View Full Version : Thermometers

21-Jul-2014, 21:06
I decided to start doing some testing, so I thought I would compare my thermometers. Off course they all give different readings of the same container of water.

I have a Paterson Colour Thermometer, A Fieldpiece stick meter with dual temp head callibrated to 32 degrees f. in ice water, and a Raytec infared digital unit.

They read as follows:
Paterson 68.0
Fieldpiece 65.1
Raytec 65.5

In hot tap water
Paterson 115.0
Fieldpiece 110.7
Raytec 105.5

How have you dealt with this issue?
I do plan on doing some color negatives and slides so accurate temperature is critical.


neil poulsen
21-Jul-2014, 21:33
Year ago, I purchased two electronic thermometers and mounted them in my darkroom. Each has a metal tube that can be inserted into baths to determine temperatures. They have a red readout that can be easily seen in the dark. They can also be calibrated by adjusting a knob. I keep a Kodak, long mercury thermometer that's accurate to a half degree on hand for calibration. As long as the Kodak doesn't break, I know that I can have consistent readings over the long term.

Tin Can
21-Jul-2014, 21:50
All measurement tools need yearly calibration.

Or pick one device as your master calibration and use it for 'your' standard.

I use a VOM with thermocouples.


21-Jul-2014, 21:57
Before you choose your thermometer, be sure you fully understand the difference between "accuracy" and "precision" on all levels, plain and subtle. :)

Arne Croell
21-Jul-2014, 23:15
Calibration is essential, as Randy and Neil pointed out. It is also important to distinguish between a fixed error and drift of an instrument. The three units you have all use different measurement principles with different error sources. The Raytec is a Pyrometer and therefore subject to variations in emissivity of the surface you measure, in addition to possible electronic drift. The Fieldpiece seems to be thermocouple-based, and thermocouples not only have large absolute errors, but also tend to drift (their advantage is a fast reaction time). Both types are not the best choice for precise darkroom measurements. A regular precise thermometer like the one Neil mentions is good, or for electronic units, thermometers based on resistance like a Pt 100 or Pt1000 type are the most accurate. Out of the three you have , I would give the benefit of the doubt to the color thermometer - it might have some absolute error, but its unlikely to drift over time, and an absolute error such as in the scale position, can be checked with ice water.

22-Jul-2014, 03:54
Try ice water, that will short them out.

Bruce Barlow
22-Jul-2014, 05:10
You'll have to pry the Zone VI Compensating Developing Timer out of my cold, dead hands. Using it, and only it, I can calibrate all my times and be consistent. Works for me.

Otherwise, a cheap thermometer for measuring wash temperature and such is way close enough for photography.

22-Jul-2014, 05:17
Use the glass one as your standard and calibrate the others to it.

Jim Noel
22-Jul-2014, 09:23
I have 5 dial thermometers which are regularly adjusted to match my Kodak Precision mercury filled one from the 1940's. I measure the boiling point, freezing point in a container of ice water left sitting for 15 minutes,and at 100 deg F.

22-Jul-2014, 09:37
Unless you break it, any glass thermometer with an engraved scale is going to retain its calibration forever. If the Paterson was made for photography, it was probably calibrated for best accuracy in the 68-75 F range, and should be entirely adequate for even color processing. If it is a general-purpose thermometer, the accuracy may not be there, but if you know someone with a Kodak Process Thermometer (there are color versions for 75F processing and B//W at 68F) you can locate your target temperature on the scale of any other suitably precise and stable thermometer. Ideally, you will have one "reference" thermometer and a "working" thermometer which can be lost or damaged without too much grief.

Bear in mind that being able to measure the temperature is not the same thing as being able to control it...

Lenny Eiger
22-Jul-2014, 17:18
Glass thermometers aren't what they seem to be. They can all be inaccurate, including the Kodak ones. I had one of the expensive ones, and it was many degrees off. I finally got a real scientific thermometer that I could depend on. Truth is, all glass, and scientific, etc., are very good at being repeatable. That's what matters.

They aren't good at being accurate. They can be as much as 6 degrees off. That's quite a bit, in photographic terms. The less expensive digital ones are calibrated at "points" which means 0 degrees C and 100 degrees C, which is useless to photographers, for the most part.

If you are interested in accuracy, you need to get something made for a science lab... there are plenty around, $100-$500.


Hans Berkhout
22-Jul-2014, 18:05
If you decide to use a glass one keep in mind that you will break it sooner or later. So always have an identical back-up one and make note how it's reading will differ from the one you are (or have been) using.

John Olsen
22-Jul-2014, 19:37
I have five thermometers, which I've tested versus my baseline Weston. The comparisons are stored in my notebook, so if I suspect the Weston is going haywire I can do a cross check. I learned my lesson when a different thermometer went belly-up a few years ago.

