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As I'm developing film at home. I have problem to keep temperature constant for: developer, stop bath, fixer, throughout the process at 20C degrees.
What's the best practice to do so ? How do people usually go about it ?
Also does final wash also require 20C degrees temperature?
A few points about film processing temperature: We control the activity of the developer, stop bath, fixing bath, and wash water, by adjusting chemical strength, time in solution and fluid temperature. In the era when most of this chemistry was established, 20⁰C (68⁰F) was typical room temperature. There is no magic in 20⁰C, you are free to process above or below. Most all modern color films are now processed at a temperature near 38⁰C (100⁰F). Charts are published cross-referencing time vs. temperate.
It is best if all fluids are adjusted to about the same temperature, this incudes wash water and washing aids. If any fluid should differ widely, reticulation may occur. This is a shattering or cracking of the film emulsion, at worst. At least, mild reticulation resembles elevated grain structure. This happens because films are sandwiches of coats. Each layer has differing rates of contraction and expansion with temperature. If films are plunged into differing temperatures, these layers may slide against each other and fissure.
Again, all fluids should be nearly the same temperature, including the running water wash. My experience passes along, measure the temperature of the running water and if possible adjust by mixing the hot and cold valves to the desired temperature.
For the home darkroom, a water bath will be the best temperature regulator. Procure a pan that will hold all the bottles or beakers of process including the developing tank. Pre-mix and measure out all solutions. Glass beakers are OK but stainless steel is best as metal beakers speed temperature equalization. Fill the pan with water and partially submerge the beakers up to about 75mm (3 inches) deep.
Use a thermometer, measure all including the water bath fluid. Add ice or hot water to the water bath to set it and keep it at the desired temperature. After the film has been loaded into the light proof developing tank, pour in the fluid; agitate continually for the first minute. Place tank in the water bath. Remove tank from water bath, agitate 5 seconds every 30 seconds and replace the tank in the water bath between agitations.
If in doubt about the temperature of your development you can place your tank in a bigger tub of water of the desired temperature. This is easiest done with the plastic ones – Paterson, Jobo & the like.
This is usually an overkill for B & W process, but necessary for color. In color process higher temperatures are required, and inconsistency can lead to a irreparable color shift.
As for the final wash, which is commonly done with tap water, it does not need to be 20C exactly.
Just keep in mind two things:
I process film in a three-reel Paterson tank. Assuming the dev goes in at around 20°C then experience shows that it doesn't change enough to worry about for processing times of up to about ten minutes. However this depends on what the ambient temperature is, and smaller tanks will drift faster: where I live it's generally not hot enough for temperature rise to be a problem, but it can occasionally be cold enough in the winter (we have no central heating so the house can get fairly cold in rooms we don't heat). In this case I fill a sink with water at around 20°C and just keep the tank half-submerged in that. Nothing about B/W is that fussy about the temperature being axactly 20°C - a degree or two either way is OK, especially if the temperature drifts slowly during the process.
No, the final rinse does not have to be at the same temperature, although you want to avoid large steps in temperature. So what I do is try and arrange successive stages in the process so they're 'heading for' the final rinse temperature. At this time of year it's all fine, but in the winter when the water is pretty cold I try and have stop, a few degrees colder than dev, fix a few degrees colder than that, and then perhaps I do the first bit of rinsing with water with a little hot water added to get that stage intermediate between the fix & the temperature from the cold-water tap.
(Parts of the process which happen at significantly lower temperatures also take longer, so if you fix at, say, 13°C in the winter then give the fix plenty of time to do its thing.)
The most common way to stabilize temperatures is to measure out all of your steps into separate containers. Place everything into a larger container filled with water, and in my case, ice. Get the larger container down to 20C and remove ice. Begin development.
The volume of the container as a whole should keep things at ~20C throughout your process.
For rinse, your method depends on the temperature available to you through your tap. If your tap water is colder than 20C, simply raise the temperature until it is at 20C and leave the tap running through the process (as said in another answer, you don't need to fully cycle the water that much so a slow run is fine).
If your tap water is warmer than 20C at its coldest, then you have to get creative. I had a buddy in Phoenix, AZ who had this issue. The pipes were so close to the surface and ran through the roof of the house so that, in the summer, the cold water tap was positively hot. The problem was solved by using a large beverage dispenser (the kind you see usually holding tea or lemonade with a spout at the bottom). Have the faucet run into the dispenser filled with ice and exit out the spigot at the bottom and into your film container. The volume should help the temp remain fairly steady once you get the temp dialed in.
