Thermometers

I have had to revert to using my remaining mercury thermometer. After 27 years the sudden death of a Kodak electronic digital one forced me to change. Now that digital one was accurate until it stopped working and I never had a problem with film developing when using it. I used to check it against the mercury one at intervals and they never ever varied by more than .25 of a degree. This was even older than the Kodak electronic one.

After I started using the mercury one again, at 1st it was fine with the developed films being spot on. Now with the past half dozen of so colour films I have had a series of under exposed negative strips (Colour Negative). There are no parts of the mercury broken away from the main column which could make it give a false reading.

I always though that a mercury thermometer was un-adjustable and once made that was it. so why suddenly the deviation? It has never been dropped!

Nothing else has changed. The development times always 3min 15 secs. Used once and discarded. The developer was Tetenal and correctly diluted. (The same developer used before the electronic one stopped working) The only difference was the film, some were Fuji, some were Kodak and few Agfa.

Last night I decided to experiment and increased the temperature to 39.5c instead of 38 and the negatives were terrific. So the question is, why has a tried and trusted mercury thermometer suddenly started under reading where before it was spot on?

I have a Kodak glass thermometer that the fluid inside has separated but the section from the metal end up is correct. I use a Taylor 9040 digital thermometer. Its within .5°F.
https://www.ebay.com/itm/Taylor-984...910448&hash=item4611f718cf:g:97wAAOSwhfdaX7s4

A fluid stem thermometer keeps its accuracy.

Unless
-) the stem got interrupted
-) one handles it differently, as for instance dipping it more or less than before.

Be careful about the 'nothing else has changed' part. Check your clock, for one thing. I have seen clocks vary with age. Room temperature variations are another area. A well looked after mercury thermometer is going to be consistent if used consistently. There should only be one moving part after all!

Now, it may be that you have been adjusting for a digital thermometer error over time, and not realized. Now that that unit is dead, you cannot re-check.

Just to kill one theory, my clock(s) are both digital and are more or less accurate with each other allowing for operator accuracy in pressing the start switch. The mercury thermometer has been compared with another digital thermometer belonging to a fellow darkroom worker and there is a small difference of approx .5 degree C with the mercury one showing the temperatures a little cooler.

Actually, this evening I found a supplier for a digital thermometer which appears the be an exact copy of the Kodak one but with a different logo on the case. It is claimed in the advertising blurb to be .25 of a degree C accurate at an ambient temperature of 70 degrees C. That is good enough for me. The only problem is the price. I paid I think around £25 (UK) for the Kodak version back in 1992, this one is 5 times the price.

I don't find digital thermometers to be more accurate than my Kodak Process Thermometer 3 unless you get a very expensive one. .

I agree with the other comments here. A mercury thermometer will always be accurate, as long as the fluid stream is not interrupted. Check that the mercury column is one continuous stream of mercury. Make sure there is not any mercury "dis-connected" to the main column. If you find that you do have some separated mercury in the column, you can put the thermometer in warm water to get the column to rise and reconnect with that disconnected mercury. You may have to put it into cool water and warmer water several times to get the column to rise and fall to touch those dis-connected bits, and get them back continuous.

You need to find some standard that is within the range of the thermometer you have. If you do have a digital thermometer with the range thru freezing to boiling. You can check the calibration of that digital with ice water and boiling water. A beaker half full with ice, rest with water, stirring constantly with the temperature sensitive portion of the thermometer touching only the water, you should get a reading of about 3 to 4 °C. Boiling water, with the temperature sensitive portion of the thermometer touching only the water and not the bottom of the pan, and you should have 100°C (unless you are like in Colorado (high altitude), but tables are available to get the boiling point of water at various altitudes. If it is in calibration at those 2 points, it is most likely in a linear calibration throughout the range between these 2 points. You could then get some water at the temperature you want to check your mercury thermometer at, and because now you feel confident in your digital, use the value of your digital and compare it to your mercury.

For a direct check on your mercury thermometer, you will need to find some standard that is within the range of that thermometer. I do not know the range of your mercury thermometer. It probably does not go down to 0°C (32°F) or up to 100°C (212°F), so you couldn't put it in ice water, or boiling water to check for accuracy at those points. Acetone boils off at about 56°C, however that is very flammable and a thermometer calibration using this method should only be done by trained personnel in laboratory safety chamber (not photo lab!).

Another thought is that a thermocouple is very inexpensive, however you would need a very accurate (calibrated within the last year) DC milli-volt meter to use this method, and probably not an instrument that everyone would have.

Does anyone else have some method that they use to check their digital or dial thermometers? Something that would not require the tools and test equipment associated with an industrial instrumentation technician?

BMbikerider said:

I have had to revert to using my remaining mercury thermometer. After 27 years the sudden death of a Kodak electronic digital one forced me to change. Now that digital one was accurate until it stopped working and I never had a problem with film developing when using it. I used to check it against the mercury one at intervals and they never ever varied by more than .25 of a degree. This was even older than the Kodak electronic one.

After I started using the mercury one again, at 1st it was fine with the developed films being spot on. Now with the past half dozen of so colour films I have had a series of under exposed negative strips (Colour Negative). There are no parts of the mercury broken away from the main column which could make it give a false reading.

I always though that a mercury thermometer was un-adjustable and once made that was it. so why suddenly the deviation? It has never been dropped!

