Different Thermometers

thefizz · Sep 23, 2012

I recently noticed a difference of 2 degrees Celsius between 2 mercury thermometers I have and when I opened a new 3rd thermometer, it was 4 degrees different. How do I know which is correct?

wiltw · Sep 23, 2012

Buy a fourth thermometer?!

Have you tried immersing it in a glass full of ice with water in it? If your thermometer will measure temperatures of 32°F/0°C, you can test it using an ice bath. An accurate thermometer will read 32°F/0°C in an ice bath at any altitude or atmospheric pressure. Fill a glass with ice, then add just enough water to cover the ice, but not so much water that the ice floats.

After a couple of minutes, insert the thermometer stem or probe into the middle of the ice bath and stir gently. Don't let the thermometer rest against the ice or you'll get a low reading.

If the thermometer goes just over 212°F/100°C, put it into a wide pot of boiling water to verify it at 212°/100°C.

bobwysiwyg · Sep 23, 2012

Reminds me of an old saying, a man with one watch always knows what time it is, whereas a man with two is never sure.

pentaxuser · Sep 23, 2012

The problem with darkroom thermometers is that they are designed for darkroom temps so don't measure either water freezing or boiling point but cover the much smaller and useable range appropriate to a darkroom. So the tests described above aren't applicable.

I can't think of any easy way except to buy or borrow a certified thermometer which is guaranteed accurate to within plus or minus 0.2 degrees C

The differences mentioned are large and certainly significant for both B&W and especially colour.

pentaxuser

Diapositivo · Sep 23, 2012

Buy 30, the one nearest the average is the most correct.
Sell the other 29 on eBay stating "checked against my other thermometres"

Seriously though, you probably mean you noticed a difference of 0.2 or 0.4 °C, I suppose. 4 °C is an enormous mistake and 2 °C also.
A common fever termometre (they cost a few Euros) is probably exact within a few tenth of °C (or it would be useless and wouldn't be sold as a medical aid) and should work very well for colour development.

mooseontheloose · Sep 23, 2012

Actually I've had the same problem -- three thermometers, all within 2 full degrees of each other, but as much as 4 degrees between the top and bottom ranges. These are all darkroom dedicated thermometers. A while ago I realized that my negs did not look right (probably due to underdevelopment), got a couple of thermometers to test against the current one, went with the one that matched my wall thermometers, and have had more luck since. That being said, I may try the ice water method to see if that confirms which one is the most accurate.

Ken Nadvornick · Sep 23, 2012

Pick one to standardize on in your darkroom. It doesn't have to be perfect. Just reasonably close. Checking against a fever thermometer is a really good starting point, especially for color temps (because it will be most accurate at or near its target of 98.6F). Once you have established your standard, then calibrate (measure the difference) for all your others. Note the differences and keep a small chart on the wall for reference.

In my darkroom my standard is a Hass Intellifaucet, since this supplies all of my mixing, tempering, and washing water.* I run it continuously to fill a large plastic container, then all my other thermometers go into that container (partially or fully submerged, per their design) and I check the offsets. Then I keep a reference list that says,

Kodak Darkroom: 68F = 69.6F
Kodak Tray: 68F = 69.0F

and so on. If I needed temperature set points other than 68F I would repeat the procedure for those specific points, since the thermometers are likely non-linear in their response.

[Edit: If you really have the need or desire to calibrate all of your darkroom thermometers to a more accurate source than an ice bath, fever thermometer, or Intellifaucet, then check out these NIST traceable fractional degree calibrated mercury and calibrated non-mercury instruments offered by ICL Calibration Laboratories, Inc. Definitely not for the faint of wallet. Note that the uncalibrated versions are, however, quite reasonable.]

Ken

* The product description for the Intellifaucet states:

"Each Intellifaucet is tested and calibrated to +/- 0.1 F at two widely separated temperatures. Reference thermometers are traceable to the National Institute of Standards and Technology."

and that's good enough for the non-analytical work we all do in a darkroom. Consistency is far more important than absolute accuracy.

jp80874 · Sep 23, 2012

In the Photo I class at the local college the students are given a supply list which includes a thermometer. As they tour the lab they are told to mark and then put their thermometer into a beaker of water with all the other student’s thermometers. The range is usually about four degrees.

John Powers

markbarendt · Sep 23, 2012

I have several that disagree but it doesn't matter as long as I know how much they differ.

If in the thermometers range, you can check 0 C/32 F with a slurry of well stirred ice & water or boiling water 100 C/212 F.

One thing to understand is that consistency is more important than correctness. If you have been working with one (errant) thermometer and you are getting good results, and you switch to a properly calibrated thermometer that reads differently you will change your photographic results.

This is not to say correctness is unimportant.

ic-racer · Sep 23, 2012

Use the thermometer that reads closest to zero centigrade.

chriscrawfordphoto · Sep 23, 2012

Good thermometers should all match. I have three Kodak Process Thermometers, one of which is 40 yrs old, and all three are exactly matched.