Eric Woodbury
22-Jul-2014, 21:24
The Raytek specs are bad. +/- 3.5 degrees and repeatability a tenth worse. IR depends on emissivity first, followed by a host of other variables. I use a mercury tube type from a chem lab. Certainly it is repeatable. Then calibrate yourself to this and call it good. Doesn't really matter after that. And what's a degree here or there?

Drew Wiley
23-Jul-2014, 08:25
I standardize on just one really good thermometer. It's a Kodak Process Thermometer Type 3. Faster to use and more reliable than any electronic thermometer I've
owned. Very accurate CERTIFIED thermometers are still readily available from scientific supply houses. The trick is to find a model optimized for the temperature
ranges typical of darkroom work. Expect to pay two or three hundred dollars. But used Kodak ones often turn up at lower prices. There were themselves expensive
new, of course. The most important thing besides the repeatability of a given instrument is simply to stick with it, as your personal standard. For fussy work like
color separation negatives I like to keep process temps within 1/10 degree F. That requires an expensive thermoregulator for the water bath. No mixing valve or
immersion heater has anywhere near that kind of accuracy. But for ordinary work it's sheer overkill, and a simple passive water jacket is sufficient, or in the case
of something like RA4 paper development, a basic Jobo tempering box for keeping the chemical flasks within parameters.

23-Jul-2014, 09:16
You'll have to pry the Zone VI Compensating Developing Timer out of my cold, dead hands. Using it, and only it, I can calibrate all my times and be consistent. Works for me.

Otherwise, a cheap thermometer for measuring wash temperature and such is way close enough for photography.

Well, if my CSI–sense serves, at least we will know accurately your time of death! :D

I love mine as well, but never thought about how to calibrate my other thermometers with it. Yeah, for most B/W procedures, I'm good to within 1ºC or so – I love B/W.

Drew Wiley
23-Jul-2014, 10:18
I use that Zone VI goodie routinely for black and white paper developing or garden-variety drift-by water jacket monitoring around sheet film trays. I would love to
have a spare. Not really suitable for critical sensitometry; but 90% of what one does in a darkroom is not that.

21-Sep-2014, 19:53
I finally found a solution that I find acceptable to me. I ordered this thermistor (http://www.gadgetcat.com/product/10k-precision-epoxy-thermistor-3950-ntc/), and a Legacy Pro adjustable dial thermometer from Freestyle. The thermistor is accurate to plus/minus one quarter degree C.

Comparing the Paterson to this show the Paterson to be fairly close but very slow to react (it is in a protective clear tube).

4-Oct-2014, 21:42
Comparing the Paterson to this show the Paterson to be fairly close but very slow to react (it is in a protective clear tube).

Protective tube is for storing, not to be used with.

4-Oct-2014, 21:44
... Kodak Process Thermometer (there are color versions for 75F processing and B//W at 68F)

How can we distinct theses 2 versions?
I have 3 Kodak Process Type 3.

5-Oct-2014, 12:19
Lab guy here, I take the liberty of tuning in.

Glass thermometers are available in two categories: semi-accurate, semi-expensive thermometers used for "checking" temperatures, and relatively accurate, relatively expensive thermometers that can be used to "measure" temperatures. There is some gray area in between, but usually, you will find either category at a lab supply store.

For the accurate temperatures needed for film processing, the checking version is not enough, as the error of the measurements with these thermometers may easily exceed one Kelvin. The measuring variety normally comes with some information on the accuracy of the measurements made. If you need a maximum error of 0.1 °C, you can buy one that will give you that. If you need 0.01°C, you may still be lucky, but they will be harder to find. If you need 0.5°C, you'll have an exceedingly large selection to choose from.

Thermometers that use a probe of some sort usually can be calibrated at the factory, but getting a decently accurate one is expensive. If you don't need the logging functions they usually provide, stick with glass. I would not recommend home-calibrating, as it can easily go wrong.

As far as the aging of thermometers is concerned: They do, and as a matter of fact the resulting error can be quite large. Mercury is especially prone to this, as small bits of the column can break away form the rest, forming small droplets in the tube. This will result in a decreased level of mercury, which in turn results in lower temperatures measured.

The only way of having accurate thermometers is buying one of the desired accuracy and replacing it every so and so years.



5-Oct-2014, 18:06
My two cents.
Pursuing an accurate measurement of the temperature is always important, specially with color development but a good thermometer is just part of the equation.
After a few comparisons I found the Jobo 3321 to be an excellent thermometer. I have a few copies and all are +/- 0.1 C. I also gathered a few "fever" thermometers and compared readings.
The key is the temperature inside the film tank, which shifts (at least with the jobo processors) a 10% towards the room temperature. I do most of the development with a mod CPE2, checking both, the room temperature and the bath under the tank.