Temperature change during tank development

I process 120 and 35mm film in stainless steel inversion tanks, typically Tri-X shot at ASA200, D76 1:1, 7 min to 10 min (depending upon contrast) at 68 degrees. I presoak at 68 degrees and keep the tank in a tray of water at 68 degrees during processing to help maintain the temperature. I am very careful with these steps and measure and record the developer temperature as it is poured from the tank.

Inevitably, the temperature drifts higher (never lower) following these procedures. Sometimes it is 1/4 degree, other times as high as 1 degree especially during the warmer months when the darkroom may be as warm as 80 degrees. Even in the dead of winter when the darkroom is perhaps 68 degrees, I get a small drift in temperature.

My question is whether some of this drift is a byproduct of the development process?

Interesting subject and something that I have considered (& researched a bit over the weekend). I would say a drift of 1/4 of a degree when developing Tri-x is going to amount to 2/10ths of no difference - heck, I'd has it a guess that 1deg F change in difference is not going to cause too many issues.

A quarter of a degree? Hmm...I wonder if it could come from simply agitating the tank and holding it in your 98.6-degree hand...I really have no idea, to be honest. Then again, I'm not militant when it comes to temperatures. I process Tri-X at 72 or 73 degrees, and I really don't pay that much attention to keeping every single chemical or wash at the same temperature...I just get close enough to prevent thermal shock or cracking or any sort of other inconvenience.

Interesting thread!

Not my field, but if I had to guess, it could probably be caused by the chemical reaction during processing. You can see this when you remove the lid from the processing tanks, there is sometimes a release of pressure. Pressure is linked to temperature. Maybe this is the connection?

I wouldn't worry about a 1 degree change myself though.

Very interesting.

I used to take few precautions to control temperatures of my 35mm and 120 tank processing.

But after getting used to temperature compensated time with my 4x5 tray processing, I realized that it is not as easy to monitor the temperature of the small tanks.

So I have a gallon tank of tempering water and I completely submerge the small tank inside of this.

I think Sundowner may have identified a potential source of temperature rise. It is remarkable how efficient stainless steel is at transferring heat.

I have also wondered about the possibility of temperature changes due to chemical reaction but have concluded that handling the SS tank with my warm hands is a more likely explanation. Since I started using TMY-2 I have tried to tighten up on my temperature control by using a short water soak to temper the film and reel. I have actually turned temp control into a game by checking the dev temp after the film is transferred to a tank of stop ( I use separate tanks filled with dev, stop and fix). By keeping the dev tank in a deep tempering bath between agitation cycles dev temperature can be very closely controlled. Kodak suggests " a deviation of plus or minus 0.5 degrees C (0.9 degrees F) is sufficient to change the characteristics of a negative unless you adjust the development time" so I try not to let my dev temp drift beyond that point.

Go and collect 4 thermometers that are used in darkroom work. Lay them side by side and you will most likely find they read 3 or 4 different temperatures. A half degree difference may be detectable with two negatives viewed side by side that were shot under very controlled studio lighting. In the real world half a degree is not an issue.

Also note that development times are ROUNDED to the nearest 1/4 minute. If you look at development time 0.25min/7min = a 4% difference. Much more then a half degree change 0.5/68 = about 1% difference. (I know I should use Kelvin here but close enough for my point)

The way you meter the shot plus or minus <1/3 of a stop will make a more difference than a 0.5 degree change in temperature over during development.

All chemical reactions cause a change in temperature. Just dissolving something in water will raise or lower the temperature even if the solid and liquid were the same temperature to begin with. Shaking a liquid will raise it's temperature. You must shake your film during development. So I really see no way to keep the temperature perfect.

It's an interesting question but I doubt that a slight temperature increase will have a noticeable effect on the negatives. I've always started with my developer at 68F and made no effort to maintain that temperature throughout development (my darkroom is in the house with AC so the temperature is rather constant). My negatives are fine. Depending on the development time there could be an increase in contrast especially if it were a short development time. But let's say Delta 400 for 9min30sec @68F ends up at 72F you could develop for the 70F time instead although I'm not sure it would make much difference unless the lighting and subject were contrasty to begin with.

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