Accurate thermometer for C41

I agree wit Diapositivo, with a Jobo, you don't need to take any measurements (well, maybe except for control measurements from the water bath every few months), but it takes a surprisingly long time for the developer to reach the water bath temperature to the tolerance of +/-0.2C. I agree it's about half an hour. That's why I'm metering the developer and giving it brief baths in really hot water to bring it up to temp in just a few minutes. (Do this carefully: you need to have the liquid in the bottle in motion by stirring it, and measure once again after 30-60 seconds you have placed it back in Jobo.)

I can heartily recommend the following: Thermoworks RT301WA:

http://www.thermoworks.com/products/low_cost/rt301wa.html

I like it because it can be easily calibrated. If you know your local air pressure (easy to get) and can boil distilled water, you can calibrate the thermometer so it reads the proper temperature at your location - in my case it was 100.2C

On the other side, you freeze some distilled water, then make a slush with a combination of crushed ice and distilled water. Stick your thermometer in, and calibrate it so it reads 0.0C. That's it. I tested this thermometer against 3 lab-grade devices, and it was bang-on accurate.

I use a Paterson Colour Thermometer (was $27 at Calumet) and then determine the corresponding temperatures on a digital thermometer (LCD with probe) that is actually used during developing (for b&w, 68F on the color thermometer = 69.1F on the digital). I do the calibration occassionally as the digital thermometer might change as the battery depletes. This has worked well for both color and b&w developing.

Arkasha said:

I can heartily recommend the following: Thermoworks RT301WA:

http://www.thermoworks.com/products/low_cost/rt301wa.html

I like it because it can be easily calibrated. If you know your local air pressure (easy to get) and can boil distilled water, you can calibrate the thermometer so it reads the proper temperature at your location - in my case it was 100.2C

On the other side, you freeze some distilled water, then make a slush with a combination of crushed ice and distilled water. Stick your thermometer in, and calibrate it so it reads 0.0C. That's it. I tested this thermometer against 3 lab-grade devices, and it was bang-on accurate.

Click to expand...

You know, we really don't need it to be that accurate, as long as it's consistent in it's error. Most mercury and spirit thermometers are, it may read 38C and actually be 37C or 39C, but it will have the same error from the factory until it gets dropped. This means that your work flow will adapt to the thermometer. The problem is that electronic ones can drift, as they age, when it says 38C, it could actually be 37.9C and a couple of years later actually be closer to 36C..... The battery power level can also affect this. I don't know about the dial type, last time I saw one being used for photography it was B&W and with black and white, you can be off as much as 2-3C and it doesn't really matter.

wogster,

You can recalibrate this thermometer, to account for drift. It's also quite inexpensive - about $25 delivered.

The most accurate device I have owned is a type K digital thermocouple from ebay from HK / China, it's dark grey with a orange cover. When I was calibrating it, surprisingly it was spot on.

I have another thermocouple, much more expensive with IR readings as well as K type thermocouple attachment.. the thermocouple over-reported by 2 degrees.

So what I recommend is test your equipment!

I also recommend thermocouples due to rapid response, inexpensive to replace (the HK one was like $20 shipped or less iirc), and accuracy once tested/calibrated.

I dislike digital thermometers due to their slow response.