thermostated water bath for C-41


I present here a simple set-up that allows to control the temperature of the chemicals when developing a color films in C-41 or E-6.


After starting again analog photography two years ago, I only shot with black and white films. One year after, in 2014, I started to use color film during my trip in Montreal. I shot there half a dozen of films (mostly super cheap Rossmann 400 mostly, i.e. Fuji Xtra 400 rebranded if I remember well — which costs about 1 € per roll), that I decided to bring to a shop in Munich, advertising doing it itself (or people said so on forums). Well, bad surprise, they just sent the whole bunch to a minilab, which came back with nice drying marks. And the price was not really super cheap too. Although Rossmann shops can develop them — for pretty cheap -, I just decided I would try developing color films myself!

The specificity of color films is that they must be processed at a precise temperature. Either 30°C, 38°C (the most standard), or 45°C, with a tolerance of ± 0.5°C. Otherwise color shifts will appear – which, I guess, can be corrected in digital post-processing for those scanning. The point here is that temperature is critical for a proper development. Development time being quite short though (3min 15s at 38°C), some people just let water run for several minutes (…), or prepare a slightly hotter bath in the sink, and wait until the chemicals are at the desired temperature. If you have money, you can also invest in a Jobo CPA2 processor. I tried something intermediate by making a thermally controlled bath. Here is how I did it.


The equipment used for this set-up costs about 60 € and consist of:

  • A temperature controller STC-1000. You can easily find it with its temperature probe on eBay for instance for about 10-15 €. I think there are two versions depending on the country (110V or 220V). The working principle is rather simple: depending on the measured temperature and set point, the controller opens or closes a relay connected to a resistor. Hence the bath is heated, or just naturally cooling down.

  • For the heater, I use a travel version like this one. A power of 300 W is enough (as explained further down in the article).

  • An aquarium pump to homogenise the bath temperature. A flow of 200-300 L/h is fine.
  • An electric plug to connect the heater to temperature controller. I suppose it is not absolutely necessary since you can also directly connect it with terminal strips.
  • Some electric wire, terminal strips, etc.
  • A box to fit it in.
  • A plastic tray for the bath.

And that’s it!


Below is a drawing of the back face of the STC-1000, with the various connectors. 1 & 2 are the power supply to the controller, while 3 & 4 are for connecting the temperature probe. 5 & 6 are going through the heater relay, and 7 & 8 are going through the cooler relay. These last two, I don’t use, because I don’t know how I could efficiently cool the bath (and so far, natural cooling was fine). In the scheme below, I show how to connect the controller and the heater together and to the electric plug. The cable colours respect the European standards.

Here is the results once everything is mounted:

The power supply cable is the thick black one on top. The plug on the right is to connect the heater.

I looked for the best place to control accurately the temperature, and it appeared to be in the bath directly, “not too close” from the heater of course. I tried in the bottle containing the developper, but the inertia is too high then, and the controller allows heating for too long, resulting in thermal cycles of up to several degrees. So, my receipt is to put the developper and the probe in the bath, and wait one or two hours that it stabilises (I then check the developper temperature with a standard thermometer).

Regarding the controller, it has a cut-off threshold of +0.3 °C above the set point, and 0.0 °C below. Therefore, setting the controller for 38.0 °C is not the best possibility, the temperature finally varying between 38.0°C and 38.3 °C. This is not a disaster of course, but it is even better to set it to 37.8°C.

Finally, a heater of 300 W appeared good enough for my small bath. I initially tried a 3 kW heater (!!! — the one in the picture above) but this is so powerful, that the bath temperature increases by several degrees before the controller updates its measurement (every few seconds)…

You should now be ready to develop your color films. I will not present the whole development process, as I think there are plenty of websites where it is already done. You can also have a look to the Tetenal user manual. It is self explanatory. Below are my own first results, where you can see the color bands on the film side, sign of a successful development – at least technically speaking.