Calibrating (digital) thermometers
Old 01-31-2019   #1
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Calibrating (digital) thermometers

It's been a while (2-3 years) since I've been serious about film photography, but I've got a darkroom now (hell yeah!) and getting back into it. I've got several thermometers, two of which I really like: a digital instant read one, and an old 'analog' Paterson meter, with a very precise scale (56/86f or 13/30c). I'd never tested them against each other, but I just did and the digital thermometer consistently measures 2 degrees C higher than the old one. How do I know which one is correct? I feel like both should be accurate, but apparently that's not the case.
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Old 01-31-2019   #2
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There is no reason both devices should be accurate. Both of them could be wrong. Still, a 2 deg C difference does indicate one of them is off.

Proper calibration is not an easy task.

Calibrating for 0 deg and 100 deg. C is straightforward. For every 500 meters in elevation the boiling point decreases by ~0.5 C. The weather affects atmospheric pressure as well. Freezing points are also affected by pressure. But you need to calibrate for a different temperature range.

Calibration using other liquids, or mixtures of liquids, with know boiling points within the range you need could be hazardous without access to a laboratory with a chemistry hood.

ThermoWorks sells calibration tools.[1] They offer safe calibration liquid mixtures for $99/liter. You would also need a way to heat the bath. Their certified calibration simulators for digital thermometers start at $159.

ThermoWorks also sells high-quality, digital, immersion thermometers for brewing. These are NSF (National Sanitation Foundation) certified. I would just use one of their immersion thermometers.

Another option would be to use one of the thermometers you own. I'd assume the error in the temperature range of interest is constant. I would run a series of tests over a small temperature range. If you see a difference in quality I'd just use the optimum indicated temperature from there on. I doubt atmospheric pressure changes would be affect film/transparency development.

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Old 01-31-2019   #3
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check for freezing and boiling point accuracy and the rest shall be linear enough in-between.
As willie says, For every 500 meters in elevation the boiling point decreases by ~0.5 C.
Meaning if oyu are at 1000m altitude your boliling temp should be 99 deg not 100 (C)
The rest is not important unless you have a storm while doing it.
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Old 01-31-2019   #4
Malcolm M
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Get a job in a lab, then you have a choice of several UKAS certified thermometers accurate to 0.1C to check against. My thermometer is spot on.
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Old 01-31-2019   #5
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I work in a lab (not a photo lab) and we have a large master thermometer which is checked regularly by the state if Illinois. A traditional mercury thermometer. We use that one to check the rest of the ones we have, the ones our technicians use.

For developing film, I use a digital thermometer I bought at Target. It seems accurate.... But I really should bring it in sometime and check it.
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Old 01-31-2019   #6
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put the thermometers in a slurry of ice & water.... they'll come down to 32F/0C. Then you can see how far off the digital ones are. Some of them can be adjusted. If not you can just mark the offset on a piece of tape & adjust the reading by that amount each time.
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thermometer calibration
Old 02-10-2019   #7
randy stewart
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thermometer calibration

An accurate thermometer is desired, however in B&W (and to a lesser degree, in color) consistency is more important. In other words, once you accept a set of process standards (time/temp/agitation), it is most important that you be able to repeat your results. Years ago I acquired a Kodak lab grade mercury thermometer (the one in the stainless steel casing) which is accurate to 1/4 degree F. I have a Kodak process thermometer, which is not quite as high a quality, but very good. I check them against each other, then against the metal dial thermometers is use day to day, which can be adjusted to calibrate. I also have a photo digital thermometer reading to 0.1 degree F. The whole bunch is good within a degree, so I good to go.
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