It’s true that the equilibrium RH test isn’t immediate. RH sensors are slower to equilibrate with the environment they are in at low RH values but they aren’t glacial. Step changes show up pretty quick. It’s waiting until the number totally stops changing that can take a while. In practice just 10-15 minutes will tell you the major answer and 30 minutes or so should get you within a point or few depending on the moisture level.
Anyone can (and should) try the test and see for themselves. It’s only if you want to get the last percentage points (of debatable use anyway with cheap hygrometers) that the time drags out.
Possibly still not practical for the OP but I would suggest instead of trying to calibrate on the fly for varying moisture content, especially since it looks impossible, @user_3764121982 might concentrate on more uniform moisture content in their filament.
That would mean humidity targets when drying as well as in storage/use. It’s something like what I already do. I don’t dry for a certain time. I dry to a target humidity and then keep my filament as dry in storage as possible. I don’t have to calibrate anything to get consistently good prints now and haven’t since I did my own little research project on drying. Water just isn’t an issue for me. Zip. Nada. Zilch.
Can’t say if better drying would get OP where he needs to go though.
And did more digging on instrumentation to provide real-time moisture content information. Conductivity is believed to be a no-go for filament and bulk/spools have issues with winding uniformity affecting the measurement - except for an annular capacitor design that only looks at the filament inside. It hasn’t been commercialized yet but could possibly be a very cheap sensor that directly measures moisture.
So no nondestructive tests known yet for filament moisture but one possibly in development. It’s at least been discussed as workable.
IF 1KG of PETG has a moisture content of 40% and is put in a sealed container with 0% RH, what will the RH% be after 48 hours
The answer basically was the same thing we have been saying.
The Relative Humidity (RH) inside the container will reach 40%
if the PETG was previously equilibrated to a 40% RH environment, or 100% if the phrasing literally meant the plastic contains 40% water by weight.
1. Identify the moisture equilibrium
Hygroscopic materials like PETG absorb and release water vapor until they match the surrounding air. This state is called Equilibrium Relative Humidity (ERH). If your filament is acclimated to 40% RH, its internal vapor pressure matches air at 40% RH. [1]
2. Compare the mass ratios
A 1 kg spool of plastic is massive compared to the air in a 1-gallon container.
At 60°F, 1 gallon of air at 40% RH holds only about \(0.02 \text{ grams}\) of water.
The 1 kg spool holds vastly more moisture than this, even at low absorption levels.
3. Determine the final RH
When placed into 0% air, the filament desorbs moisture. It only needs to shed \(0.02 \text{ grams}\) of water to bring the entire 1-gallon container up to 40% RH. Because \(0.02 \text{ grams}\) is an imperceptible fraction of the spool’s mass, the filament’s internal moisture does not drop significantly. It will quickly drive the closed air up to 40% RH and stop.
Note: In the literal (but physically impossible) scenario where a solid 1 kg PETG block contains 40% water by weight (\(400 \text{ grams}\) of liquid water), the container would completely saturate to 100% RH almost immediately.
Final Result
After 48 hours, the closed container will stabilize at a final reading of 40% RH because the massive filament spool dictates the humidity of the tiny volume of air.
I think the biggest hang up is the time reference, you cannot determine quickly how much moisture content a spool of filament contains, but the blanket statement many are using that you can’t use the AMS RH% to determine the level of moisture in a spool is false. You can, if you give it enough time.
I think the conversation has deviated slightly from the original ask, which was moisture detection of the filament itself. What you and MZip are discussing (using equilibrium RH as a dryness indicator) is a separate, already-established practice widely used in 3D printing.
You couldn’t tell that from most posts about water on this board, though. The incorrect lore about various aspects of drying that cause people problems has been very pervasive. It would seem Bambu even discontinued PETG HF at least in part from people thinking it was dry yet having all sorts of print issues. Many posts here if you search. I dry my filament thoroughly and know it’s dry and my PETG HF prints have been great.
My original reason for pushing the RH measurement was for people having problems with drying (or not realizing they are having problems with drying). It was a diagnostic that would give definitive answers on filament moisture and as a diagnostic so what if it takes a while for the reading to settle.
Few here even know what a Karl-Fischer titration is (I actually do) and were instead convinced that 0 weight difference during a dry meant 0 water was left which is simply false. But it was the accepted method here and left many with bad prints and frustrations high enough some said they were returning or selling printers.
So while RH may be accepted at other forums, it sure hasn’t been here. And it’s truly valuable as a diagnostic and sorting if a drying method is effective or just wasting time and energy.
I’m not sure who you are replying to but if it’s to me it’s pretty funny.
RH% can be used to indirectly measure the moisture content of a spool of filament. That has been my whole point here.
If the AMS has desiccant in it (always active), or is an AMS that’s drying filament things get complicated because the system is no longer really in equilibrium. Also, the AMS units hold four spools. Put two spools in an AMS and read the humidity and it will be a combination of those two spools and anything else going on. And it will be a weighted average of sorts depending on how much filament is on each spool.
If you only put one spool in an AMS, and there is no desiccant or running drying membrane, and it is sealed well enough for the time it takes to settle, then yes, you absolutely can measure humidity in there and indirectly, filament moisture. As an aside, though — dead volume is a killer. For faster response you would want to have cylinders that displace air in the other slots to minimize dead volume effects.
I was speaking practically. In actual use, most have desiccant in there, multiple spools, and there may even be an active drying membrane chugging away. In those cases, no, you can’t use an AMS to measure filament “humidity”.
Its more like a high school science fair sharing a room with a quantum physics conference, mostly the two groups keep to themselves but every once in a while the interactions are predictably hilarious.
This. It’s really all the humidity in an AMS tells you. Desiccant is much stronger at influencing humidity than filament is. If there is desiccant in the AMS, that’s the biggest contributor by far to the humidity in the AMS.
You can see spikes if you put a wet spool in, or dips if you put a dry spool in, but the humidity mostly tracks the desiccant.
It’s really all the humidity in an AMS tells you. Desiccant is much stronger at influencing humidity than filament is. If there is desiccant in the AMS, that’s the biggest contributor by far to the humidity in the AMS.