Which thermometer?

Since my last mercury thermometer broke, I am down to 2 of the blue liquid filled types and 1 with a round dial on the top. Unfortunately they all read slightly differently. The type that clip on with the dial that can be easily seen are great but not if they are not accurate. Can anyone recommend an accurate dial type available in UK please?

Also, what are thoughts on digital thermometers?

Digital much more accurate than mercury.

Actually, I was gifted with a wonderful electronic cooking thermometer, the Thermopen. I would absolutely trust it in the additional role of darkroom thermometer. They are not inexpensive, but earn their keep in the cooking role alone.

It is really not possible to generalise on the precision of technologies in this case, it is all about calibration and tolerances.

In my experience, there is a steep price jump from consumer products to lab grade products and the question is if it is worth it.

 

Put all your thermometers in your house next to each other and I am almost certain you will get as many different readings.

I would suggest that you assign a reference thermometer - it doesn't really matter if it measures correctly as long as it is consistent; because it doesn't matter if you develop @19.5°C or 20.5°C as long as you do the same every time.

Make sure that you know how much your other thermometers deviate from the reference thermometer to mark and use them accordingly.

 

As far as thermometer properties goes, I would personally look for a thermometer that can be submerged fully in water - anything else would be impractical for me.

Responsiveness is also something I would consider important.

Edited April 25, 2022 by NHSN

"Make sure that you know how much your other thermometers deviate from the reference thermometer to mark and use them accordingly."

....In practical terms; I put a little piece of waterproof P-touch tape to indicate the "correct" 20°C mark on all the non-reference meters I use for photo chemistry.

It is a little less intuitive to do with digital meters, but on my digital room thermometers I put a deviation sticker for example "+0.7°C", indicating that I should add 0.7 degree to the reading to reach the real temperature according the the reference room thermometer.

It is really not possible to generalise on the precision of technologies in this case, it is all about calibration and tolerances.

Agreed 100%.

Liquid-in-glass thermometers also need to be immersed to the correct level to be totally accurate - even if certified. This could be a fixed immersion line or up to the level of the spirit or Mercury.

 

Digital thermometers have no such restriction as long as the small sensor tip is full immersed. However, cheap digital thermometers can well be out by a degree or two and definitely need 'calibrating' before serious use. But once you know the error, their repeatability is very good and they're extremely robust.

 

I used a cheap digital thermometer for years, knowing that it had been checked against a laboratory standard mercury thermometer. I just stuck a label on it saying 'subtract 0.7 degrees from reading @ 20 C'. As long as any error is known it's not an issue.

 

P. S. Seems like that pesky +/- 0.7 degrees is quite common then NHSN.

Edited April 25, 2022 by rodeo_joe|1

I use one of these in my Jobo Tank: https://www.amazon.com/Zacro-Aquarium-Thermometer-Terrarium-Temperature/dp/B01A0TMS6Y

When I was processing B&W film regularly for my studio I used Weston dial thermometers that I calibrated once a month with a Kodak process thermometer (mercury). Unless I dropped them they never deviated by more than 1/2 degree F over that period of time. I still use that Weston occasionally and my film comes out well.

I use exactly the same process as AJG. I calibrate my Weston Dial thermometer to Kodak process thermometer. I only use the Kodak thermometer for calibration purposes. There has been very little deviation over the last 40 years.

Pick one, try it, adjust according to results, then stick with that one.

 

Like light meters, it is rare for commercial-grade items to agree.

 

Generally speaking, where n=number of measuring devices, the number of different readings will also be n.

Thanks all. Helpful replies there.

When I was processing B&W film regularly for my studio I used Weston dial thermometers that I calibrated once a month with a Kodak process thermometer (mercury). Unless I dropped them they never deviated by more than 1/2 degree F over that period of time. I still use that Weston occasionally and my film comes out well.

 

I found dial thermometers just more convenient to use. No batteries, no switches, always ready to go.

They respond quickly vs. some of the liquid thermometers, especially the ones which are inside a glass bulb.

As @AJG said, as long as you calibrate it against a "reference" thermometer, you are fine.

An interesting discussion item.

I talked to an engineer once, and he told me that too much accuracy is a practical problem.

 

Example, if you have a lab grade digital thermometer that has 0.01 degree accuracy.

You might end up "trying" to get the temp EXACTLY to say 68F +/- 0.01 degree.

When in practice +/- 2 degree F is "good enough."

Why, cuz the developer table for developer X has negligible time difference for until there is a 2 degree difference.

So why go through the extra effort to nail the temp to 0.01F ?? Because the accuracy of your thermometer tempts you to do so.

 

So, for a digital thermometer, I would ignore anything below the decimal point.

 

Different developers have a steeper or shallower temp/time curve, you have to study the developing table for YOUR developer, to determine how much accuracy you really need.

And then again, unless you have a thermostatically controlled tempering bath, and pre-warm your tank and reels, who knows what temperature your developer ends up being when it hits the film? Or what temperature it is at the end of the process?

 

It's well-known that holding an SS tank in the hand can actually raise the temperature of the liquid inside above 20 degrees, but maybe this compensates for a temperature loss in a cold room?

For B&W film, there's little point getting paranoid about these things, 'cos generally "close enough is good enough".

 

Plus, the major variable and cause of developing faults, from posts on this forum, is not grasping the importance and physics of a good agitation regimen.

Accuracy is how well the thermometer indicates the actual temperature. Precision is how closely it reads the temperature - how many decimal places or fractions of a degree it can read. But for darkroom work, accuracy and precision aren't as important as the third characteristic - repeatability.

 

To achieve optimum results, you want the process to become calibrated over time. Until the process is calibrated, you have no way of knowing which processing variable to adjust to get desired results (normally, the variable that would be tweaked is time with all other processing variables held constant). And to have a calibrated process, the thermometer must always read X when the actual temperature is X - but you don't really need to know what X is.