Interesting thread, and your comment here Roger. On this side of the pond, the ideal would be to obtain a thermometer certified by the NBS (National Bureau of Standards) and compare all others to it - the master/slave thing. However, not sure we need that extreme in our darkrooms (the NBS thermometers, like everything else are quite pricey).

Like most things darkroom, we can cater to the analytical side and spend all of our time checking, calibarting, etc. NOTE:I am as bad about this as anyone, so I do not think it is a bad thing. On the other hand we should consider what we can do - in this case trying to match at least 2 "good" thermometers and keeping one as the master, then always match to it should work very well.

One question, those that use the digital thermometers, have you noticed any changes in readings when the battery starts to go? I would think that at some point the readings would become "off" just prior to the battery failure.

Quote Originally Posted by Roger Hicks
What surprises me is that no-one has suggested the 'master/slave' model.

The master thermometer does not even need to be accurate, merely consistent, though accuracy is nice if you can get it. Mercury-in-glass lab thermometers (incliding the Kodak ones) are probably the most reliable; bimetallic dial drift most in my experience; and electronics may do almost anything without warning, no matter how reliable. Most don't, it's true, but they can.

All 'slaves' are calibrated to the 'master'. Let's say you have three thermometers, one 'master' and two 'slaves'. When the 'master' reads 20, one slave reads 19,5, the other, 20,5. For a standard 20, you therefore use the slaves accordingly.

Break a slave; replace it; recalibrate against the master. This one's 19.7. Fine. Still the same temperature...

My 'master' is a mercury-in-glass Brannan that agrees with another, identical, Brannan to 0.1 C. My 'slaves' vary widely for different purposes: spirit-in-glass, dial (for water baths), digital... Digital and dial are checked periodically; spirit-in-glass only when I acquire them.


Roger (