Why Does AMS 2 Pro Humidity Rise After Drying?
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By Lee Zhong, GrandCube3D
Your AMS finished drying last night, but the humidity reading is higher this morning. That rise can have three different causes: the chamber has cooled, moisture is still moving out of a damp spool, or humid room air has entered. More than one can happen together. A higher reading alone does not prove that drying failed or the filament became wet again overnight. Compare readings at similar temperatures first. If the enclosure closes properly and fresh desiccant helps only briefly, additional desiccant capacity may help maintain a dry storage environment for longer.
This guide focuses on the AMS 2 Pro and AMS HT after heated drying. The storage checks also apply to an original AMS loaded with externally dried filament.
1. Cooling can raise RH without adding moisture
The AMS humidity percentage describes the air around the filament. It does not measure water inside the filament itself.
Bambu Lab explicitly explains that humidity can rise after drying because the chamber cools. Relative humidity, or RH, depends on temperature: as air cools, its saturation vapor pressure falls, so the RH percentage can increase without an addition of water. Bambu Lab: AMS humidity detection; Vaisala: humidity and temperature.
That makes a hot reading at the end of drying a poor comparison with a cool reading the following morning. Let the chamber temperature settle, then compare it with another reading at a similar temperature.
If RH rises during cooling and then levels off, cooling may explain much of the change. If RH keeps climbing while temperature remains steady, investigate moisture sources and desiccant condition as well.
2. A drying cycle may leave moisture in the spool
Completing the timer does not establish how much moisture remains inside the filament. A cycle that is too cool or too short for the material may leave a damp spool in apparently dry air.
Jabil's filament guide explains that drying rate depends on temperature, surrounding humidity and the material's tendency to retain water. Its guidance also shows why room-temperature desiccant storage can be much slower than an appropriate heated drying process. Jabil: filament drying and storage guide, pages 4–5.
This provides a possible explanation for continued rebound: residual moisture in a spool can move into sufficiently dry surrounding air. It is a diagnosis to investigate, rather than something the humidity display can confirm by itself.
Check the recommended procedure for the exact filament and the dryer's capabilities. Bambu notes that some materials need drying temperatures beyond the AMS 2 Pro's range. Adding beads cannot make an unsuitable drying cycle equivalent to the specified process. Bambu Lab: filament drying recommendations.
3. Moisture can enter again after drying
Opening the lid exchanges chamber air with room air. Loading another damp spool also adds an internal moisture source. A poorly closed lid or displaced seal deserves attention if humidity continues rising during otherwise steady storage.
Bambu's drying guide says the AMS 2 Pro and AMS HT close their intake and exhaust when drying is off. Its humidity guide recommends checking that the seals fit and the lid closes completely. Those features support moisture control; they do not establish that your particular enclosure has no moisture entry. Bambu Lab: drying workflow; humidity checks.
Record lid openings alongside temperature and RH. A rise after a spool change should be interpreted differently from a rise during uninterrupted closed storage.
4. The desiccant may have little capacity left
Desiccant holds a finite amount of water. A container can look full while its contents have little useful capacity remaining. AGM's technical guidance explains both the finite capacity of desiccants and the importance of using them inside a sealed enclosure. AGM: how desiccants work.
Start with known-effective material. Replace spent desiccant, or regenerate it using the supplier's instructions. Do not assume a filament-drying cycle fully restores every silica gel or molecular sieve formulation. Check the holder's heat limits separately.
How to check your humidity rebound
Use the following observations to choose the next check. Several causes may overlap.
| What you observe | What to check next |
|---|---|
| RH rises as temperature falls, then settles | Compare later readings at similar temperatures before changing the setup. |
| RH keeps rising after temperature settles | Check drying history, lid openings, seals and desiccant condition. |
| Rebound becomes more noticeable after adding one spool | Investigate that spool's moisture history and drying requirements. |
| Fresh desiccant helps, but needs servicing sooner than you want | Check continuing moisture entry; then consider additional effective capacity. |
For a useful record, note temperature and RH before drying, at the end of heating, after cooling, and later during closed storage. Add the filament type, drying settings, desiccant condition and any lid openings.
If you suspect a spool, repeat the observation with that spool removed or with a known-dry compatible spool. Keep the desiccant and room conditions as consistent as practical. This is a troubleshooting comparison, not a measurement of filament water content or a definitive leak test.
When does adding more desiccant make sense?
Consider more capacity when the lid and seals are in good condition, the filament has received suitable drying, and effective desiccant controls humidity but needs refreshing too often for your routine.
The practical reason is straightforward: after drying ends, there is still a storage period to protect. Desiccant provides somewhere for moisture in the chamber air to go. More usable capacity can provide a larger reserve for that continuing moisture load.
For an AMS owner, the implication is that adding effective desiccant may extend the interval between refreshes under comparable conditions. It does not establish a fixed number of extra days. Material choice, initial dryness, room humidity, lid openings and airflow all affect the result.
A suitable spool-core holder can make additional space available for that reserve. Its value comes from holding an appropriate quantity of effective desiccant, allowing air access and making refills convenient. Check spool fit, moving clearance and secure bead containment. Keep the AMS air passages clear and follow its operating guidance.
Evaluate the upgrade by recording the dry fill mass and how long the setup maintains your chosen storage conditions at comparable temperatures. The useful outcome is a maintenance interval that fits your printing routine.
Do you need to reach the lowest possible number?
A single RH number cannot certify a dry spool. Follow the exact material's storage and drying guidance, and read the humidity trend alongside its drying history. Prusa likewise recommends protected storage with desiccant and material-specific drying procedures. Prusa: drying filament.
For a freshly dried spool, a sensible routine is to allow the prescribed cooldown, keep the enclosure properly closed, maintain effective desiccant and compare readings at similar temperatures. Add capacity when the existing reserve is too short-lived for that routine.