All three of my light meters (Minolta Flash Meter IV, Flash Meter VI, and Autometer IIIf) all match exactly, not even 1/10 stop variation. I mention that because the joke about owning multiple watches is often applied to handheld meters too.

Photo Engineer · Sep 23, 2012

One of the big problems is that each thermometer is marked with an immersion depth. For example, there is a 2" thermometer in the US which means that the thermometer must be inserted 2" into the liquid being tested. There ar Bulb thermometers and others as well. Inserting a thermometer to the wrong depth can mean a serious error.

PE

tkamiya · Sep 23, 2012

bobwysiwyg said:
Reminds me of an old saying, a man with one watch always knows what time it is, whereas a man with two is never sure.

Conversely.... a man with one watch will never know if the watch is off but a man with two watches will at least know, not to trust either one. I have something like a dozen watches. I'm a time zone all by myself.

As far as thermometer goes, I have multiple but I only use the same one every time. For B&W work, it really doesn't matter if it's degree or two off, as long as your process is calibrated and the reading is consistent.

Gerald C Koch · Sep 23, 2012

First check each of the thermometers for any column separation. Then if they are OK place them in an ice-water bath. Each thermometer must be inserted in the bath up to their marked immersion depth. Good lab thermometers should be accurate to plus/minus 1oC. To avoid any parallax error your eye should be immediately above the 0o mark.

Uncle Goose · Sep 24, 2012

when I was working in the Pharmaceutical business we had a Benchmark thermometer, when a new thermometer came in it was compared with the Benchmark and the difference was then marked. Once you know the deviation it doesn't matter as you can compensate. The deviation should always be the same though, if that's not the case it's time for a new thermometer. Like people said here before, immersion depth is important as is looking at the scale in direct line and not at an angled line.

It's not uncommon to find deviations, even in new thermometers. You can buy them with guaranteed 0.1 deviation at most but they will cost you dearly, those are needed for very precise analytical procedures. For photographic development a 1°C deviation isn't a disaster as long as you know it's there.

Photo Engineer · Sep 24, 2012

What you need is a Beckmann thermometer. It can be adjusted!

PE

Rafal Lukawiecki · Sep 24, 2012

Peter, I have a lab-calibrated higher precision mercury thermometer, certificate issued just a month ago for 18, 20, and 24 C points. I use it to check my other 4 thermometers, including the colour Paterson one I bought from you recently! All bar one are spot on, including the one from you. If you can meet up, I'd be glad to check yours.

scheimfluger_77 · Sep 25, 2012

tkamiya said:
...I'm a time zone all by myself...

You know, I have this issue myself but I'm finding a little more physical activity and a little less dinner are helping....

Steve

Smudger · Sep 26, 2012

Orson Welles "My doctor recently advised I should stop having dinners for four,unless three other diners were invited".

AgX · Sep 26, 2012

pentaxuser said:
The problem with darkroom thermometers is that they are designed for darkroom temps so don't measure either water freezing or boiling point but cover the much smaller and useable range appropriate to a darkroom. So the tests described above aren't applicable.

Yes, and a good glass lab thermometer has indicated this themperature.
On german ones that is "mittlere Fadentemperatur".

Photo Engineer said:
One of the big problems is that each thermometer is marked with an immersion depth. For example, there is a 2" thermometer in the US which means that the thermometer must be inserted 2" into the liquid being tested. There ar Bulb thermometers and others as well. Inserting a thermometer to the wrong depth can mean a serious error.

PE

But that should only deliver right results at the calibration temperatures (test object temperature and ambient temperature).

For different situations see here:

http://translate.google.de/translate?hl=de&sl=de&tl=en&u=http%3A%2F%2Fde.wikipedia.org%2Fwiki%2FFadenkorrektur (automated translation from German)

Photo Engineer · Sep 26, 2012

AgX said:
But that should only deliver right results at the calibration temperatures (test object temperature and ambient temperature).

For different situations see here:

http://translate.google.de/translate?hl=de&sl=de&tl=en&u=http%3A%2F%2Fde.wikipedia.org%2Fwiki%2FFadenkorrektur (automated translation from German)

That is correct, but the fundamental "stem correction" may cause variations at the outset and this is often why people see differences in thermometers.

Another problem is the use of alcohol and mercury in thermometers. Even though the markings on the thermometer may have been adjusted for the different liquids, the use of different liquids (and dyes in the alcohol) may affect the coefficient of expansion.

PE

ROL · Sep 26, 2012

thefizz said:
I recently noticed a difference of 2 degrees Celsius between 2 mercury thermometers I have and when I opened a new 3rd thermometer, it was 4 degrees different. How do I know which is correct?

I feel your pain, but consistency is usually much more important than accuracy, at least in normal B/W processes.

Photo Engineer · Sep 26, 2012

Consistency is important, of course, but if you break the thermometer, heaven help you!

PE

Different Thermometers

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