Note to self on digital thermometers

Andrew Moxom · Feb 27, 2010

After trying the new digital thermometer, I am happy to say neg densities are visibly (to the naked eye as I don't own a densitometer) up to about where they should be normally for the way I print. I tried Neopan 400 in HC-110 dilution H @ 68 Deg F for 10 minutes. Lovely tonal scale. Same for TMY-II in TMAX dev 1:4 for 7 minutes. Lots of good shadow detail and easy printable negs overall. I will be getting a good glass thermometer as the test one in the next few days to understand how much drift occurs over time to the new purchased digital one.

Larry Bullis · Mar 2, 2010

Last night I noticed that I had a pan of water that had been sitting in my trailer darkroom for a few days, and two thermometers - a small dial thermometer, and a mercury thermometer. I stuck them both in the pan, and wonder of wonders; while the dial had a lot less precision, they seemed to be right on at 59°F. It occurred to me that the most important comparison test would be the temperature that you actually use, especially if you use just one. Like 68°F/20°C. The range might possibly be different due to manufacturing standards, etc. between two thermometers, but if both give the same working temperature, it wouldn't matter. Would be important to repeat the test now and then, though.

Martin Aislabie · Mar 2, 2010

bowzart said:
Last night I noticed that I had a pan of water that had been sitting in my trailer darkroom for a few days, and two thermometers - a small dial thermometer, and a mercury thermometer. I stuck them both in the pan, and wonder of wonders; while the dial had a lot less precision, they seemed to be right on at 59°F. It occurred to me that the most important comparison test would be the temperature that you actually use, especially if you use just one. Like 68°F/20°C. The range might possibly be different due to manufacturing standards, etc. between two thermometers, but if both give the same working temperature, it wouldn't matter. Would be important to repeat the test now and then, though.

Couldn't have put it better myself

The only temperature range you are really interested in is 20C +/-3C or what ever your chosen processing temperature happens to be.

Knowing accuracy at the freezing point and boiling point of water is of limited value.

Personally, most of my very accurate thermometers don't have such a range, as I have sacrificed overall temperature range for an expanded scale around a chosen temperature range.

Martin

wogster · Mar 7, 2010

Martin Aislabie said:
Reinhold is absolutely on the money with this

Consistency in B&W is the key

It doesn't actually matter is your reference thermometer is actually accurately reading 20C - what matters in B&W is that you always use the same indicated 20C consistently

We all adjust our film and print processing time away from the manufacturers recommended starting point for time/temperature to give us the results we desire.

So choose a good thermometer and calibrate all your others to it.

Then use the temperature equivalent to 20C on your reference thermometer for all your other work.

Keep your master/reference thermometer safe and don't use it for anything except calibrating work- as accidents sadly do all too frequently happen

Martin

Calibration can be as simple as taking your reference thermometer, sticking it in a pan or bowl of water adjusted to 20℃, putting another thermometer into the same pan or bowl of water, and seeing what matches up. It doesn't really matter if it says 20℃ or not, get a little bit of ladies nail polish and mark a line on it at your 20℃. Now it's calibrated. For a digital, if it can be adjusted, adjust it to read the same temperature. If it can't be, stick a label on it which tells you what 20℃ is equal to, include the calibration date.

JayGannon · Feb 15, 2011

Reinhold said:
The coefficient of expansion of mercury is based on the unvarying laws of God.

Those impressive LED numbers on that hi-falootin elektronik gizmo are only a read-out from a collection of parts put together by monkeys.

Get a good gla$$ thermometer and use it to confirm all your temperature dependent instruments and processes. Once everything is confirmed, put it away in a safe place as a standard reference. Do not use it routinely, it can be broken...

Reinhold

www.classicBWphoto.com
if that doesn't work, try: http://mysite.verizon.net/res14rg7y/

I guarentee my Fluke thermometer is more accurate than any chem thermometer.
I use a Fluke as reference for some Brannan 76mm Immersion Thermometers that sit in trays/tanks etc. The Brannans are reasonably accurate the Fluke is hyper accurate and calibrated professionally every year.

John Shriver · Feb 16, 2011

The Kodak Process Thermometers are readily available, and are a great calibration standard for the home. Technically illegal to ship by USPS, however. (HazMat.) At least be sure the shipper puts a lot of plastic bags around it, so it doesn't drip if it breaks. (The USPS won't have a good sense of humor about Mercury dripping out of a package.)

The Weston "Miroband" dial thermometers are readily calibrated against one, they have the nut to turn the shaft with.

Mainecoonmaniac · Feb 16, 2011

I thought digital thermometers were always accurate and more accurate than dial thermometers. I guess this case cleared any misconceptions on digital thermometers. I have a spirit type glass thermometer that I know is accurate to verify my dial thermometers. Don't like to use my glass one because I know I'll break it

JayGannon · Feb 17, 2011

They can be more accurate but they do need to be cabibrated regularly, as does any thermometer, no thermometer holds its calibration forever digital or traditional.

Pat Erson · Mar 17, 2011

Overnight my trusty long-column alcohol thermometer suddenly lost ONE whole Celsius degree. I was processing at 19°C without knowing it (of course my negs were a bit thin).

I had to check it against another tiny throwaway thermometer to realize what happened. Now I process my TRI-X at 21-sth and it seems to work.

So YES, buy two identical thermometers and check one against the other regularly.

greybeard · Mar 17, 2011

Ummmmm....buy three. If you have two, they will probably never agree, and you won't know which one is wrong. If you have three, you can at least find the majority opinion

Actually, if you have one unbroken, etched-stem liquid-filled thermometer, you are all set. There is practically nothing that you can do to one of these to change its inherent calibration more than a tiny fraction of a degree, and even the immersion depth correction is usually negligible.

It is the type which has a cardboard scale glued to the thermometer inside a housing that are really problematic. If you are lucky, the cardboard falls off completely rather than shifting lengthwise slightly.

Your reference thermometer doesn't even have to be all that accurate (in the sense of indicating temperature with respect to absolute zero) as long as the reading precision will let you see an error of a degree or so.

Note to self on digital thermometers

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