Thermometer calibration for C-41

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I recently acquired a load of darkroom equipment, including an old Jobo water bath and rotary tank, so I'm going to give C-41 development a whirl this weekend using a Tetenal kit. I've got three thermometers - a glass tube (B&W range), a metal dial type and a digital probe...and of course the damn things are all giving me different readings!

The digital device is reading around 2 degrees C above the dial thermometer, but this is not a consistent difference - it comes to within 1 degree when the temperature is up around 40 deg C. In the B&W range, the dial and glass devices agree to within half a degree, so I'm inclined to think it's the digital thermometer that's inaccurate, but that's just a guess.

As C-41 is so temperature sensitive, I'm worried about relying on something that's out of whack. Is there any way to accurately calibrate thermometers at home? I know you can do boiling water/ice calibration, but how do you test something with a 10 to 60 degree C scale? Also, does anyone know which type of thermometer is generally more accurate?

(BTW, neither the digital or dial seem to have a calibration button or knob).
 
Sorry J I can't help with the thermometer question (funnily enough I've just bought a new digital one myself which seems very accurate) but I would be very interested to hear about how you get on with the devving as this is my next step as well.

Andy
 
its not as temperature sensative as people think ,,,,just do it
 
donut said:
its not as temperature sensative as people think ,,,,just do it
Click to expand...

Really? I've read a fair bit online where people say it only has a 1 degree tolerance (i.e. half degree either side), so I've been getting myself prepared for super-accuracy. How much tolerance do you think it has without impacting on the colour?
 
Loads, Rellei give development times from 20 - 40 degreess. You need to keep the temperature consistent though, you can't have it wandering about during development, though the actual temperature doesn't really matter so much. All C41 negatives will need colour balancing for printing anyway.
 
i had a three degree C difference from when i started developement until i poured the developer out and it came out fine ,,, (tetenal kit )
 
donut said:
i had a three degree C difference from when i started developement until i poured the developer out and it came out fine ,,, (tetenal kit )
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That's good to know. In that case I think I'll go with the dial thermometer and aim for a consistent temperature, which should be okay as the water baths seem to be working ok.

Thanks for the info - I feel a lot less nervous about ruining the film :)
 
How accurate they are will depend on the type (see my reasoning for using digital) and manufacturing quality.

I would always prefer to use a digital thermometer, because it gives you a definite figure (whether it's right or wrong that's another matter, solved by having it calibrated properly). But you look at your glass tube, then tilt it just the slightest angle and it could look like a different reading.

The same can apply to the dial thermometer...

Then there's manufacturing quality... You can get some utter garbage that is made by well known supposedly good quality companies.

You can calibrate them at home, but you need a thermometer that you already know is correct to test them against.

I work for a calibration lab, only as office staff, but after nearly a decade there I hope I've picked something up lol.

If you google it (either thermometer calibration or temperature calibration) you'll come up with loads of companies that do this if you really wanted to have them "properly" done and have a certificate to prove it. There are cheap ones, and of course there are more expensive ones. It just depends on how serious you want to get. Generally the more expensive, the better they can tell you how accurate your thermometers are. I'd at least go with a company that has some an ISO accreditation (as in they are audited and keep to a certain standard, ISO9001 for example, not ISO film speed lol). The top level of calibration is ISO17025, known as UKAS.
 
Just thought of something, my dad never had digital thermometers, only mercury in glass and he coped just fine for the 40+ years he had his darkroom so they can't be all bad lol. As others have said (not that I'm film person though) just go for it :)
 
Good point about using the digital as it's easier to read. I guess if I get it close to the required temp on the digital and stop it drifting, I should be okay. We'll see how it goes on Sunday :-)
 
The reason people like good quality mercury in glass thermometers is they can be fairly accurate and consistent over time sadly it has been illegal to sell them since 2009 in the UK, fortunately I brought mine in 2008 a 0-50C lab grade 12" its easy to read marked in 0.1 increments and you can check the calibration at 0c by water in ice easily enough.

Digital temperature meters tend to be highly accurate its easy to get a spec of -/+ 0.2c, however that is the meter not the probe. If its a wide range unit typicaly the probe will be a K type thermocouple whose accuracy is something like -/ + 2.5c or 1.5 for some better ones. You can get very accurate digitail temperature meters / probes but most of the Platinium resistance ones start off at around £200. Some digitail meters use silicon probes wich can be fairly accurate over limited ranges I have a Hanna Instruments HI9025C pH meter the spec given for temperature accuracy with this is 0.5c using a silicon probe.

Some years ago I worked for a calibration company calibrating instruments typically meters are calibrated by feeding them accurate mV voltages and probes / thermometers are calibrated using a thermal block calibrator I would imagine for most people here the cost would be unnecessary and with regard to K type probes hey temperature accuracy and repeatability is pretty poor anyway. If you can get your hands on a good mercury in glass treasure it failing that with digital look carefully at the specs especially of the probe rather than the meter.

I know people like the Tetanol kits for ease of use and availability however I would stick to 38C if you can.
Article from apug that is relevant

http://www.apug.org/forums/forum40/40584-c41-quality-slightly-lower-temperature.html

Photo Engineer is Ron Mowrey a retired Kodak Labs researcher
 
Thanks for all the tips. We ran a couple of films through the Tetenal process on Sunday and stuck to the 38C routine using the dial thermometer. I'm still working on my colour scanning so I haven't anything to share right now, but it does seem to have worked okay. I say, 'seem to' because the colours are skewed, which may be my scanning process, but is more likely to be because the film we processed was discovered at the back of a drawer and was shot 14 years ago! I'll post something in the film shots thread when I've figured out a good colour scanning routine.
 
...and the 2nd film we processed was completely blank. Obviously unused :rolleyes:
 
Maybe sticking the thermometer under your arm (or up your bum :eek:) would be a good way for testing...your body is quite good at temp control providing you don't have a cold or whatever. Is there a doctor in the house.
 
excalibur2 said:
Maybe sticking the thermometer under your arm (or up your bum :eek:) would be a good way for testing...your body is quite good at temp control providing you don't have a cold or whatever. Is there a doctor in the house.
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The things I have to do for art :mooning:
 
Best thing to do with three thermometers is throw two of them away.....

The best suggestions are melting ice (0'C), boiling water (100'C) and body temperature (37'C). These will give you three decent points to be going on with, though boiling point is affected by air pressure (100'C at 1.01325 Bar or 1013.25 mBar), so useful to know what the barometric pressure is at the time.
 
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excalibur2 said:
Maybe sticking the thermometer under your arm (or up your bum :eek:) would be a good way for testing...your body is quite good at temp control providing you don't have a cold or whatever. Is there a doctor in the house.
Click to expand...

It's not bad, but the "normal" body temperature has a spread of over one degree C.
 
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