Thermometers

Brian Jeffery · Aug 10, 2006

I recently acquired a Paterson darkroom thermometer and I was using it to monitor the temperature of the water in my Jobo CPE. However, I started to have my suspicions about the accuracy of the thermometer when I had to turn up the temperature on the Jobo in order to obtain the desired 24 degrees C. I dug out my Jessops thermometer and I was amazed to find that it had an indicated reading of 26 degrees C. I would expect half a degree difference between the thermometers but two degrees, thats a lot. I then had to dig out a brewing thermometer to see if I had two thermometers that agreed. The brewing thermometer concurred with the Jessops thermometer.

I now find myself in the market for a decent thermometer. My priorities are reliability, consistency and long life. Reasonable accuracy would also be helpful

Im thinking of getting a digital thermometer as Ive managed to break the glass type in the past. Does anyone have any experience of the Kaiser or Ilford digital thermometers? The Ilford model is almost three times the price of the Kaiser thermometer. Is it worth it? Whats everyone else using out there?

Brian

Chan Tran · Aug 10, 2006

I had the Ilford DT30 a long time ago. I mean over 20 years ago. I like it because it very fast. I don't think it's very accurate though. Right now I use a Kodak Process Thermometer Type III, which is a simple mercury unit but seems to work well.

glbeas · Aug 10, 2006

I found a digital lab thermometer on Ebay a while back that seems pretty accurate and has a very respectable range. It was pretty cheap but can't remember what the brand was.

raucousimages · Aug 10, 2006

I use an old dial type from Besseler. I seems to work good, I check it against a glass kodak now and then. The kodak is great just harder to read.

roy · Aug 10, 2006

jeffeb said:
. I now find myself in the market for a decent thermometer. My priorities are reliability, consistency and long life. Reasonable accuracy would also be helpful Brian

Brian, try RH Designs.

wildbill · Aug 10, 2006

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. $13

vinny

Bob F. · Aug 10, 2006

Paterson also do a "certified" thermometer (it isn't - it's just guaranteed accurate to +/- 0.3C) and so do Kaiser (to =/- 0.2C) - http://www.silverprint.co.uk/dark18.html.

Cheers, Bob.

Jim Jones · Aug 10, 2006

raucousimages said:
I use an old dial type from Besseler. I seems to work good, I check it against a glass kodak now and then. The kodak is great just harder to read.

That may be the best solution. I do use glass Kodak and Unicolor thermometers, but try to keep the darkroom temperature, tanks, and chemicals within a practical range so a wall thermometer often suffices.

Claire Senft · Aug 10, 2006

If you can find one at am affordable price get a Kodak Process thermometer 3. It is a stainless steel clad mercury thermoeter. It is not delicate. It offers accuracy of 1/4 degree at 68 and 100 ºF. These are, in my opinion, the nicest darkroom thermometers ever made.

Tom Hoskinson · Aug 10, 2006

roy said:
Brian, try RH Designs.

I strongly agree with Roy's recommendation. The RH Designs digital thermometer is an excellent design at a very reasonable price. Nice people, in addition!

PeterB · Aug 11, 2006

jeffeb said:
I recently acquired a Paterson darkroom thermometer and I was using it to monitor the temperature of the water in my Jobo CPE. However, I started to have my suspicions about the accuracy of the thermometer when I had to turn up the temperature on the Jobo in order to obtain the desired 24 degrees C. I dug out my Jessops thermometer and I was amazed to find that it had an indicated reading of 26 degrees C. I would expect half a degree difference between the thermometers but two degrees, thats a lot. I then had to dig out a brewing thermometer to see if I had two thermometers that agreed. The brewing thermometer concurred with the Jessops thermometer.

I now find myself in the market for a decent thermometer. My priorities are reliability, consistency and long life. Reasonable accuracy would also be helpful

Im thinking of getting a digital thermometer as Ive managed to break the glass type in the past. Does anyone have any experience of the Kaiser or Ilford digital thermometers? The Ilford model is almost three times the price of the Kaiser thermometer. Is it worth it? Whats everyone else using out there?

Brian

Brian,

If your liquid-in-glass (LIG) thermometer is not calibrated or certified (to NIST or ASTM), then you cannot know its accuracy. Because you do not know its accuracy, how do you know which one is reading correctly? Hypothetically, if your thermometers were only accurate to +/-1 deg C, then a difference of 2 deg C would still be within specification, because (24+1)=(26-1).

Your scale divisions will most likely be every 0.5 degrees (or 0.2 or 1.0). This scale solely determines the thermometer's resolution, not its accuracy.

Your thermometer will also be the partial immersion type which is inherently less accurate than a full immersion type.

