick. Absolute values are irrelevant.
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I use a similar one to the R.H designs for everything, bought from Maplins. Recently, I introduced another one on the wall. Again, a digital one from Maplins, with a difference of 0.3*C from the first. Close enough and saves me dipping my chems every session.

Hope this helps

Regards

Stoo