I mean C41
Depends on what you mean by color photography. Making consistently matched color separations via black and white film might ideally demand tolerances within plus or minus 1/10th F. I actually have a thermoregulator capable of that kind of accuracy. But I rarely need it. And it in turn needs to be calibrated using a truly trustworthy, accurate thermometer, not some cheap toy thing.
Ordinary RA4 processing, just like Cibachrome of the past, can easily tolerate plus or minus 3 degrees, though I have always kept it tighter than that. With routine black and white film developing, I just use one of those Zone VI compensating developing timers with its water jacket probe. As long as I start out a little above 20C, any drift down in water temp seems to be correctly factored in.
What do you test it on, and how?
Calibration of a mercury thermometer is impossible.
The way to test is is to put ice cubes in a glass and wait for it to melt a little.
Then start stirring the thermometer without touching the glass.
It should read 0c 32F.
I have a Kodak color thermometer (not sure if it is mercury) and a bunch of dial thermometers. Every now and then I take out the Kodak and using that as the constant I check all the dial thermometers. Usually they are all super close. Good enough for black and white work. Never had an issue with color either but I scan that so it isn't as critical for me as someone who prints I suppose. At some point good enough is good enough. I guess we each have to decide at what point that is.
Anybody has a Mercury thermometer recommendation and where to source it?
I purchased a thermoworks one but I feel it got a little off with time.
I simple mercury thermometer with give me the most consistent and accurate reading.
But that boiling water test? Ha! Any of us who have done a degree of mountaineering know that when water begins to boil at 10,000 ft it's still so lukewarm you can put your finger into it without getting hurt. A "rolling boil" is necessary to cook your freeze-dried dinner or whatever. To calibrate a thermometer relative to that standard, one has to factor in a number of variables not only including what "sea-level" actually means, but ambient barometric pressure as well.
I have had my Kodak process thermometer for 55 years and I got it used, I've used it for C22, C41, R4 and what R4 replaced, E4, E6, and GAF/Anscochrome and color negative chemistry. As it is sealed and mercury does not react with glass, never worried about a false reading, negatives and slides are spot on. My thinking is that temperature fluctuations are more of a concern than being a degree off as long I am off a constant degree every time and my times are fixed. For ease of viewing I use a dial type, a Patterson I think, I use my Kodak to keep it calibrated. I expect that a good certified digital thermometer will be as accurate.
I was given an expensive digital thermometer by a lab chemist. The instruction leaflet said it would be guaranteed accurate for two years. After around eighteen months it was way out, useless, even with new batteries.
So how do you know your mercury is faulty and it is not the digital?
What do you test it on, and how?
The only spirit thermometer that I use is small five inch one kept in the corner of the print developer dish.
Spirit is good enough for my printing.
An interesting short article link to darkroom thermometers.
A few of my mercury thermometers have engraved on the back "76mm immersion" etc.
One reads "total immersion"
Apart from me, does anyone take any notice of these instructions.
Mercury thermometers may no more be sold in the EEC due environmental regulations...
Mercury is highly toxic and does not degrade in the environment, when it gets in the water/river it destroys aquatic life and can not be stopped doing so.
Calibration of a mercury thermometer is impossible.
The way to test is is to put ice cubes in a glass and wait for it to melt a little.
Then start stirring the thermometer without touching the glass.
It should read 0c 32F.
I have read some where that a drop on those can bang them out alignment And show different temperatureI do not know about your digital thermometer, but generally they are based on the thermocouple. A thermocouple is a pair of wires of different compositions, which, when twisted together at one end, produce a voltage at the other (not twisted) end. This can be read by a voltmeter, chart recorder, etc. This voltage is proportional to temperature at the thermocouple. Tables of voltage vs. temperature have been accurately and precisely determined since long ago.
Basically, a digital thermometer is a voltmeter calibrated to read out in degrees (F or C or whatever). "Back in the day" we used to measure voltage directly and read the temperature off of the appropriate chart. Some may remember the "J" chart.
Thermocouple wires had "names"; the J type was iron-constantan ( I don't remember what this alloy was.) Another popular one was type K. Dial thermometers use the same principle; the dial replaces the voltmeter.
All this is really just for info. The voltage generated by the thermocouple should be stable, and not drift with time. I can not say about the voltmeter, although I never heard of one drifting. For the perfectionist physical chemist, I believe there might be slight corrections necessary, such as for how much of the thermocouple is immersed, and how much is not.
water freezing point and boiling point can't be use with the Kodak Process Thermometer type 3 as it only read from 55F to 140F.
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