The Problem with Existing Film Tanks
If you develop film in a darkroom, you know the frustration: almost every tank on the market leaks chemicals from the top when inverted. Paterson, AP, and most other systems all have this issue. The only exception is Jobo — but it comes at a cost. Inversion agitation is inconvenient, messy, and completely incompatible with automated film development setups where precise chemical dosing matters.
I needed a better solution — one that separates the agitation method from the chemical timing problem. The answer was dip-and-dunk.
How Dip-and-Dunk Works
The core idea is simple: instead of inverting the tank, you lift the film out of the chemicals. When the reel is raised above the solution, the chemical reaction stops immediately as the droplets shake off. You then pour the next chemical into the drum, lower the film back in, and development continues.
This gives you precise control over development timing without any inversion — and without a single drop of chemical going where it shouldn’t.
Design and Features
The OpenTank is fully 3D-printed and open source for non-commercial use. Every part of the design was built with real darkroom use in mind.
Universal Mounting System
The drum has standardized mounting holes on its sides, allowing you to build custom stands, lifting attachments, or integrate the tank directly into an automated film development machine. These holes are designed for future expansion — whatever attachment you need, the interface is already there.
Dual-Port Top
The drum features two ports on the lid:
- Automatic dosing port — for integration with a film development machine
- Manual dump port — in case something goes wrong, you can pour chemicals manually without disassembly
- Both ports also accept a thermometer, so you can monitor the actual temperature of the chemicals inside the drum in real time — something almost no other system supports cleanly
Light-Safe Lid with Release Mechanism
The lid uses a 90° angle light baffle, making the tank permanently light-safe even while the film is being raised and lowered. A simple release mechanism lets you drop the film into the chemicals or lift it out in one motion. A 2 mm gasket rim prevents any dripping along the lid edges if you tilt the tank.
Collapsible Reel System
The internal stem collapses to fit both 35mm and 120 film reels. Two stem versions are included:
- Standard stem — compatible with Paterson reels
- Slim stem — compatible with Jobo reels (1 mm smaller core diameter)
No need to reprint the whole drum — just swap the stem for whichever reel system you use.
Light-Safe Funnel
A side-mounted funnel allows you to pour chemicals in and out without exposing the film to light. The funnel has two visible level markers:
- Lower mark (~320 ml) — correct volume for one roll of 35mm film
- Upper mark — correct volume for one roll of 120 film
Customizable Exterior
The textured exterior shown in photos is not part of the base design — it was added directly in the slicer using extrusion (not displacement mapping, which would be slow and add no value). You can create any grip texture you like, paint sections in different colors, or add logos. The design is intentionally left open for personalization.
How to Use
- Load your film onto the reel using the standard Paterson or Jobo loading method
- Drop the lid onto the stem — the assembly locks into one rigid unit
- In a dark bag, click the drum closed — it’s now light-safe
- Place the tank on your work surface and pour chemicals through the funnel to the appropriate level marker
- Open the release and lower the film into the solution
- Agitate using any combination of: slow rotation, dip-and-dunk strokes, or a mix of both — the tank supports all methods
- When development time is complete, lift the film above the solution and shake off the droplets — reaction stops immediately
- Pour the chemicals back out through the funnel into your storage bottle or waste container
- Repeat for each chemical step (stop bath, fixer, wash)
- Open the drum, lift both sides, remove the reel
The tank has been tested with C-41 color chemistry and works equally well for black-and-white processes.
Current Limitations
The tank is currently designed for one roll at a time — either 35mm or 120. The dip-and-dunk method requires the film to travel above the chemical level, which means the tank height scales with film count. Two rolls of 120 would make the tank impractically tall. Single-roll development is intentional: paired with an automated machine, you develop each roll as soon as you shoot it rather than batching.
Part of a Larger System
OpenTank is designed as a standalone darkroom tool, but it’s also the development module for an upcoming automatic film developing machine project. The mounting holes, dual ports, and precise volume calibration are all designed with automation in mind.
Download and Support
The full STEP files are available on Ko-fi. The design is free for non-commercial use under a share-alike license — you can modify it, remix it, and print it for personal use.
If this project is useful to you, a small donation keeps the channel going and funds the next iteration of the automatic developer. Every contribution helps directly.