Here’s a quick tip when it comes to developing temperatures and how to control them without a lot of fancy equipment. The idea is to allow for the loss/gain of heat between your developing solution and the surrounding environment (it’s called an “open system” in heat transfer lingo) when there are differences in temperature between the two.
That was the complicated part, the rest is a lot easier. Lets say for example that your room temperature is 22 degrees Celsius, and your development charts call for 20 degrees. What you do then is that you deliberately make your developer slightly cooler than what’s call for, maybe 19 degrees. This way, you’ll initially be too cold, but as your developer sucks in heat from it’s surroundings, it’ll heat up to and past the temperature your process calls for, but will as a mean over time have been around what you wanted it to be. This is due to the fact that the reaction speed of chemical processes is temperature dependent, i.e. they work faster at higher temperatures and slower at lower temperatures. So your development will start out slightly slow and end slightly fast, evening out each other.
Here’s a graphic that I made that probably explains this a lot better than I just did.
But what do you do if your room is colder than the process temperature? Easy, just start of hotter than called for instead.
One thing to keep in mind is that this only works if you’re only one or two degrees off the ideal temperature, if you can’t for example get cold enough water or the room temperature is significantly hotter you should start looking into compensating for the higher temperatures, and develop at the elevated room temperature instead. Most popular developers have a chart for doing this in their data sheets, and it’s really easy to use. If you’re working at temperatures over 26 degrees Celsius, then you’ll have to look into getting hold of so called “tropical” developers. These are special developers formulated to work predictably in hot environments, like the tropics, therefore the name, but this are really high level voodoo that I have no intimate knowledge of, so you’re on your own in that case.