Accurate digital thermometer for chemicals

mklw1954 · Sep 26, 2020

I use a digital thermometer (Extech TM20, with a wire probe) after calibrating it to a Paterson Colour Thermometer, simply by seeing what the digital value is when I get a pot of water to the temperatures I want as measured by the Paterson thermometer. For example, 68F on the Paterson = 69.2F on the Extech. I also calibrate for C41 color film and RA-4 color print temperatures.

Michael Teresko · Sep 27, 2020

AgX said:
Do not let you fool by that decimal. It does not tell anything on precision.

Certified +/- 0.5°C at tested points. The certificate is probably expired, but as long as it's consistant (which it seems to be), I'm happy.

russell_w_b · Sep 27, 2020

If you have a digital multimeter you could always buy a Fluke thermometer probe for it. Something like 1.2% accuracy and can be calibrated?

AgX · Sep 27, 2020

If one considers the digital thermometer as the one in current use, then having one dedicated to the darkroom seems the better option to me.
Moreover one needs a dedicated or at least modern Multimeter to have a direct read-out of the temperature and not a voltage to convert by table into a temperature.

Bikerider · Sep 27, 2020

To be honest, absolute precision is not really needed, what is more important is consistency of measurement. If a thermometer reads 34c instead of 35c and the next day the measurement is the same and so on for ever after, there is no problem For colour neg you do have a certain amount of latitude and testing one film development gives slightly under development then increase the time by about 10 seconds for a 1 degree discrepancy. I have 3 working thermometers, a Kodak digital, a mercury certified and a dial type. they all vary by no more than .5 of a degree between each other, but the readings are consistent and I have no problem with that. If you develop regularly then you soon get to realise when the temp is too low or too high.

thuggins · Sep 27, 2020

Go to the drugstore and pick up a $2 oral thermometer. It will be as accurate as any "calubrated" lab thermometer.

Chan Tran · Sep 27, 2020

thuggins said:
Go to the drugstore and pick up a $2 oral thermometer. It will be as accurate as any "calubrated" lab thermometer.

If you your process need temperature around the human body temperature. These thermometers are accurate within a narrow range so that they can be accurate and cheap.

radiant · Sep 27, 2020

Pretty accurate sensors are really cheap so I assume most of digital thermometers are pretty accurate.

I tested one cheap indoor/outdoor meter with calibrated thermometer and the difference was under 0.5 degrees celcius.

Donald Qualls · Sep 27, 2020

Chan Tran said:
If you your process need temperature around the human body temperature. These thermometers are accurate within a narrow range so that they can be accurate and cheap.

Human body temperature is just over 1C below recommended temperature for C-41 and E-6 -- well within the calibration range of fever thermometers. The problem with these for our purposes is that they're not continuous reading. The old mercury ones had a restriction in the capillary so they showed only the maximum, then had to be "shaken down" once they cooled, to return the mercury to the bulb for another run. In emulation of that, modern digital fever thermometers have an internal timer, or (for the more expensive ones) actually track when the temperature stops rising, and then sound an alarm, while holding the display so you can read off the result. Once again, you get a single, maximum reading for each run.

We really need a continuous reading thermometer for what we do; we don't want to know that our tempering bath has, sometime, reached 38.5C; we need to know what temperature it is right now on a frequent basis.

grat · Sep 27, 2020

The Amazon version of that thermometer (DT310LAB), once I tracked down a manual (at Home Depot of all places), says:

Measurement Accuracy: ±1.8°F (±1°C) between -4° and 248°F (-20° and 120°C); ±3.6°F (±2°C) elsewhere

That's a bit much for C41, in my opinion.

DREW WILEY · Sep 27, 2020

First, typical electronic thermometers tend to respond slower than glass ones. Second, you don't get something for nothing ... Which third, explains why a lot of things showing up on sites like Amazon have their specifications calculated using a substantial BS coefficient. In the long run, it's probably going to save you money as well as time to seek out a deal on a Kodak Process thermometer rather than potentially wasting a lot of film and chemistry learning the hard way.

grat · Sep 27, 2020

I have never seen a quality thermistor with a slower response time than a liquid thermometer-- which isn't surprising, since you've got to heat up the entire column of liquid to get an accurate measurement.