Nothing else has changed. The development times always 3min 15 secs. Used once and discarded. The developer was Tetenal and correctly diluted. (The same developer used before the electronic one stopped working) The only difference was the film, some were Fuji, some were Kodak and few Agfa.

Last night I decided to experiment and increased the temperature to 39.5c instead of 38 and the negatives were terrific. So the question is, why has a tried and trusted mercury thermometer suddenly started under reading where before it was spot on?

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a man with one thermometer knows exactly what temperature it isa man with two can never be sure.

I would always consider a quality mercury thermometer in good working order a valid reference point to calibrate any digital thermometer against (unless the digital one is a calibrated high end device), not the other way round.

Mercury thermometers should not be used for actual measuring in a darkroom as the risk of accidentally breaking them and spilling mercury is simply too high. They should be put aside in a safe place and kept as reference. Also, digital immersion thermometers are usually much quicker in readout than fluid ones, which is a real benefit when you are dealing with quickly drifting temperatures in a color process. The cheap digital ones are not accurate enough for color processing and should be calibrated.

It is difficult to tell what exactly caused your problems, as there are so many possible influental factors outside the accuracy of your thermometer that come to my mind. Also, the way you are taking the actual measurements can have influence on the results. Fluid thermometer need time to expand or contract to the correct value. The depth of immersion will also have influence on the results.

RalphLambrecht said:

a man with one thermometer knows exactly what temperature it isa man with two can never be sure.

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Sounds good, but works only as long as your only thermometer doesn't break! Then you can hear the wise man with one thermometer yelling and swearing because he realizes that he has no idea how the measurements of the new thermometer compare to those of the old one and he wished he rather was the foolish man who had kept a second thermometer as reference point in his drawer ... ;-)

REAndy said:

A mercury thermometer will always be accurate, as long as the fluid stream is not interrupted.

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And as long as the glass tube has not been displaced/moved with respect to the scale.

MattKing said:

And as long as the glass tube has not been displaced/moved with respect to the scale.

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MattKing, you are absolutely correct. In my limited darkroom experience, nor in my work experience I have never seen/used a mercury thermometer that was attached to a card. The scale graduations was marked on the glass rod of the thermometer itself.

Now, like you describe, my oven thermometer, is a fluid-stem thermometer (think regular old fashioned mercury thermometer, but not mercury as the expanding fluid) is exactly like that. A glass rod with bulb on one end and filled with expanding fluid (not mercury in this case), and it is attached to a metallic scale. And yes, you can slide that. The reason probably is that these can easily mass produced.

An ANSI certified thermometer will never be attached to scale. Because as you point out, it can be moved/slid with respect to the scale. (which, by the way, is how I calibrated that cheap oven thermometer I was just talking about. Unfortunately, you can only do a single point calibration with this type of thermometer, so you have to calibrate at the point of most critical work.

RalphLambrecht said:

a man with one thermometer knows exactly what temperature it isa man with two can never be sure.

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Great reply!
I have been using the same Kodak glass thermometer since early 1970s. Several darkroom dial thermometers show very slight variations, but if variations are consistent it makes no difference when iprocessing, since consistency is the key to successful development.
As for the hazards associated with a mercury thermometer, I went to high school when real chemistry was still taught and we conducted experiments with mercury. Over 65 yrs later most of my classmates are still alive and kicking. We were taught how to handle hazardous chemicals. My high school chemistry class prepared me sufficiently for a mineralogy class in my freshman year at university.

I use a cheap Chinese food probe/thermometer, my negatives always come out fine, both C-41 and Black and white, if it broke I wouldn't hesitate to get another one.

RalphLambrecht said:

a man with one thermometer knows exactly what temperature it isa man with two can never be sure.

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But with 3 odds are 2 will be correct.

I distinctly prefer the Kodak Process Thermometer (later style) to any digital thermometer. Faster reading, more accurate. Originally pricey, but they still turn up from
time to time. Otherwise, scientific lab supply houses offer equivalent thermometers for around $250

TonyB65 said:

I use a cheap Chinese food probe/thermometer, my negatives always come out fine, both C-41 and Black and white, if it broke I wouldn't hesitate to get another one.

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Me too. However, I had to throw away one because it was never reliable. I didn’t use it for the darkroom though, as it’s scale didn’t even start until 100 F. One thing with the cheap ones if you have to submerge a large portion of the shaft to get an accurate reading. I also have a pair of mercury thermometers. I don’t use them because they’re too large and bulky. But they agree with the grocery store thermometers I do use (within a half degree Fahrenheit).

I wouldn’t worry about the health risks of mercury. If one breaks, don’t eat it. Yeah, it can absorb through your skin, but if you eat seafood or drink tap water, you’re likely being exposed to larger amounts of mercury per year than if you had a thermometer accident.

Don't forget most thermometers are calibrated to 76 mm immersion. Back in my working chemist days we used a thermometer clamp attached to a ring stand. It also takes longer than most people think for a proper lab thermometer to stabilize.

Some of the inexpensive electronic digital thermometers are quite good. A thermocouple is a darn good primary standard.

I have been using a nice color by Beseler, Japan made dial thermometer for 30 years. Every now and then I get out a half a dozen much more expensive thermometers to check, I adjusted it once many many years back.

I bought 3 lcd electronic aquarium thermometers off ebay. Damn things are dead on to my Kodak process thermometers and all 3 match each other. Response time is dismally slow. I use these for big water baths etc.