thermometer calibration?

[Mike Anderson]

I'm ready to process film for the first time. I have a new thermometer. Does it need to be calibrated?

I've worked as a butcher, and we calibrated our thermometers by putting them in a cup of ice, and ensured they read 32 degrees. Is that how you do it in photography? Edited: can't do it that way with this thermometer, it doesn't go down to 32F.

...Mike

[Eric Woodbury]

No. Your thermometer could be fine at 32F and way off at 70F.

I assume you are doing BW and as such, it need not be perfect, just consistent, altho it should be within a degree F. I just bought a new dial thermo and compared it to all my others, some alcohol, my old dial, and a mercury thermometer I have left from college chemistry. The mercury has been my standard of comparison for 30 years. They all matched, even the cheapo alcohol. That's my calibration. So they could be off, but all off together. The new dial thermo was about $30.

[BarryS]

Hi Mike--If it's for black&white work, I wouldn't worry. Precision (repeatability) is more important than absolute accuracy. If you're doing color work, you'll need some sort of reference standard.

[Heroique]

Yes, a consistent temperature from job to job is what's key.

For example, if you like results when your thermometer reads "68 F," then you want to be sure that each time your thermometer says "68 F," you're getting a consistent temperature, whatever the "real" temperature happens to be.

What makes inferior thermometers so exasperating is that w/ each indication of, say, "68 F," you may actually be getting varying temperatures. Maybe 67 for one job, and 71 for another, even though your thermometer always says 68...

I used to use analog dial thermometers, because, well, I just like them the best. But then I noticed they had a tendency to vary w/ barometric pressure changes! Maybe the better dial thermometers don't do this.

[Doremus Scudder]

The standard way of calibrating your various thermometers is to have a standard, factory-calibrated, mercury thermometer and then compare your process thermometers to it. The usual base-line photo thermometer is the Kodak Process Thermometer. They are factory checked to be accurate to 1/4° F.

FWIW, if you have a thermometer that reacts linearly, like a mercury thermometer with etched scale or even a non-mercury glass thermometer from a reputable firm, the calibration method you mention is still valid, i.e., using a container of distilled water and crushed ice and seeing if the thermometer reads 32°F or 0°C. Using boiling water is less accurate, since the actual boiling temperature is strongly influenced by altitude and air pressure. If your thermometer is off at freezing point, it may still read linearly, in which case it simply needs an adjustment factor. This, however, would have to be determined against a known, accurate, thermometer.

Mercury glass thermometers are very linear as long as the mercury column has not separated and the scale is etched accurately at the factory. A spirit thermometer with an accurate scale and tight manufacturing controls is also fairly accurate (e.g., Kodak Color Thermometer). That's why a good thermometer from a reputable firm is needed for a base-line.

Unfortunately, metal probe/dial thermometers are notoriously inaccurate and non-linear. What I do is use a base-line thermometer that is reliable and has been checked with the above method (I have both Process and Color glass thermometers) and then bring a volume of water to processing temp. (e.g., 20°C/68°F) using the base-line thermometer. I then immerse all my other thermometers and either adjust them (some have rotating dials for calibration) or use stickers to indicate the error at processing temp (e.g., "reads 2°F over"). I do this once every 6 mos/yr.

This way, I'm assured of being within margin. As mentioned above, you only need to be really accurate for color work.

That said, for my B&W processing, I use a Zone VI compensating developing timer, which adjusts the "seconds" according to the actual temperature; faster "seconds" for warmer temps, slower for lower. This allows me to process at room temperature the whole year, as long as I heat a bit in the winter and keep the darkroom under 80°F or so in the summer. I just then have to make sure that freshly-mixed solutions and the wash water are at ambient temp before processing.

The main thing you need to be careful of with B&W processing is that the solution temperatures are close. Reticulation and other problems can occur if there is a too-large disparity in temps, say more than 2°C or so (I might be a bit conservative here...)

Hope this helps,

Doremus Scudder

[Jon Shiu]

Can thermometers be calibrated against a digital fever themometer in a warm glass of water?

Jon

[Doremus Scudder]

Jon,

The only issue in your suggestion is the accuracy and linearity of the digital fever thermometer that you are using. If these type of thermometers are accurate in the range they are designed to work in, i.e., around body temperature, then you could calibrate against them in that range. Whether such a thermometer reads accurately enough at 68°F or not is another question. Such thermometers may not be linear enough to be accurate at temperatures they are not designed for.

Maybe someone who is more familiar with digital thermometers can comment on the accuracy and linearity of fever thermometers.

Best,

Doremus Scudder

[Lenny Eiger]

Everyone else is correct that repeatability is the key vs accuracy. This is a good thing, every thermometer at the photographic level is garbage. I'll repeat: crap. I have half a dozen glass ones, including a Kodak Process, very old one, and they are all inaccurate - up to 6 degrees off (yes, including the Kodak fancy one). When attempting to purchase a real thermometer I was willing to spend even $80-100. I tried abut 6 of them , they were are supposed to be accurate to within .2 or something, but oh, that only meant at "points" which means 32 and 212. Near 70 they could be a couple of degrees off. A lot of false advertising. I spent a lot over time to get good thermometers and these folks piss me off.

I complained about this last year and a fellow sent me one that is perfectly accurate, probably about $450 or so new, I think. It's a sorry state of affairs. You can not imagine for a minute that the thermometer you are using is accurate, unless you've spent a lot of money.

Be happy about repeatability. At least that's there, and have your own development times for your film.


Lenny

[Bruce Watson]

Can thermometers be calibrated against a digital fever themometer in a warm glass of water?

There is no reason to think a fever thermometer is an accurate standard to calibrate against.

[Bruce Watson]

Everyone else is correct that repeatability is the key vs accuracy.

No, it's not. Repeatability and accuracy are two different and independent concepts.

A thermometer can exhibit excellent precision (repeatability) and horrible accuracy (read 20C when the actual temperature is something else, like 22C or 17C). Such a thermometer would be thought precise, but not accurate. It would work fine for photography once you calibrate your workflow like you would with the Zone System or other method.

On the other hand, you might find a thermometer that you can test over and over and over and find that on average it's very accurate. But it's not precise -- it takes many measurements (that all fall in a range of say 17 - 22C) to get enough data points to average together to arrive at the correct temperature. Such a thermometer would be thought accurate, but imprecise. It would be a nightmare for photography because it would be nearly impossible to correctly calibrate your workflow with a method like the Zone System -- every run would give you different results. Talk about frustration!

I'll take the first one over the second one any day for photography. Precision is my friend. Of course, what one really wants is an accurate and precise thermometer. And as you correctly point out, those can be quite expensive and hard to find.

Edit: Darn -- such a nice rant, and all because I couldn't read that Lenny and I are in violent agreement. Sigh... Sorry Lenny.