Ditto that. I have two, and both read exactly the same. For High temperature readings (100F/38C) the digital oral thermometers are excellent and accurate. I used to use those for processing color film (C-41).
Originally Posted by Claire Senft
Thermocouple with a submersible wire probe. I've seen Chinese-made ones for about $25. Real accurate, cheap, easy to read, easy to calibrate.
I've a waterproof one made by Cooper. It has a hard-wired probe. It works great.
I'd rather have one with a type K replaceable probe.
Omega and Cooper sell fairly well on Ebay.. The handheld ones are nice.
The bench ones will monitor 8 to 12 probes at a time (neat!)
I've given up on regular thermometers since having found my thermocouple.
While the digital oral thermometers are often accurate to +/-0.1degC (by using a thermistor), they are only so over a narrow range of say 32.0 - 42.0 degC.
Originally Posted by donbga
What surprises me is that no-one has suggested the 'master/slave' model.
The master thermometer does not even need to be accurate, merely consistent, though accuracy is nice if you can get it. Mercury-in-glass lab thermometers (incliding the Kodak ones) are probably the most reliable; bimetallic dial drift most in my experience; and electronics may do almost anything without warning, no matter how reliable. Most don't, it's true, but they can.
All 'slaves' are calibrated to the 'master'. Let's say you have three thermometers, one 'master' and two 'slaves'. When the 'master' reads 20, one slave reads 19,5, the other, 20,5. For a standard 20, you therefore use the slaves accordingly.
Break a slave; replace it; recalibrate against the master. This one's 19.7. Fine. Still the same temperature...
My 'master' is a mercury-in-glass Brannan that agrees with another, identical, Brannan to 0.1 C. My 'slaves' vary widely for different purposes: spirit-in-glass, dial (for water baths), digital... Digital and dial are checked periodically; spirit-in-glass only when I acquire them.
The only Kodak Process Thermometers that were calibrated and NIST certified were the "Type 3". They come supported in a stainless steel exo-skeleton for want of a better term.
Originally Posted by Bruce Appel
BUT.... how do you still know that it is in calibration? Without a recent NIST traceable calibration you really don't know it is in calibration. Are you wondering whether an LIG thermometer can go out of cal? It sure can, check out this useful page of information as to how they can and do go out of cal.
Most liquid in glass (LIG) thermometers are 'repeatable' as you say (the more correct term is 'precise'). An LIG thermometer will most likely have high precision, but unless it has been recently calibrated with an NIST or equivalent certificate to prove it, then the accuracy cannot be guaranteed.
Unfortunately in light of this, the OP (Brian) is still left scratching his head, because he probably has no convenient way of ensuring that any LIG thermometer he buys is reading accurately. And unless he has a recent cal. certificate, unfortunately he can be none the wiser.
If you go for a digital one, how do you know that the Chinese brand $5 one is accurate to its stated specifications? The ones I would tend to trust more are the digital oral thermometers which unfortunately don't go down below 32.0degC.
Of all the digital thermometers mentioned in this thread, only one mentions an accuracy claim (by design) of +/- 1degC (http://www.rhdesigns.co.uk/darkroom/...ermometer.html). So just because your new fangled digital thermometer has 0.1 degC resolution, does NOT mean it is accurate to +/- 0.1 degC !! It is most likely accurate to something between +/-1 to +/-2 degC. Really !!!
I also wanted to highlight two other statements about digital thermometers in this thread that are rather misleading:
1. "I found a digital lab thermometer on Ebay a while back that seems pretty accurate"
according to who or what !
2. "I've been using a pyrex digital thermometer from Linens and Things for about 2 years and it's dead on with my glass one every time. Accurate w/in a tenth and it's quick as well"
just because it has a 0.1 degC resolution does NOT imply it has 0.1 degC accuracy ! Secondly, how do you know that your glass one is dead on with a traceable standard ???
Never be fooled into thinking that an LIG or digital thermometer's scale divisions are in any way a statement of its accuracy.
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Originally Posted by Roger Hicks
The digital clock in my car agrees with my bedside radio clock too.
They are both 2 minutes slow.
I don't know if this rings true to you but for me; use what you use, and as long as it is consistent from session to session and gives me the results I want, I don't bother with it.
You'd need a laboratory and a bunch of test engineers to be even remotely sure about anything. But even then, we are talking about shutter times, developing times, agitation schemes, temperatures, and so on and so forth - all the ways to mess up the ideal, perfect process, from making the photograph until it hangs on the wall. Listen to anyone but choose for yourself.
Prints reveals truths that negative scans obscures.
Originally Posted by PeterB
Well, I did say that consistency matters more than accuracy...
It is always a good idea to have two thermometers, preferably identical, in the darkroom, and compare them once in a while. If one is broken, you can just pick up the other without having to bother about any possible change in process. Just remember to get a new second thermometer and test it against the one in use, so it is ready when the next mishap occurs.
Interesting thread, and your comment here Roger. On this side of the pond, the ideal would be to obtain a thermometer certified by the NBS (National Bureau of Standards) and compare all others to it - the master/slave thing. However, not sure we need that extreme in our darkrooms (the NBS thermometers, like everything else are quite pricey).
Like most things darkroom, we can cater to the analytical side and spend all of our time checking, calibarting, etc. NOTE:I am as bad about this as anyone, so I do not think it is a bad thing. On the other hand we should consider what we can do - in this case trying to match at least 2 "good" thermometers and keeping one as the master, then always match to it should work very well.
One question, those that use the digital thermometers, have you noticed any changes in readings when the battery starts to go? I would think that at some point the readings would become "off" just prior to the battery failure.
Originally Posted by Roger Hicks