Different Thermometers

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?

Buy a fourth thermometer?! :whistling: :blink:

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.

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.;)

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

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.

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.

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.

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

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.

Attachment 57617
Use the thermometer that reads closest to zero centigrade.