Is drying function on (new) AMS HT working correctly

Bambu Lab AMS AMS HT
1

Newbie question: Just bought an AMS HT (to go with a month-old P2S and AMS 2 Pro). Mostly to try to print with TPU using the new TPU Assist module which I just installed. Anyway, in prep for my first TPU print I opened up a fresh pack of Bambu 90A TPU and immediately put it in the HT to dry it using the recommended TPU setting for 18 hours. According to the HT panel, at start the RH was 36% at 22C. After about 30 minutes I noticed the RH was at 14% and the temp at 75C. Now after over 16 hours of drying the RH is 13% at 75C. That seems like a miniscule drop in RH for that many hours of drying. In fact, RH of 13% at 75C would be over 100% RH at room temperature (by my calculations). The RH in the room (at 22C) is 45%. So either the new spool of TPU was really wet to begin with (right out of the packaging?) or something is wrong with the HT and it’s not drying effectively for some reason.

For comparison sake, I put the AMS Pro 2 in drying mode (with 4 rolls of new PETG) and it drove the RH down to 6% at 22C in a little over 8 hours.

Any thoughts on this? Is this normal with TPU and the HT? I’m very new at this, but something just doesn’t seem right. Thanks for any help.

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Hi Kenton, a few things to consider (before throwing the HT under the bus)…

This could be normal… The only way the RH inside the HT is really going to drop is with the desiccant in there sucking out moisture. The filament may still be releasing moisture, which is the point, and that air is being exchanged with the outside air and in theory being removed, but in exchange it’s just sucking in ambient air. And with the rather minuscule amount of desiccant in the HT, only so much moisture can be removed before the air is exchanged. In contrast, the large AMS units can hold a lot of desiccant, especially if one adds extra storage bins.

Getting extra desiccant into the HT, eg. in a container mounted in the center of the spool being dried, can definitely help.

The filament could already have been dry, or as dry as the HT will ever get it in the current environment, so there’s not going to be any observable RH change.

Now for the issues… folks here have posted varying experiences with the HT, so definitely YMMV (there are threads on here worth reading). Personally I gave up on the HT as a dryer, but I’m trying not to let that influence my advice too much.

One thing reported is the inaccuracy of the humidity sensor, which for at least some folks reads high (possibly due to its position within the unit, sometimes due to packing tape still being left during assembly, covering the sensor… lol). The humidity towards the back of the unit also tends to be lower (where the exhaust vent and desiccant packets are).

Another thing is the venting itself, which may not be entirely effective. For example myself and others have better results if we briefly opened the HT once in a while to release trapped moisture… the humidity reading can drop several percent in some cases (then slowly build back up). The vents themselves have also known to fail or be defective (and not open/close correctly), which should and in most cases does throw an error at you, but apparently not always (can be visually checked).

So is the filament getting dried? IMHO the simplest and most direct way to check is to print with it. Another common measure besides monitoring environmental RH is to weigh the spool (on a scale with at least 1g resolution) before drying and then compare to weight after/during drying. It’s still a relative metric (doesn’t tell you how much moisture is actually in there), but in theory once it stops loosing weight then it’s as dry as the drying solution can get it. That doesn’t measure ultimately how effective the drying was, but it’s something, and quick to do.

HTH,
-Max

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That confuses a lot of people. In a running dryer you are really looking at a process and not a static equilibrium.

Water is on the move. It’s leaving the filament to enter the air and then the dryer is trying to move it outside of the dryer. The humidity reading is a result of the two processes competing. More water comes off the filament than the dryer eliminates and the humidity goes up. When the dryer can move more water out, the humidity drops.

It’s also confusing because if the dryer is doing well and removing lots of water the humidity can seem artificially low and people can think the spool is dry when all that has happened is water is coming off slower than it’s being removed.

Hopefully that makes sense but in sealed storage you can see equilibrium in static systems but in dryers it’s dynamic and more complicated.

