Temp measurement and accuracy

[tgtaylor]

I just checked my therometers and the Jobo "color" therometer reads 18.6C (65.48F) and the Jobo "B&W" reads 65.5F. Both are of the mercury type in glass tubes and are kept together. The Patterson dial theromenter read ~65.3F but is was stored inside a cabinet away from the 2 Jobo's but in the same room. Since I live at sea level I instinctively trust the Jobo readings which I use to calibrate the Patterson's. Guessing the temperature with your finger I have found to be several - as much as 10 degrees off mark. Your nose would make for a better probe as it is more sensitive than the finger.

Judgng from the replies it looks like I lucked out and got 3 good therometers

[Bruce Watson]

I have been struggling with temp measurement.
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.
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Which one do I trust ?? Just how accurate are the thermometers ?? I know the thermometers are certainly slower to repond to variations.

You can only trust the precise ones. That is, the ones that are repeatable. They may not be terribly accurate, but if they are within a degree or two they should work fine for photography.

Now, pick one and get rid of the rest. Comparing thermometers will drive you crazy.

[swmcl]

Vaughn, I know you are correct and I do appreciate your reply.

Thanks Leigh for your insights. This thermometer is not able to go below 15C so I can't do the iced water trick. Plus I'm 600m above sea level for the 100C as well ... I might go the McMaster one though ... I should've mentioned the new Extech multimeter is NIST calibrated. McMaster-Carr seems to only ship to the US ... dumb heh!

Bruce ... I'm building a precessing tank on the side. One that is like a big Jobo. I need to find something accurate and repeatable for when I eventually get the project up and running. The project uses thermistors which can also theoretically offer a great resolution.

Thanks all !

(Now I need to find a way of getting the McMaster-Carr probe to Australia ... anyone got a PayPal account with the inclination ? )

[Leigh]

(Now I need to find a way of getting the McMaster-Carr probe to Australia ...

Hi Steve,

I have an open account with McM, so no problem getting the thermometer.

Don't know how much it would cost to ship it and such.

- Leigh

[photobymike]

i use a Kodak Process Thermometer either a type 2 or 3 ... i prefer type 2... I like to buy on ebay but they always send us mail. Besides getting broken the USPS specifically lists the them as hazard and forbids them in the mail system. Ten thousand dollar fine and responsible for clean up is what they say on there website....i have 3 broken when i have received them. Well anyway they are accurate.

[Jim Jones]

Many good labs keep an expensive calibrated thermometer in a safe place to check other thermometers against occasionally and make adjustments.

I've got an old Kodak Process Thermometer that I use for that purpose and a few of the Kodak color thermometers as well. They all seem to read within less than 0.5°C of each other.

The analogue dial thermometers are another story. I've got stickers on them telling me the error at my processing temperature (e.g., "reads +2°F") for the ones with no adjustment. For the others that can be twisted to adjust the reading, I adjust against the process thermometer at processing temp. If it's wrong, at least I'm consistent.

Acquiring a good, calibrated thermometer (the one Leigh suggests would be fine if it is that accurate through its entire range) and using that to calibrate other thermometers to.

BTW, there are lots of other threads on this topic here. Do a search for even more than you wanted to know.

Best,

Doremus

Yes, indeed. Analog dial thermometers should be checked often against a reliable meter. If they are stored together in a darkroom with stable temperature, this is easy to do at room temperature whenever you use the dial thermometer. Keeping the darkroom, chemicals, film tanks, and wash water at the desired developing temperature saves a lot of bother.

[swmcl]

Leigh,

It doesn't matter much ... if McMaster is the only place we know of then the cost must be borne ...

I'll send a PM to discuss further ..

Cheers,

Steve

[Drew Wiley]

Certified thermometers can be obtained from just about any serious lab supply firm, typcially for around $250. Just be sure it's a model optimized for the mid-ranges typical of photo processes. I personally use the Kodak Process Thermometer Type II. Never had good luck with electronic probes due to slow response times. McMaster sells direct to end
user with a credit card. I'd probably try Lab Safety Supply first.

[Louie Powell]

It does not matter how accurate they are (at least within several degrees of "normal"), as long as they have good precision. In other words, as long as they give the same temperature under the same circumstances every time.

Do all your work and testing with the one thermometer and you are set. If you get another thermometer, calibrate it to your original.

Vaughn

Vaughn has the right idea, but it's not expressed clearly.

The issue isn't precision. The term 'precision' refers to the 'fine-ness' of the measurement. A thermometer that measures to 0.1 deg us sufficient for photochemical processing.

The critical issue that Vaughn meant to say is 'repeatability'. That is, if the liquid is 100deg, then the thermometer should register 100 deg every time it measures the temperature of that liquid.

The third measurement concept is 'accuracy' which refers to how closely one instrument compares to an absolute reference instrument. Accuracy is important to achieve predicable results, but the photochemical process does allow for minor variations in temperature if compensating changes are made in time.

But as Vaughn said, of the three concepts - precision, accuracy and repeatability - the most important is repeatability because if your thermometer is repeatable, then the results that you will achieve will be both predictable and repeatable. No one cares about how accurately you measured the temperature of your processing solutions - all they care about is whether your negatives are properly developed.

[Leigh]

...of the three concepts - precision, accuracy and repeatability - the most important is repeatability...

Absolutely correct.

As long as a thermometer gives you the same reading every time for a given temperature,
it will work fine for process control.

Any actual error between what the thermometer says and the true temperature will be compensated automatically
when you calibrate your process. We all calibrate our development process, yes?

- Leigh