Calibrating a thermometer?

[._._z]

I have in its original box a 20-year-old, unused Beseler model #8935 'Precision Photographic Thermometer' that allows one to calibrate adjustment.

 

I am interested in using it, but am not sure as to the best way to check its temperature accuracy.

[Nicholas Barry]

Fill a glass with crushed ice and put the thermometer in the ice. Keep the ice as well packed against the thermometer stem as possible. Check that the thermometer reads 0C or 32F.

[._._z]

I should have mentioned that this thermometer only goes from 60-160 F (supposedly accurate +/- 1 degree F from 70-130 F, and accurate to 1/2 degree F from 90-110 F).

[kevin_schoedel]

<p>Actually, to check at 0°C, wait until some of the crushed ice has melted, or alternatively, add a little cold water and wait a few minutes for it to cool. Then clear a space in the middle of the ice, and put the thermometer in the liquid water, <i>not</i> touching the ice. The temperature of the ice could be below 0°C -- say perhaps -10°C if taken from a home freezer -- but the water can't be below 0°C, or else it'd be ice. (It can't be significantly above 0°C either, while surrounded by ice.)</p>

<p>If you have already been using a different thermometer for development, or you can borrow one, then it would probably be simpler to adjust this thermometer to the other <i>at the temperature(s) you actually use</i>, e.g. 20°C. Even though the other thermometer might also be also off, you can still use the same developing time and conditions as with it, and expect the same results.</p>

[ryuji_suzuki]

Calibrate the termometer at the temperature you use most often. It is best to calibrate between 20C and 25C for regular photographic purposes. For this, you obviously need a reference thermometer; the real accurate one is very expensive. If you have a thermometer that you use for your reference, it is acceptable to calibrate against it for most photographic purposes. Your reading will be at least consistent but not absolutely accurate.

[mark_sampson]

You don't have to buy a reference thermometer. Go to a) local photo lab or b) high school chem lab and measure against theirs. I'm sure people would be willing to help in either place.

[david_goldfarb]

Oral thermometers are highly accurate and a good standard for calibration, since they are in the range of most photographic thermometers, and you probably have one at home or can get one for a few dollars at a drug store.

[kevin_schoedel]

<p>

OK, now I've read your description of the thermometer, posted while I was writing. There isn't a simple reference at these temperatures, other than another thermometer.

</p>

<p>

If you don't have access to an accurate reference thermometer, here is a method that is relatively cheap and most likely good enough for development, given the variations that differences in agitation technique and so on could cause. Go to a 'dollar store' or similar place, and find a rack with a dozen or more cheap and nasty glass thermometers -- best if there are different varieties. You'll probably find that adjacent thermometers disagree with each other by a few degrees. Lay out all the thermometers in a row, arranged in order from the lowest reading to the highest. The ones in the centre of the row will probably agree to a fraction of a degree; buy the middle one.

</p>

<p>

I've read somewhere that some (?) oral thermometers are actually offset from the true temperature to account for a difference between mouth temperature and internal body temperature, but I don't know whether this is true.</p>

[esben_jensen]

The crushed-ice-with-water method is independent of other thermometers, since 0 degrees celsius is defined as this. I can't imagine you'll see any difference in your prints, if the scale on the thermometer isn't calibrated EXACTLY at 20 degrees.

[craig_bridge]

If you'll accept +/- 1/2 degree F, most of the digital thermostats will be that close if you slew the temperature through several degrees and track the display changes and pick the midpoint. If you want closer in the 20-25 degree C range, you'll have to find a standards lab. If your willing to calibrate in the 98 degree F range, a basil thermometer (ones used to plot temperture to predict ovulation) will be accurate to better than 1/4 degree F.

 

The primary reason for the calibration adjustment was to get multiple thermometers to agree and not to calibrate them to an absolute standard.

 

BTW: The ice melting point equilibrium method is only accurate if your using distilled water for the ice cubes and the water bath.

[conrad_hoffman]

Agree with all the above. I'd contend that you want a calibrated thermometer so when you break it, you can replace it without having to redo all your development tests. Since the DUT doesn't cover freezing or boiling, you could find another thermometer that does, calibrate it at those points then, with a bit of math, have a good idea of its accuracy at 20C. Now compare with that.

[ryuji_suzuki]

I don't think many of these are necessary for regular b&w photographic processing, but here's something.

 

Temperature sensing devices are nonlinear devices and calibrating at a point far away from your usage range doesn't help much in terms of accuracy.

 

For most photographic processing, consistency across processing runs in the lab is more important than absolute accuracy provided that absolute accuracy is not too bad. (within 1C or so is ok; although this number seems big, many thermometers I've seen are this much off, but still usable as long as you do your own darkroom calibration.)

 

Digital thermometers may report to tenth of a degree but that doesn't mean that digit is accurate. Indeed, my digital thermometer I regularly use in darkroom is about 0.9C off against my standard thermometer. My standard is an old Paterson glass one (the long one for color processing) calibrated in lab at work. By calibration I mean I made a list of correction factors at a few points around the range I'm interested.

 

Water's phase change points (meltin and boiling) are also susceptible to atmospheric pressure besides impurities and I wouldn't take them to be accurate to the extent some claim. It's probably good to 1C if you pay some attention to these factors. Again, calibrating to 1C at 0C does not mean the termometer has the same accuracy at 20C. Farther away from your calibrating point, the deviation tends to increase.

 

I don't think high school's chem labs have thermometers that's more accurate than ones sold for darkroom use. Even in college chemistry accurate thermometers are only used in some upper level chemistry labs that deal with topics that are highly related to temperature like kinetics.

 

Again, you really don't need that kind of accuracy. I'd just calibrate it against other thermometers in the darkroom near 20C and be done with it. Even if the temperature is 1C off, if you make your personal processing calibration to adjust temperature by a minute or two, the net result is pretty much the same, and the whole thing is at least repeatable.

[pete_andrews]

Got any freezing Platinum anybody?<br>I've got this international standard that needs calibrating.

[ryuji_suzuki]

Dear Mr Physicist,

I think there might be a thread on light meter calibration somewhere.

 

:-)

 

But one stop is doubling of energy so how much (little) accuracy is needed is intuitively understood.

[._._z]

Thanks to everyone for his assistance. I found Beseler instructions for calibrating the thermometer and thought I'd pass it along, since I was the one who originally brought up this question. the suggested instructions are close to those made here by David and Kevin:

 

"CALIBRATION PROCEDURE: Immerse 2 1/2 inches of Color_By_Beseler Thermometer stem into running warm tap water and adjust hot water until thermometer reads 105 degrees Fahrenheit. Fill a beaker with this water and and put thermometer in the beaker. Insert a glass-type fever thermometer into the beaker, read the temperature on the fever thermometer (once the thermometer fluid has stopped rising) and compare to the Color_By_Beseler thermometer reading. Adjust the hex nut at the base of the Color_By_Beseler thermometer dial until both temperature readings are identical."