DREW WILEY · Sep 27, 2020

Egad. What are you trying to temper anyway - a mudpuddle? The point is to mix your water just under specified temp, as indicated by a quick reading, then allow the water bath heater to bring it up to spec, along with the chem containers. But even if you have the luxury of a precise thermoregulator or mixing valve, you still need an accurate thermometer to check it. Within it's specified range of usage, a certified process thermometer takes about 3 sec !

AgX · Sep 27, 2020

DREW WILEY said:
Second, you don't get something for nothing ... Which third, explains why a lot of things showing up on sites like Amazon have their specifications calculated using a substantial BS coefficient.

That Is why far above I stated a price of what I think It will be hard to find something god below. Although that meter design is from the 90's and likely the device could with current design and automated production made cheaper. And the prices for digital oral thermometers are plain amazing. But Drew showed that not all cheap is that precise.

Bikerider · Sep 28, 2020

grat said:
I have never seen a quality thermistor with a slower response time than a liquid thermometer-- which isn't surprising, since you've got to heat up the entire column of liquid to get an accurate measurement.

I beg to differ. The expansion of the Mercury or spirit in the bulb at the base is enough to get your reading. What happens in the tube is of little consequence! Actually my Kodal colour thermometer is quite slow - perhaps it has something to do with the age of the instrument - it is at least 40 years old but when it gets there it is still accurate.

radiant · Sep 28, 2020

Good therapy for this thread to watch this video:

(developing starts at 3:48)

AgX · Sep 28, 2020

Bikerider said:
I beg to differ. The expansion of the Mercury or spirit in the bulb at the base is enough to get your reading. What happens in the tube is of little consequence! Actually my Kodal colour thermometer is quite slow - perhaps it has something to do with the age of the instrument - it is at least 40 years old but when it gets there it is still accurate.

Not quite. With precision fluid-stem thermometers it is prescribed per model how deep to insert the stem.

jim10219 · Sep 28, 2020

Bikerider said:
To be honest, absolute precision is not really needed, what is more important is consistency of measurement. If a thermometer reads 34c instead of 35c and the next day the measurement is the same and so on for ever after, there is no problem For colour neg you do have a certain amount of latitude and testing one film development gives slightly under development then increase the time by about 10 seconds for a 1 degree discrepancy. I have 3 working thermometers, a Kodak digital, a mercury certified and a dial type. they all vary by no more than .5 of a degree between each other, but the readings are consistent and I have no problem with that. If you develop regularly then you soon get to realise when the temp is too low or too high.

This. I use a cheap digital meat thermometer off of Amazon. You can even calibrate it if you like with a glass of ice water. There are like 100 different brands of this thermometer out there, but they're all the same internally. I got the one with the bottle opener attachment.

But there's not really a point in worrying about calibration for this sort of thing. All you need is repeatability. And most digital thermometers shouldn't have any problem with that, so long as you use the right battery. You shouldn't just blindly just follow instructions if absolute precision is your goal anyway. You need to run experiments to find out what times and temperatures work best for your equipment and workflow. Otherwise, good enough is good enough.

Donald Qualls · Sep 28, 2020

Unfortunately, for color, absolute temperature does matter. You can be absolutely consistent at 97F (because your thermometer reads consistently high) and you'll get color casts and/or crossover. Absolute consistency at 103F will get you the same kind of problem in the opposite direction. And adjusting time for temperature, as I routinely do with B&W (4% per degree F has been burned in my brain for many years), will just complicate figuring out what went wrong from the end result.

That said, if you have a process that's working for you, giving you prints or scans you're happy with, then there's no reason to spend a bunch of money on equipment that, by its nature, will change the process that's working for you.

Bormental · Sep 28, 2020

Well, I actually tried the same experiment as on the video, "let's shoot a roll and develop worry-free" and yes, that roll had a cool/cyan color cast. It worked for some photos but absolutely not for others. Here's an example:

This was developed without temperature compensation, i.e. the chemicals were heated to 100F and the tank was held under hot water for maybe 20 seconds before pouring the developer in. The tank was also oversized (3-reel tank, with just 1 loaded reel) so my guess is that the temperature dropped 4-6F and possibly more.

