Is your thermometer really accurate?

[Demetrius Latchis]

I recently bought a new digital thermometer and found it was 2.5 degrees off, reading low. I tested it with boiling water and it read 209.5 . I was ready to return it but checked the web for testing methods. I was doing C41 process after a lapse of 20 years

To perform a boiling water test you must know two things, the barometric pressure at the time of the test and the elevation in feet at your testing place. Go online to

http://www.virtualweberbullet.com/boilingpoint.html

and enter the numbers. The approximate boiling water temperature will be shown and will be accurate enough for darkroom use.

As it turned out my two old dial standby thermometers were reading too high and the new one was correct at 209.5 degrees.
It is no wonder then that my recently processed C41 negatives were off in color. I had used 97.5 degrees instead of the 100 specified. I had to do some color correcting in the scanner and fine tuned it in Photoshop. It took some time to do.

As everything else seems to be tested on this forum, film speeds, developing times, etc. it should be just as important to use the correct tested temperature. Getting a digital thermometer that reads in the 200's range for the boiling water test can be used to test your other ones at your working temperatures.

Perhaps others have their own methods.

[Jim Jones]

A mercury or a spirit darkroom thermometer is usually accurate or has an obvious problem. I've had troubles with dial thermometers in the past. They can often be calibrated against a reliable thermometer. This is best done at a temperature close to what you use for your most critical processes. Checking a thermometer at 200+ degrees does not ensure that it is still accurate at 68 or 100 degrees.

[Ed Richards]

You can also check the freezing point with a water ice mix made with distilled water. Just give it time to equilibrate. That will give you two data points.

[Scott Knowles]

Digital thermometers should be checked routinely with a multi-point test (minimum freezing and boiling) with a "standard" thermometer (don't assume icy or boiling water is the temperature you think it is). The USGS tests every digital thermometer at least twice a year against a NBS thermometer, which is in turn checked every few years. In addition digital thermometers are often checked before and after long field data collection work with a two point test. You should keep a history (date, time, air temperature, barometer optional) of the checks to note drift or problems.

[Joseph O'Neil]

I've never foudn dial thermometers to be totoally accurate myself, but if you know yoru theermometer is off by a degree or two, at least for B&W, I can live with that.

I ahve an old mercury thermometer I guard with my life. Sealed up, and well looked after, they are perfectly safe. The demise of the mercury thermometer is something of an error IMO.

Another option I have not tried are some of the new digital thermometers. I have not seen or used one specifically for photogrphy, but I have seen them in other applications, and they seem to be pretty good. If you can find one with the proper range, it might be worth a look.

joe

[Michael Gudzinowicz]

For 100F, check it against an accurate fever thermometer.

Also, see if there is a mark specifying how far the thermometer should be immersed. There are calibration procedures for mercury thermometers that account for immersion distance, and the differences between ambient and measured temperatures. Some of those problems may carry over to the digital thermomenters.

[Brian Ellis]

Buy two thermometers, one good mercury-based such as Kodak's if they still make it(mine cost about $100 ten or twelve years ago) or maybe one more scientifically oriented and presumably even more expensive, and a second inexpensive digital dial thermometer that can be calibrated. Don't use the good one except as a check for the digital, periodically adjust the digital based on readings from the good one. In my obsessive/compulsive/fanatic BTZS days that's what I did and it seemed to work o.k. though of course I had no way of being sure that the mercury one was perfect after five or ten years.

[Demetrius Latchis]

I agree with the posts that say to check all working thermometers with a good glass one.

I located my VWR scientific mercury thermometer Cat#61010-020 that I also guard carefully. I did two comparison tests against a new multi digital thermometer from B&H.

As the mercury one shows Celsius, the readings are that.
At 38 degrees (100F) the mercury read 38 and the digital read 37.9. At 20 degrees (68F) the mercury read 20 and the digital 19.9.

As it is much easier and quicker to read a large digital dial I prefer the digitals, and they can be very accurate. But, as the concensus agrees, check them now and then.

[Demetrius Latchis]

As an afterthought, it is recommended not to read any thermometer with the probe or bulb touching anything but the liquid for accuracy.

Kodak's currently advertised deluxe darkroom thermometer shows an accuracy of plus/minus 1/2 degree at 68 and 75 degrees, and plus/minus one degree at all other points.