Other folks have suggested purchasing the RH Designs digital thermometer http://www.rhdesigns.co.uk/darkroom/html/thermometer.html. I can't see what the advantage of this is, because its quoted accuracy is still only
+/-1 degC over the temp. range of darkroom chemicals.

regards
Peter

rjs003 · Aug 11, 2006

Back in my test lab days, we would check the accuracy of thermometers by first measuring the temp. of ice water ( water and ice in a beaker) then we would measure the temp. of water in a steaming state.
I now use a digital oven thermometer that is accurate to within one degree. Cost $20, at the local Walmart.

Papa Tango · Aug 11, 2006

Think outside the box

RJS has made an important observation. Years ago when boats floated away more than a few of my dollars, I learned that it was not necessary to buy marine grade parts at a boat store, paying a premium. There are quite a few darkroom items that fit this bill also.

I have a conventional dial Beseler thermometer, and it is used inline to check water line temperature. For solutions, etcetera, I use a Polder digital oven thermometer. Reads in tenths, has an alarm to warn when a certain temperature has been reached, and a repeating timer as well. Its calibration was tested against a Beckman Ph/Temp rig and was within slightly one-quarter of a degree of dead on all along the range. Price: under $30 at places like bed, bath, and beyond.

For quicky measurements, one of the little digital cooking thermometers works well, and are usually within one-half a degree accuracy. Price: under $10 at most grocery stores and big box retailers.

Gerald Koch · Aug 11, 2006

I picked up a digital thermometer used by food inspectors. It's quite accurate, I checked it against a mercury lab thermometer. It reads in either F or C. It looks almost exactly like the RH Designs thermometer and it set me back a whole $13.

ricksplace · Aug 11, 2006

I picked up a digital thermometer at a surplus store a few years ago for about $5. The damn thing is spot on to my mercury filled Kodak "Professional" from the 60's. I use the digital for everything now. Easily converts F to C.

pentaxuser · Aug 11, 2006

Brian As you have a Jobo, you could try the Jobo thermometers which fit into either end of the Jobo water bath. I obtained two with my secondhand CPE 2 plus. Both agreed with each other and the Jobo temp dial which sets the element heater. I managed to break one so got another from Nova Darkroom but I am sure there are other stockists as well It was either £3 or £5- I cannot remember which. That one agreed exactly with the one remaining thermometer. That indicated enough consistency for me.

pentaxuser

RJS · Aug 11, 2006

If you check Richard Henry'sbook, "Controls in Black and White Photogaphy" (he was a research chemist with a PhD. and an M.D.) he says almost all thermometers are inaccurate. It takes avery expensive, certifiied one to be accurate. But all you need is one that is reliable (i.e. consistent) then work out your processing times according to your inaccurate thermometer. When the negative is right, the time and temp. are right!
Tp paraphrase O.J's lawyer.

Stoo Batchelor · Aug 11, 2006

Brian

Just a quick paragraph or two from a book by George E Todd:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The digital thermometer seen here (fancy one with coiled wire and probe sitting in sollution) is one of five temperature measuring devices in my darkroom. One is a murcery thermometer calibrated to 0.1* accuracy, while the electronic model differs from the latter by 0.4*C. Between the lowest and highest of all five is a range of 1.6 degrees. This reminds me of a saying attributed to Confucious: Man with one clock knows time; with two clocks not. My point is that, within reason, it doesn't matter, as long as you always use the same yardstick. Absolute values are irrelevant.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

I use a similar one to the R.H designs for everything, bought from Maplins. Recently, I introduced another one on the wall. Again, a digital one from Maplins, with a difference of 0.3*C from the first. Close enough and saves me dipping my chems every session.

Hope this helps

Regards

Stoo

johndeere · Aug 11, 2006

I personaly have so many different ones that my wife thinks I am a collector. I consider the Kodak Process Thermometer 3 the standard. They can be found on ebay for a fraction of their original cost.

I use it often to check my other thermometers.

Bruce Appel · Aug 12, 2006

Another vote for a Kodak proccess thermometer. I went through the same thing a few months back, got one cheap on ebay and have been delighted. In absolute terms I don't know how accurate it is, but it is repeatable, easy to read, built like a tank.

donbga · Aug 12, 2006

Claire Senft said:
If you can find one at am affordable price get a Kodak Process thermometer 3. It is a stainless steel clad mercury thermoeter. It is not delicate. It offers accuracy of 1/4 degree at 68 and 100 ºF. These are, in my opinion, the nicest darkroom thermometers ever made.

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).

Phillip P. Dimor · Aug 12, 2006

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.

PeterB · Aug 13, 2006

donbga said:
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).

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.

regards
Peter

Roger Hicks · Aug 13, 2006

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.

Cheers,

Roger (www.rogerandfrances.com)

PeterB · Aug 13, 2006

Bruce Appel said:
Another vote for a Kodak proccess thermometer. I went through the same thing a few months back, got one cheap on ebay and have been delighted. In absolute terms I don't know how accurate it is, but it is repeatable, easy to read, built like a tank.

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.
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.

http://www.icllabs.com/whyrecalibrate.html

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/html/thermometer.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.

regards
Peter

Thermometers

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