Water Bath Temperature Control

When I started developing film at home, I was relegated to black and white only, since I didn't have a way to accurately control temperature. Trust me, I tried developing C41 with poor consistency and results. Monitoring chemical temperature is extremely important, with most developing kits recommending ±2 Deg C, but even slight temperature changes would change the development times. This project now allows me to develop C41, E6, and ECN2 process films at home with excellent consistency and color reproduction. Much of my personal life gallery photos are ECN2 images.

Previously, I had created a version that worked by just switching full power on/off to the heating coil. Unfortunately while this could warm the bath quickly to the correct temperature, it could not hold steady within the margin of error.

This solution uses an ESP32 microcontroller connected up to a K-type thermocouple to read in the temperature, a solid state relay (SSR) to allow for PID control of the current sent to the 1000W heating coil, and a simple encoder/button and display to control the setpoint and temperature. An aquarium water pump is also powered by the unit to stir the water. I added a light switch to cut power to the heating coil and a breaker is inline with the whole unit.

As this was just a one-off prototype for myself, I didn't optimize the design to use a PCB or opt to pack the electronics in tightly. With the inclusion of the SSR and a Kalman filter, the film chemical water bath can sustain a ±1 Deg C temperature.