Measuring chemistry temperature..

[fotoobscura]

Evening.

I've been using digital thermometers to measure *film* developing chemistry for about 15 years. I have two "Tru Temp" thermometers that are supposed to be accurate to +/- 1 degree and are fast read (100ms refresh).

Anyhow I decided to treat myself and buy a Kintrex IR digital infrared thermometer for both photography and cooking (testing evenness of saute pans, etc). The IR thermometer is supposed to be extremely accurate (+/- 2 degrees). However, when I take an (opaque) cup of water and measure it with all three thermometers get THREE different readings. Even the two of the same brand with brand new batteries are degrees off. The IR thermometer is 5 degrees off from the mean difference of those two!

Anyhow, the question is- does anyone use IR thermometers for measuring chemistry- if so, how accurate have they been for you? Is it worth it? I don't want to screw the pooch on a spreadsheet I've been keeping for 10+ years with development times and temperatures. I understand analog thermometers could potentially be more accurate but they don't have the level of resolution I'm seeking (+/- a degree or two).


Thanks!

 

[Jon Shiu]

+/- 2 degrees is a range of 4 degrees, so not very accurate. I use a lab thermometer (glass) that you can easily see 0.5 degree resolution.

Jon

 

[BMbikerider]

Even +/- 1 degree is a bit much for photographic purposes unless you know the error and can compensate. I have a Kodak Digital which is about 20 years old and a mercury thermometer which I am guarding with my life 'cos it is irreplaceable. They were bought with colour processing in mind and were both certified to be +/-.25 degrees accurate, either centigrade of Fahrenheit when I bought them. Even after all these years they still correspond equally so I have no doubt as to their accuracy.

Thermometers used for cooking are not going to be anywhere near as accurate as the ones that are designed for photography. 1 or even 2 degrees out is not going to make any difference, but with colour printing or slide processing they would be the death knell

 

[Gerald C Koch]

Any digital thermometer should be checked against a conventional thermometer. Even a cheap student grade laboratory thermometer is accurate to 0.5 C. When using one makes sure to observe the immersion line for the best results.

A thermometer may be calibrated by measuring two known points. The easiest is with an ice and water slurry and the other the melting point of a particular pure chemical. Using boiling water is harder since the observed temperature must be corrected for atmospheric.

 

[MattKing]

I use my Kodak Process thermometer as a calibration standard.

On a day-to-day basis, I use an inexpensive digital thermometer with a probe at the end of a cord.

As long as I check it reasonably regularly against the process thermometer, and it passes that check, I am confident that it serves my purposes well.

 

[jp498]

I've got a weston dial thermometer and have had it for 20 years and it's still working well. It's not hard to get things within a degree with it.

I do have a Kodak process thermometer that doesn't get much use if I ever think I need to verify the weston's function.

 

[Light Guru]

Even the two of the same brand with brand new batteries are degrees off. The IR thermometer is 5 degrees off from the mean difference of those two!

Variances between thermometers is not all that uncommon. The important thing is to be consistent. Pick one thermometer and stick with that one thermometer.

 

[heterolysis]

I use two good old fashioned mercury thermometers with narrow ranges, one for RT and the other C-41. Accurate and reliable.

 

[removed account4]

i use a kodak thermometer
something cheep with a clothes pin
keeping it from wandering
nothing fancy
and it seems to work ..

after a few years (30something), i have come to realize
black / white does not seem extra sensitive
to a small amount of temperature variance

i use crappy coffee for my developer though

YM will V

 

[Dr Croubie]

Just a question, you said you measured an opaque cup of water, but how exactly?
One thing I learned a few years ago about IR thermometers, is that they're a bit colour-dependent. We were trying to measure the temperature of a heatsink (we had some big RF transistors to cool), and pointing at the silver gave a massively different reading to the black (we couldn't use a thermocouple reliably because the RF interference made it read a few hundred degrees). In the end the conclusion was (by testing on various other non-RF heat sources) that pointing the IR at a black spot (even a bit of black texta on the silver heatsink was enough) was the only accurate way of measuring.
So by extension, to measure temp of chemicals, I'd be putting the liquids in a black beaker or something, letting the temperatures settle for a few minutes (most important, because you're still probably going to be measuring the temp of the container, not the chemicals), then point the IR thermometer vertically through the liquid to the bottom of the beaker.

Or just use an immersible (mercury, digital, whatever) thermometer, still probably more accurate...

 

[MartinP]

It seems very odd that the OP can only measure to a precision of a few degrees using an alchohol or mercury thermometer. The ones he has used are perhaps not photographic thermometers?

The photographic alchohol thermometers I have used are scaled in half-degrees and can easily be interpolated to a quarter due to the size of the scale. The certification is for 0,25 C and that is more than adequate for most use.

Also, beware electronic thermometers quoting (or confusing) the difference between precision and accuracy. Just because the scale goes up and down in, say, 0,1 C increments does not mean that the unit is telling the truth . . .