Attention Chemists: Dial Thermometer Calibration

[Rob-F]

I have several darkroom dial thermometers. One is a Cibachrome, used to check developer temperature. And then I have a VWR and a Tell-True, both mounted in my water supply. I've never suspected any discrepancies among them. But recently when I checked the Cibachrome against my Kodak mercury color thermometer, it read two degrees warmer than the Kodak. Since I consider the Kodak mercury thermometer to be closer to being a laboratory standard than most dial thermometers, I decided to look into it closer. I prepared a glass of ice water with plenty of ice and gave it time for the temperature to stabilize. Then I put both the Kodak and the Cibachrome into the water. The Kodak read 31 degrees, the Cibachrome 33.

Well, it seems to me that ideally, in the cold water next to the ice, they should both read 32 degrees, or just above. But the Kodak read 31. Question: Could the Kodak color thermometer be that far off? Or is there something wrong with my method? Is it possible for the water to be below 32 degrees?

 

[RObert Budding]

Well, I was educated as a chemist, so I'll have a go at this observation.

Pure water freezes at 32 F, but seawater freezes at a lower temperature (typically about 28 F) because it contains salts and minerals. It's likely that your tap water contains some minerals, too, so it should freeze a bit below 32 F. So I'd believe the Kodak thermometer.

 

[Beemermark]

As a non chemist engineer I'd say both of your thermometers are off by 1 deg. Put both is a "large" container of crushed ice (not ice cubes) and take a reading. The put a pot of water on the stove and bring it to boil. Assuming you're at sea level the thermometer should read 212 F.

Then again; practicality. Develop your film using ONE thermometer consistently, developed for the amount of time that gives your negative the density that prints well for you. Maintain that consistency and don't worry about how accurate the thermometer is. Just ensure that everything is repeatable.

 

[someonenameddavid]

It's a matter of precision rather than accuracy

 

[Steve M.]

Just as the man or woman who has several watches never knows what time it is, the same applies to thermometers. Actually, it doesn't really matter if one thermometer is accurate as long as it's giving consistent readings. My recommendation is to decide on one and keep accurate notes on developing and printing.

Are you shooting black and white? A few degrees one way or the other is not going to have any effect on your film or prints.

 

[joeswe]

"A few degrees more or less" ARE going to have an influence on your negative density and contrast.

While it is basically true that for b&w processing accuracy of the thermometer is less relevant than its precision, for color processing you need a thermometer that is both accurate and precise.

The often quoted proverb that a "a man who owns one thermometer knows his temperature bla bla" is unfortunately only true until that thermometer breaks. When he buys a new one he can re-do his whole film/developer testing from scratch! Better to own a second thermometer to keep as reference.

 

[Dan Daniel]

I was taught to adjust a darkroom thermometer using a medical thermometer at ~98-99 degrees for reference. I assume that a medical thermometer is the closest thing most of us have at home that could be considered a reliable reference.

 

[David Hughes]

Hi,


There are a lot of labs. about that can, surely, calibrate a thermometer. A search ought to find one nearby.


Regards, David

 

[Bill Clark]

As I recall, the thermometer I use had, in the instructions, put the sensing shaft into a glass of ice and let it sit for a few minutes. Look at the temp. and adjust. There is a nut on the side with the sensing shaft that is used to adjust.

 

[retinax]

As I recall, the thermometer I use had, in the instructions, put the sensing shaft into a glass of ice and let it sit for a few minutes. Look at the temp. and adjust. There is a nut on the side with the sensing shaft that is used to adjust.

Ice absolutely won't cut it, you need ice water. Pure ice can be anywhere below freezing point.

 

[Bill Clark]

To me if water is liquid it isn’t at 32 degrees.

I’ll continue with my method.

 

[joeswe]

I was taught to adjust a darkroom thermometer using a medical thermometer at ~98-99 degrees for reference. I assume that a medical thermometer is the closest thing most of us have at home that could be considered a reliable reference.

The way that medical thermometers work makes them a bit difficult to use for darkroom purposes but generally the idea is correct to use them as a reference since they ought to be accurate and precise in the temperature range that is also relevant for color processing. I have two cheap digital medical thermometers bought from a drugstore chain and they are very close in their readings (both +0.1C) to my reference (Kodak Process Thermometer #3). I have seen rather large deviations in cheap general use immersion digital thermometers. I still use them for practical processing, but have tested and calibrated them against my reference.

 

[joeswe]

To me if water is liquid it isn’t at 32 degrees.

Only melting ice has a surface temperature of exactly 0°C or 32 F (provided it doesn't contain any salts). The temperature of solid ice will be any temperature below that.

So, you have to let ice cubes melt and measure the temperature of the water between the melting ice cubes to get a reference of exactly 0°C. The temperature of the water will not rise above 0°C as long as the ice keeps melting because the melting process itself will take up any surplus energy present.

 

[dfdann]

I’m a retired Physical Chemist. I agree with the above explanation. Impurities lower the freezing point. You would need extremely pure water/ice. I cannot believe such a slight difference effects devepment.
Cheers, Dan

 

[willie_901]

I’m a retired Physical Chemist. I agree with the above explanation. Impurities lower the freezing point. You would need extremely pure water/ice. I cannot believe such a slight difference effects devepment.
Cheers, Dan

I am also a retired physical chemist. I agree with Dan.

I left physical chemistry in 2000 and absolutely no one used mercury thermometers for any reason. This was partially due to environmental concerns, but also because thermistor based devices are more accurate.

I would add that if I was really concerned about temperature measurement I would use a Thermoworks device. Here are specs for the Thermopen.

Accuracy: ±0.7°F (±0.4°C) from -58 to 392°F (-49.9 to 199.9°C) otherwise ±1.8°F (±1.0°C)

Resolution: Factory set to 1° (user reconfigurable to 0.1°)

I'm sure devices with higher accuracy exist, but they probably are rather expensive.

Note: I have no affiliation with Thermoworks except I use their products for cooking.

 

[Roger Hicks]

There's a piece on thermometers, addressing accuracy and repeatability, on my old .com site.

Cheers,

R.

 

[Rob-F]

Well, I was educated as a chemist, so I'll have a go at this observation.

Pure water freezes at 32 F, but seawater freezes at a lower temperature (typically about 28 F) because it contains salts and minerals. It's likely that your tap water contains some minerals, too, so it should freeze a bit below 32 F. So I'd believe the Kodak thermometer.

I’m a retired Physical Chemist. I agree with the above explanation. Impurities lower the freezing point.
Cheers, Dan

I did use tap water (processed by the city water works, from water drawn from the Mississippi River: Ole muddy). So it would have minerals and chlorine, making it reasonable that the water temp would have been at least a little below 32 degrees? OK then I will use the Kodak thermometer, which read 31, as my "laboratory standard!"

Thanks, guys!