[timsumma]

I use Kodak Process Thermometers to check accuracy of my other thermometers. All the dial thermometers in my dark room are exactly on the button, the reason for this is that they are assigned to specific tasks and ALL have a hex nut on their base, I turn the dial face while holding the nut to set each dial to the purpose it is intended. By the way a good dial thermometer has an emersion position for it’s stem, just like a good quality glass thermometer has an emersion line.

[John Berry]

I use a fluke meter with a thermocouple adapter and a stainless probe. I guess I have about $500 into it. May I also add that I used it as a heavy duty mechanic, thus the justification for that level of tool. Degrees to the tenth. Damn well better be for that kind of money. Is it accurate? Hell I don't know for sure. I do know that it is the thermometer that I used to dial in my film, mixed in the same pyrex pitcher that I have been using for 25 yrs. My personal opinion is that a tighter grip on consistancy with what you have is more fruitful that knowing the absolute of your thermometer. I mix my developer to within .2 degrees then I set the timer to 20 min for a reference and develop by DBI, go figure...

[Dan Jolicoeur]

Is not the purpose of optaining your own film speed to take into the account; your thermometer, your shutter speed, your technique of using a timer to dump the developer, and add the stop, etc.?

It is only a measuring tool of your process control.
If you recalibrate your thermometer; "do you not have to do another film speed test"?
Dan

[John Kasaian]

I have a thermometer but I seldom use it much unless I'm mixing something like Dektol or Rodinal. All my liquid chemicals and distilled water for mixing sits in amber glass jugs in the same location, so the temperature equalizes. It works for me and my B&W world.

[Leon Jester]

I've never found dial thermometers to be totally accurate myself, but if you know your thermometer is off by a degree or two, at least for B&W, I can live with that.

I have an old mercury thermometer I guard with my life. Sealed up, and well looked after, they are perfectly safe. The demise of the mercury thermometer is something of an error IMO.joe

Fisher Scientific offers mercury, laboratory grade thermometers at a reasonable cost.

https://www1.fishersci.com/Coupon?cid=1328&gid=201042&details=Y

[ppisczak]

I took a class in 2005 on how to calibrate one's film. A couple things I took away from that class are:

Check shutter speed of all your lenses periodically

Use your best, and only one light meter.

Use your best and only one thermometer when developing.

I liked to use multiple thermometers in different baths. Though this sounds convenient, once I started using just one thermometer, my negatives got more consistent. The thermometer I used can be purchased online from McMaster-Carr. It is a non-mercury (presumably red colored kerosen or the like), is about 12 inches long (re: good scale gradiation) and is protected in a nickel plated brass sleeve. For more details please see the attached link which will get you to their home page. The one I purchased in on page 530. You will need to go there manually as they will not give me a link. Also I purchased the red colored one...not a good idea working under a red light in that it is hard to read. I'll purchase a blue one next. Good luck with your calibration!



http://www.mcmaster.com

[Nick_3536]

For B&W I basically agree with Dan. It doesn't matter if my thermometer is accurate. What matters is the the thing always reads the same way. If it tells it it's 18C when it's really 19C but does it the same way every time I'm very happy.

For colour I use a fever thermometer to make sure everything is right.

[Joe S]

I use a fluke meter with a thermocouple adapter and a stainless probe. I guess I have about $500 into it. May I also add that I used it as a heavy duty mechanic, thus the justification for that level of tool. Degrees to the tenth. Damn well better be for that kind of money. Is it accurate? Hell I don't know for sure.

I use Fluke for my job as well (service tech) and 1 degree either way can mean its working right or not, If you look at your paper work with the meter you will see on most Flukes that you have an adjustment point inside the unit and that you need to keep them adjusted, some of the other Flukes did not come so you can adjust them and you have to send them back for adjusting (pain in the you know what). I used to parkerize rifles and I had to be very accurate (just as accurate as with photography) with my temperatures and when reading in a book on how to do it, they gave a great idea for a cheap but very accurate thermometer to test with and that is to use a CANDY thermometer that you can purchase at most grocery stores. Candy is very very sensitive to temperature and has to be done accurately so the thermometers designed for them are made so.