AMS (1) and AMS HT Pro – Humidity rises back to >30 % RH, not as advertised
I would normally have opened a new thread for this topic, but at the moment I do not have the option to create new topics in the forum. Therefore, I am posting my feedback here in the feedback thread.
The issue:
Both AMS (1) and AMS HT Pro do not behave as advertised. AMS is marketed as “keeps filament dry” and AMS HT as “airtight”. In my tests, both units stabilize above 30 % RH within hours, even when empty, closed, and equipped with new original Bambu desiccant. This makes them unsuitable for long-term storage of hygroscopic materials like PAHT-CF (PA12-CF), which require ≤15–20 % RH.
Test setup (20 Aug 2025)
Both AMS (1) and AMS HT completely emptied (no filament inside).
New original Bambu desiccant packs installed.
At the start (16:49), both lids were opened briefly so the units were “contaminated” with room air (≈58 % RH), then properly closed.
AMS HT ran the PLA drying program, AMS (1) has no active drying.
Next morning (21 Aug, 05:19):
AMS HT: 34 % RH
AMS (1): 30 % RH
(Both remained closed, still empty, OEM desiccant inside.)
Observations
AMS (1) showed only minimal change, raising the question whether its humidity sensor works correctly.
AMS HT dropped to 22 % during drying but rebounded to >30 % in less than 12 hours without opening.
This contradicts the advertised functionality of “airtight” / “keeps filament dry”.
My questions to Bambu Lab and the community
Are these values (24–34 % RH) considered within specification?
Has anyone else observed the same rebound in AMS HT or the limited response in AMS (1)?
Will there be an official solution (improved sealing, more effective desiccant, or otherwise) to actually meet the advertised performance?
I believe this issue affects many users who purchased AMS HT expecting reliable long-term storage. Input from both the Bambu team and other community members would be highly appreciated.
In my AMS HT, without heating or drying, the RH has remained at 9% for more than two weeks.
However, do you actually understand how RH works?
It is physically expected that RH will rise when the chamber cools down. To determine the actual amount of moisture present, you need to look at absolute humidity (AH), not relative humidity (RH).
That’s why I ran an empty-box test with stable room conditions and documented timestamps. A short RH rise during cool-down is expected while the HT chamber temperature equalizes. However, in my case the RH in the AMS HT continued to climb for hours after stopping the program (22% → 30% → 32% → 34%) while the room stayed ~56–58% RH and the unit remained closed and empty. After thermal equilibrium, a further RH increase implies added moisture (higher AH) from either leakage air ingress or outgassing from internal surfaces/saturated desiccant.
Control: the AMS (1) had no heating/cool-down at all and still drifted from 24% to 30% overnight while closed and empty. That again suggests moisture ingress/capacity issues rather than a pure temperature effect.
To make this reproducible, I’m happy to convert to AH if we can get temperatures inside the chamber. Does Bambu expose an internal temp for the AMS HT, or a spec for airtightness (e.g., leak rate or target RH/AH over time with OEM desiccant)?
If you have long-term values, could you share:
your room temp/RH (and ideally dew point),
the exact desiccant type/amount and regeneration method,
whether the AMS was empty or filled with dried spools (which reduces free air volume and adds polymer adsorption)?
That would help determine whether I’m seeing a unit-specific leak or if the OEM desiccant capacity is simply insufficient for ≤15–20% RH over 24–48 h.
You’re right that after thermal equilibrium, a continuing RH rise can’t be explained by RH vs. AH alone — it implies added moisture from leakage or desorption. The AMS HT isn’t hermetic, so small ingress plus outgassing from polymers, seals, or even the desiccant itself explains the drift you’re seeing. Your control case is consistent with that as well.
Since both the AMS HT and AMS 2 Pro display temperature and RH, it’s straightforward to convert those readings to absolute humidity (AH) for reproducibility. What’s missing is direct telemetry/export, which would make long-term analysis easier.
Full airtightness is possible, but it’s complex and costly — and personally I wouldn’t want to pay for that level of design. What would make a real difference is if the AMS HT/AMS 2 Pro occasionally cycled their heaters to maintain a user-set RH target, similar to how dedicated dryers like EIBOS handle it. That way you’d get stability without chasing laboratory-grade sealing.
I know the AMS2 Pro is a different beast again, but I was also interested in using it to keep filament dry (after a drying cycle in the AMS, although you still have the caveat of needing to open it after drying in order to insert the filament back into the feeders)…
I don’t have any hard measurements like you do, but initially my AMS2pro only seemed to drop the humidity (after two 4-6 hour drying cycles) to greater than 20%. I’ve since printed a bunch of these Desiccant Boxes and run another cycle, and dropped to around 8-9%… and has held it at 10% for the last week.
My second AMS2 Pro doesn’t seem to be doing quite as well, it’s at 19%, but might just need another drying cycle. But it would seem to me at the moment that the OEM desiccant just isn’t enough to help really pull/keep it down.
Incomplete or irregular drying is endemic to people using filament dryers and probably to the AMS 2 also. I’m guessing it affects us forced-air people too, but I see less rebound then when using a filament dryer.
So possibly what you might be seeing is moisture from the inner layers of the roll diffusing out. What happens if you just run it on empty? How long does it stay low?
Thank you @krellboy. I would have bought into the theory; however, the filament I have was taken out from my regular AMS, which showed 11% RH, and I moved it to the HT and put it to dry immediately. My room RH is at 37%. So dont think that would be the cause
Just stumbled across this thread because I was having the same issue, although slightly different. I’ve modified the AMS2Pro slightly to fit one of those solid state dehumidifiers from Rosahl (check out Stefan’s/CNC Kitchen’s Video about this).
Although I’ve used a smaller unit than in his video (M-2J1R)it should be plenty enough to bring down humidity to way below 20% RH…until it didn’t. Same thing as for you, I’ve been stuck at around 27%.
Since I had to drill some holes to fit the dehumidifier I tried blowing into the AMS to one hole (while all other openings sealed of course) and suprise: The AMS was leaking heavily out of everywhere.
So, tldr: Solution or root cause is the very poor sealing of the AMS which require loads amounts of desiccant or a large solid state humidifier unit. Will try to seal the AMS in the next coming days and let you guys know, if somebody’s interested.
We have a new system and I’m a printing newbee. We purchased 2 AMS Pro 2’s and 2 AMS HT’s for our H2C. Only 1 of the HT’s is having this issue. I’m able to get the others to steady at about 10 to 13%. But this one HT is giving me problems. I have dried my ABS twice in it and before I can use it, it has creeped up to 20%, pretty much over night. I recently added a desiccant insert for the slot in the back and changed the spool desiccant, but it still creeps back up over night after being dried. I think I’m going to turn in a support ticket.