Originally Posted by jeffeb
If your liquid-in-glass (LIG) thermometer is not calibrated or certified (to NIST or ASTM), then you cannot know its accuracy. Because you do not know its accuracy, how do you know which one is reading correctly? Hypothetically, if your thermometers were only accurate to +/-1 deg C, then a difference of 2 deg C would still be within specification, because (24+1)=(26-1).
Your scale divisions will most likely be every 0.5 degrees (or 0.2 or 1.0). This scale solely determines the thermometer's resolution, not its accuracy.
Your thermometer will also be the partial immersion type which is inherently less accurate than a full immersion type.
Other folks have suggested purchasing the RH Designs digital thermometer http://www.rhdesigns.co.uk/darkroom/...ermometer.html. I can't see what the advantage of this is, because its quoted accuracy is still only
+/-1 degC over the temp. range of darkroom chemicals.
Back in my test lab days, we would check the accuracy of thermometers by first measuring the temp. of ice water ( water and ice in a beaker) then we would measure the temp. of water in a steaming state.
I now use a digital oven thermometer that is accurate to within one degree. Cost $20, at the local Walmart.
Think outside the box
RJS has made an important observation. Years ago when boats floated away more than a few of my dollars, I learned that it was not necessary to buy marine grade parts at a boat store, paying a premium. There are quite a few darkroom items that fit this bill also.
I have a conventional dial Beseler thermometer, and it is used inline to check water line temperature. For solutions, etcetera, I use a Polder digital oven thermometer. Reads in tenths, has an alarm to warn when a certain temperature has been reached, and a repeating timer as well. Its calibration was tested against a Beckman Ph/Temp rig and was within slightly one-quarter of a degree of dead on all along the range. Price: under $30 at places like bed, bath, and beyond.
For quicky measurements, one of the little digital cooking thermometers works well, and are usually within one-half a degree accuracy. Price: under $10 at most grocery stores and big box retailers.
When you come to a fork in the road, take it...
I picked up a digital thermometer used by food inspectors. It's quite accurate, I checked it against a mercury lab thermometer. It reads in either F or C. It looks almost exactly like the RH Designs thermometer and it set me back a whole $13.
I picked up a digital thermometer at a surplus store a few years ago for about $5. The damn thing is spot on to my mercury filled Kodak "Professional" from the 60's. I use the digital for everything now. Easily converts F to C.
"I'm still developing"
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Brian As you have a Jobo, you could try the Jobo thermometers which fit into either end of the Jobo water bath. I obtained two with my secondhand CPE 2 plus. Both agreed with each other and the Jobo temp dial which sets the element heater. I managed to break one so got another from Nova Darkroom but I am sure there are other stockists as well It was either £3 or £5- I cannot remember which. That one agreed exactly with the one remaining thermometer. That indicated enough consistency for me.
If you check Richard Henry'sbook, "Controls in Black and White Photogaphy" (he was a research chemist with a PhD. and an M.D.) he says almost all thermometers are inaccurate. It takes avery expensive, certifiied one to be accurate. But all you need is one that is reliable (i.e. consistent) then work out your processing times according to your inaccurate thermometer. When the negative is right, the time and temp. are right!
Tp paraphrase O.J's lawyer.
Just a quick paragraph or two from a book by George E Todd:
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.
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
I personaly have so many different ones that my wife thinks I am a collector. I consider the Kodak Process Thermometer 3 the standard. They can be found on ebay for a fraction of their original cost.
I use it often to check my other thermometers.
[COLOR=Lime]Let Kodak Go Now[/COLOR]
Another vote for a Kodak proccess thermometer. I went through the same thing a few months back, got one cheap on ebay and have been delighted. In absolute terms I don't know how accurate it is, but it is repeatable, easy to read, built like a tank.