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That makes sense. But if there is no filament in the dryer, should the RH percent drop to, let’s say, at least 10%? I was trying to figure out if my AMS HT works, so I only left 2 humidity meters in the dryer. And having it dry for 12 hours at 45c. When my humidity meters showed 10%, the AMS HT meter still showed 25%… That’s a big difference… I do not like AMS HT, and I wish I had spent the money to buy Sunlu E2 instead.

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Ah. With no desiccant and no filament in there you should be seeing just what the dryer itself can do ideally. There will probably be variations since the dryer has to exhaust to room/ambient humidity, but in general you should see the best performance possible because there is no real load.

As an aside, just saw where someone found the tiny film seal on some humidity sensors in some Bambu units had been left on after the boards were cleaned which totally invalidates any humidity readings. Those seals prevent process washings and coating applications from contaminating the sensor during board manufacturing. If readings don’t change or barely change, or don’t make sense, maybe that’s a cause?

I personally wouldn’t put desiccant in any active dryer. If the desiccant is well-dried, it will soak up any liberated water from the filament possibly better than the dryer can — so in that case you don’t need the active dryer.

If the desiccant is loaded up, the dryer will be trying to dry filament and desiccant and the desiccant is a much bigger load. It hobbles the dryer performance.

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First of all a big thanks to all who replied. I’m actually overwhelmed by the number and quality of responses in a short time (essentially just a day). My first time on this forum and so far it’s been great.

Anyway, thought I’d report back after the whole drying process actually finished: After 18 hours of drying (at 75C) the RH was 13%. So I’m thinking, “Oh no, by the time the HT cools down to room temp (22C) that RH is going to be a lot higher – maybe even higher than the 36% it started out at”. But that’s not what happened. In the 5 hours or so it took for the HT to cool, the RH did rise, but only to 16% where it has now stayed for the last 12 hours. So it seemed a lot of humidity disappeared after the drying process ended – but to where? It wasn’t going outside the box because now the HT is just acting like a sealed container. After reading through and thinking about all your replies, the answer is obvious to me now: It must have gone into the desiccant (and perhaps a little bit back into the TPU). I should mention when I first got the HT, I printed off one of the HT-compatible desiccant boxes from MW and filled it with reusable orange silica beads. I’m thinking the drying process baked those beads and released any moisture they had accumulated. When the drying process ended and the temperature started to drop, those beads started doing their thing again and sucked up a lot of moisture that was hanging around and are now holding on to it. Hence the relatively low final RH of 16% at room temp.

So the net, I guess, is that the HT is working as I hoped: With a full spool of TPU, the RH has stabilized around 16% at 22C, which is pretty acceptable and as soon as BL delivers my new hot end, we’ll maybe see how dry that filament really is.

Thanks again everybody!

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The humidity you read in a running filament dryer is showing you the balance between water being liberated from the filament and removed from the drying chamber by the dryer.

If the dryer is doing a good job of eliminating water then the reading will trend toward a low humidity. A spool not giving off much water will also do this. If the filament is giving off lots of water it can trend to higher humidity - same as if the filament dryer isn’t removing the water as effectively. It’s a balance.

When you hit 13% RH in your dryer and turned it off to cool down, you are correct that a container of air at 13% RH would move to a higher RH as it cooled, but there are other things happening in a dryer, as you suspect.

As the dryer contents (air and filament) cool, the filament is starting to act like a desiccant. It will move from being a water source to being a water sink.

And the water amounts are small. It’s tiny fractions of a gram of water in the air and at least a reduced amount of water in the filament but still maybe on the order of grams. But it has had some water removed and becomes hungry to get it back.

The ultimate humidity you saw when it was all cooled down was a result of the air cooling and the RH going up, the filament cooling and starting to act like a desiccant itself and pulling the RH down, and whatever desiccant in the chamber doing what it does and pulling water out and presumably lowering RH as well. Thing is if the desiccant wasn’t more dry than the filament it could turn into a water source.