A small web-based toolkit for analog film photographers, designed to run locally on a Raspberry Pi or any machine with Docker. It consists of two tools: a thermodynamic temperature mixing calculator for preparing chemicals at the correct temperature, and a development timer with audio agitation reminders.
Temperature control is critical in film development. Black-and-white developers typically require 20°C, while colour processes (C-41, E-6) demand 38°C with tolerances as tight as ±0.3°C. The traditional approach — placing chemical bottles in a water bath and waiting — is slow.
A chemical working solution is mixed from concentrate + water anyway. That water doesn't have to be room temperature. If you keep a bottle of distilled water in the fridge (typically ~4°C) alongside your room-temperature water (~20–22°C), you can calculate the exact volumes of each to mix so the final solution lands precisely at your target temperature. No waiting, no thermostatic inertia.
The calculation is a simple heat balance equation. Since all components are water-based and share approximately the same specific heat capacity, it cancels out and the problem reduces to a weighted average of temperatures by volume.
For colour processes, the same principle applies in reverse: pre-warm water to ~45°C, then blend it with cold water to hit 38°C precisely. A brief equilibration in a sous-vide bath can correct for any residual error from the concentrate's slightly different specific heat — but the blending step dramatically reduces the time spent waiting.
m_cold · T_cold + m_warm · T_warm + m_chem · T_chem = m_total · T_target
m_cold + m_warm + m_chem = m_total
Where m is volume in ml and T is temperature in °C. The chemical volume is fixed by the dilution ratio (e.g. 1+9 means 1 part concentrate to 9 parts water, so 30 ml chemical in 300 ml total). Solving for m_cold gives:
m_cold = (m_total · T_target − m_chem · T_chem − m_water · T_warm) / (T_cold − T_warm)
m_warm = m_water − m_cold
Chemical Mixer
- Interactive CLI — run without arguments, enter values one by one
- Non-interactive CLI — pass all arguments as flags, scriptable
- Web interface — minimal dark-themed UI served locally, works in any browser
- Input validation — catches impossible temperature targets and warns about negative volumes
- Verification — always reports the actual calculated mixture temperature as a sanity check
Development Timer
- Countdown with configurable minutes and seconds
- Three distinct audio signals synthesised entirely in the browser via Web Audio API — no server connection required
- Overdevelopment tracking — at 0:00 the timer continues upward with a negative sign, so you always know how far over you are
- Event log — timestamped record of every signal fired during the session
- Docker support — runs on Raspberry Pi or any ARM/x86 machine
Requires Python 3.10+ and Flask (only needed for the web interface).
pip install flaskdocker compose up -dThe web interface will be available at http://localhost:5000.
For Raspberry Pi, the standard python:3.12-slim image supports ARM64 out of the box.
Run without arguments and the calculator will prompt for each value:
$ python app.py
╔══════════════════════════════════════╗
║ Darkroom Chemical Mixer Calculator ║
╚══════════════════════════════════════╝
Cold water temperature [°C]: 4
Warm water temperature [°C]: 22
Chemical concentrate temperature [°C]: 22
Target temperature [°C]: 20
Dilution ratio (e.g. 1+9, 1+14): 1+9
Desired total volume [ml]: 300
════════════════════════════════════════
RESULT
════════════════════════════════════════
Cold water: 33.3 ml
Warm water: 236.7 ml
Chemical: 30.0 ml
────────────────────────────────────
Total: 300.0 ml
Mix temp: 20.00 °C (target: 20°C)
════════════════════════════════════════
All inputs as flags — useful for scripting or keyboard shortcuts:
python app.py --cold 4 --warm 22 --chem 22 --target 20 --ratio 1+9 --volume 300Short flags:
python app.py -c 4 -w 22 -k 22 -t 20 -r 1+9 -v 300| Flag | Short | Description |
|---|---|---|
--cold |
-c |
Cold water temperature (°C) |
--warm |
-w |
Warm water temperature (°C) |
--chem |
-k |
Chemical concentrate temperature (°C) |
--target |
-t |
Desired final temperature (°C) |
--ratio |
-r |
Dilution ratio, e.g. 1+9 or 1:9 |
--volume |
-v |
Desired total volume (ml) |
Start the server:
python server.pyThen open http://localhost:5000 in a browser. On a Raspberry Pi on your local network, access it from any device at http://<pi-ip>:5000.
The mixer and timer are linked — a button at the bottom of each page navigates between them.
The timer is available at /timer.html and runs entirely client-side — the audio engine uses the Web Audio API and requires no server connection after the page loads.
| Signal | When | Purpose |
|---|---|---|
| Beep 1 (double pip, 880 Hz) | At 0:30, then every 60 s | Agitation reminder |
| Beep 2 (descending, 660→550 Hz) | At 10 s remaining; every 60 s if overdeveloping | Pour-off warning |
| Beep 3 (triple tone, descending) | At 0:00 | End of development |
The first 30 seconds of development require continuous agitation. At the 0:30 mark, Beep 1 fires and the log reads "Stop agitation — let tank rest". After that, Beep 1 fires every 60 seconds as a prompt to agitate again for approximately 10 seconds.
When the timer reaches 0:00, Beep 3 fires and the display turns red and continues counting upward with a negative sign (e.g. -0:05, -0:10...). Beep 2 fires every 60 seconds as a reminder. Press Stop to end the session.
Black-and-white developer at 20°C (e.g. Kodak HC-110, dilution B = 1+31)
python app.py -c 4 -w 22 -k 22 -t 20 -r 1+31 -v 300Colour developer at 38°C (C-41, dilution 1+4, pre-warmed water at 45°C)
python app.py -c 4 -w 45 -k 22 -t 38 -r 1+4 -v 500Hybrid method for colour processes: blend to approximately 38°C using this calculator, then place the beaker in a sous-vide bath set to exactly 38°C for a minute or two. The calculator eliminates the bulk of the warming time; the sous-vide corrects for the concentrate's slightly different specific heat capacity and any measurement imprecision.
The calculation assumes all components have the same specific heat capacity as water (4.18 J/g·K). Chemical concentrates deviate slightly from this, which introduces a small error — typically less than 0.5°C in normal dilutions. For black-and-white work this is negligible. For colour processes using the hybrid method described above, the sous-vide step corrects for it automatically.
.
├── app.py # Calculator logic + CLI + Flask API
├── server.py # Web server entry point
├── static/
│ ├── index.html # Chemical mixer web UI
│ └── timer.html # Development timer web UI
├── Dockerfile
├── docker-compose.yml
└── requirements.txt
Built with the help of Claude (Anthropic).
MIT