Also, this color shift was hard to correct after scanning. Here's one of the images that was completely ruined IMO:

Another experiment I want to do is to develop at a higher temperature and see what kind of color shifts appear. Some color casts are more pleasant to the eye, so maybe artifacts of over-developing are more desirable.

Donald Qualls · Sep 28, 2020

The temperature drop is a large part of why (for instance) the Cinestill C-41 kit gives a solution temperature of 102F -- if you have a 4F drop over the 3:15 process time, your average will have been 100F (= "just right").

This kind of drop is much more likely than not with any home process -- we don't have a 5-10 L thermostatically managed tank of nothing but developer, and bring the film to the solution; we have a small tank starting at room temperature (along with the film) and bring the developer to the film. Getting around this with tempering baths is greatly complicated by the fact that both partially filled tanks and partially filled solution containers will float in the tempering bath, leading to a lot of chaos at a time when you really, really don't need chaos.

It doesn't help that the process time of 3:15 is most of two minutes below the minimum time Kodak recommends for B&W processes to avoid uneven or inconsistent results due to fill and drain times being too large a fraction of the process time. That short time is great for a machine that needs to go dry to dry in under twenty minutes (so they can then print your images and get your negatives and prints back in your hands within an hour); it's not so great for hand processing. If we could get a C-41 compatible process that would give results that pass the various quality hurdles, and takes 5;30 with agitation every minute, instead of 3:15 with agitation every 0:15, everything about it would be easier.

Nodda Duma · Sep 28, 2020

Witold said:
Sure there is, search for Termoprodukt DT-1and be sure to search for english site. I have it. It can be had with cal certificate too. High precision, fast response. I spoke about it some time back on this forum, so if you search it here it should come up too.

I was intrigued so I checked this thermometer out on their website. They don’t seem to ship to the United States, though?

-Jason

Nodda Duma · Sep 28, 2020

Donald Qualls said:
It doesn't help that the process time of 3:15 is most of two minutes below the minimum time Kodak recommends for B&W processes to avoid uneven or inconsistent results due to fill and drain times being too large a fraction of the process time. That short time is great for a machine that needs to go dry to dry in under twenty minutes (so they can then print your images and get your negatives and prints back in your hands within an hour); it's not so great for hand processing. If we could get a C-41 compatible process that would give results that pass the various quality hurdles, and takes 5;30 with agitation every minute, instead of 3:15 with agitation every 0:15, everything about it would be easier.

Drop your dev temp to 95F or lower.

Donald Qualls · Sep 28, 2020

Nodda Duma said:
Drop your dev temp to 95F or lower.

But then we get color shifts and crossover, because the relative development of the color layers depends on diffusion, as well as activity, and both depend on temperature. Just a degree or two will give a a cyan cast (in the positive) and crossover, as in Bormental's example ("hard to correct in postprocessing" is one of the marks of a subtle crossover). This isn't passing the quality hurdles.

I've heard of running C-41 chemistry at 75F for C-22 films; this will actually get color images without melting the emulsion, if you have a formalin-bearing stabilizer to convert the dyes -- but there's no expectation, in this case, of getting accurately represented colors (wrong color developer for the couplers anyway); "It's enough of a wonder that the bear dances at all." Further, the Dignan 2-bath formula works at temperature from 75F on up, but 2-bath works very differently from canonical C-41; all of the diffusion takes place before the development starts, and then the development runs to exhaustion.

Nodda Duma · Sep 28, 2020

If you say so.

Placing such stringent expectations on color consistency means a) you’re a major lab, or b) you don’t believe in color correction filters / photoshop for printing. Even then, I’ve never seen issues with inconsistent color shifts at different temps. Repeatability of process ie consistency from session to session is far more important for your personal workflow.

My opinion, of course. But I’m pretty comfortable with C-41 developing so there’s that

Accurate digital thermometer for chemicals

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