Note that there are thermometers with different ranges, intended for different purposes! I have one covering 15 to 35 Centigrade with a very precise scale (intended for black-and-white standard temperatures) and another for colour work that goes to 50 Centigrade.
If the OP buys a thermometer randomly it might be sub-optimal for one purpose or the other, or perhaps he already has a perfectly good thermometer which doesn't measure up to 50C. The suggestion to use "hot" tap water is perhaps the most practical - and is probably what most of us do(?).
My understanding of the need for "hot" water is just to allow the powder to dissolve easier, I don't believe that the exact temperature is all that critical. I just use hot tap water, but my tap water is from the City of Ottawa and is supposed to be in the top ten for North American Cities
Charles MacDonald email@example.com
I still live just beyond the fringe in Stittsville
As a matter of good mixing practice, the temperature should not be higher than 50-60F. At higher temperatures, oxidation is accelerated, and at significantly higher temperatures certain developer components may be broken down altogether. One should never be without a thermometer when doing any mixing if chemicals. Even a very cheap thermometer is better than nothing.
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