We have just launched our brand-new Prusa USS Drybox – a truly useful accessory to keep your filaments organized, neatly stacked, and above all: dry. Using it is as straightforward as it can be, and thanks to its tightly-sealed design, the results are top-notch. It’s the perfect fit for most filaments.

It has a unique quick-access seal, it sits perfectly on our new universal spoolholder, and offers up to 5 times better sealing compared to other available dryboxes.

Using the CO2 Decay method, we measured the Air Exchange Rate (AER) to test airtightness. The Prusa USS Drybox achieved an impressive 0.15 AER – a level of sealing typically reserved for museum cases preserving fragile artifacts.

AER Comparison:

• Prusa USS Drybox: 0.15
• Generic Drybox: 0.74
• Zip-lock Bag: 5.40

And we’re ready to take it one step further – but let us first explain why.

Humidity has always been one of the primary enemies of 3D printing. Some filaments soak up moisture faster than a kitchen sponge. However, the universal “dry your filaments” advice might be a double-edged sword. Maybe you’ve experienced it too. You kept drying your ASA over and over again, only to end up with stringy prints. And then, you picked a random filament spool that’s been sitting on the shelf for months, and it printed perfectly. There is a reason for that.

Introducing the Prusa Pro ACU – Active Conditioning Unit

Our engineering team began internal research into active humidity management roughly 18 months ago, after a pattern emerged in customer support data: Overdried filament (brought below 12–15% of its optimal relative humidity levels) becomes brittle at the molecular level, losing the slight plasticization that ambient moisture provides. For hygroscopic materials like Nylon and TPU, the effect is pronounced. The results are more visible layers and brittle prints.

Around the same time, ALL3DP published an article titled “Yes, You Can Over Dry Filament. Here’s How Not To,” mentioning that “Too much drying — whether it’s at the wrong temperature, for too long, or too often — can damage the molecular structure of some polymers. This negatively affects strength, layer adhesion, and results in brittleness.”

So, we knew we were onto something. The same team that brought the Prusa Pro HT90 to the market started experimenting with dryboxes and something we call “Active Filament Conditioning.”

Early testing proved that properly conditioned filament showed measurably better bridging performance and, in flexible materials, a reduction in surface micro-cracking that conventional drying actually introduces.

And now, we’re finally ready to introduce the Prusa Pro ACU – Active Conditioning Unit!

How the ACU Works

The ACU is a desktop filament conditioning enclosure that actively manages internal humidity to match the target range for your material (loads via OpenPrintTag). It’s based on the Prusa USS DryBox, but with additional electronics. Because they share the same base, you can actually upgrade your DryBox to the ACU!

The Prusa Pro ACU is a self-contained device powered by a single-core 160MHz RISC-V CPU. It features integrated Wi-Fi, Bluetooth 5.0 LE, USB-C, hardware cryptographic acceleration, and its ecosystem is pretty groovy – it’s a kind of seed that has the potential to grow into something even bigger.

The ACU offers three main operating modes.

Ambient mode creates a controlled environment without introducing active moisture.

Saturated mode reintroduces native moisture back into the filament. Soak a standard silica gel bag for approximately 90 seconds, place it in the conditioning tray, and the ACU brings internal relative humidity into the 65–85% range over several hours – optimal for ASA, PA11 Nylon, carbon fiber composites, and most woodfill materials. With the ACU on your desk, you will soon become a master of unlocking the hidden potential within your most difficult materials.

And before we get to the Submerged Mode, we would like to briefly mention that the ACU uses the widely accepted 3/4’’ BSP standard for all connections, ensuring compatibility with a broad ecosystem of third-party accessories, including the widely-known garden hose to HDMI adapter. The hardware design files will be available on Printables.

Fully Submerged Mode for Advanced Users.

Some of the filaments absorb moisture more than others. Generally speaking, they need moisture – this is why they try to absorb as much as possible. For example, PEI is one of these “thirsty” materials, and it’s the perfect example of how Fully Submerged mode works.

In Fully Submerged mode, the spool sits in a water bath inside the ACU’s chamber, which holds up to 2 liters of liquid. This delivers maximum conditioning, uniform across the entire spool, in a way that standard exposure to air simply cannot match.

A huge benefit is that as the filament soaks up water, you’ll end up with more filament compared to a dry spool.

You can, of course, use additives in the Fully Submerged mode. We recommend adding 40-60 grams of table salt per liter of water. This protects the extruder gears from surface oxidation – steel and moisture have a complicated relationship, and the saline solution keeps it professional. Check out the video in this article for more information.

All fluid dynamics data, like bath temperature, moisture gradient, and saline concentration drift, are monitored through Prusa Connect. The ACU calculates real-time Reynolds number for the fluid bath, alerting the user when turbulent flow conditions (Re > 10⁷) may affect uniform moisture distribution across the spool and become a hydrological event.

Firmware Improvements

Firmware version 6.6.7-beta introduces a companion feature: Automatic Bed Wetting.

At the start of each print, the extruder lays a 2mm layer of water across the print surface before the first layer begins. This enables aquaplaning-assisted nozzle guidance – the nozzle rides across the moisture layer with a frictionless contact profile that eliminates the micro-vibration artifacts causing first-layer inconsistencies on textured PEI sheets.

We recommend demineralized or distilled water to prevent mineral scale deposits on the print sheet surface over time.

The Automatic Bed Wetting module is still in beta and may still trigger randomly. If you find yourself dealing with Automatic Bed Wetting repeatedly throughout the night, you are not alone – this is a known teething issue affecting a small percentage of users, and usually goes away as the user matures. In the meantime, we recommend not filling the reservoir immediately before starting an overnight print and placing an absorbent mat under the sheet.

Something for the Community

We had a great time designing the Prusa Pro ACU – it took nearly one entire evening after work. And we decided that it would be a shame not to share the project files with you. So if you want to have some fun and play around with the STEP files, go right ahead – everything’s on Printables.

One of our team members already created a custom firmware and added some really tasty new features.

If there is one thing the science of hydrodynamics has taught us, it’s that moisture will not be contained. Moisture breaks free, it expands to new territories and crashes through barriers, painfully, maybe even dangerously, but, well, there it is. And because we understand that electronics and water don’t mix well, please don’t try this at home. 😉

However, you can certainly let us know in the comments how you would, theoretically, utilize the fully submerged mode!

Happy printing!