Interesting humidity and filament experiment

[WobblyHand]

I tried out some of those vacuum bags and filament, with my home made RH meter. The meter is powered by a single 18650 battery. Putting the meter in the bag with silica gel and sealing it, but not pumping it down, you could see the RH plummet downwards from the initial room RH = 17%. The RH in the bag was below 6% within a couple of minutes.

However, when the bag is evacuated, the RH went up. It went up to 14%. Eventually, over 24 hours it declined back down to 6%. I think this is telling me that evacuating is a good idea as it is pulling moisture not just out of the air, but the filament. I think under vacuum conditions the filament is finally out-gassing the absorbed water. And the silica gel is slowly absorbing it. So simply putting your filament in dry air doesn't really pull the moisture out, you need to lower the ambient air pressure as well.

Anyways, thought it was an interesting experiment.

 

[francist]

Those are exceptionally low values for Relative Humidity, what did you calibrate your meter against? Looking at Nashua RH for today I see numbers around 68%. That’s outdoors of course, but I’m still wondering how you’re seeing values like 17% inside your house, not to mention 6%.

 

[great white]

What's happening is you are lowering the “boiling point” of the water.

Water in a vacuum transitions to gas at a much lower temperature under a vacuum, sometimes as low as room temp.

The same principle is used when servicing auto ac systems: put it under a vacuum and let it sit to remove the harmful water vapours from the system. Leave it sit long enough and it will start drying out the desiccant bag in the accumulator.

By putting the filament under vacuum, you are allowing the entrained water in the filament change state to gas and escape the plastic.

As a side bonus, as you remove the air from the bag, you are also removing whatever RH existed within that air.

Vacuum storing filament is just a “win-win” scenario….

 

[WobblyHand]

Those are exceptionally low values for Relative Humidity, what did you calibrate your meter against? Looking at Nashua RH for today I see numbers around 68%. That’s outdoors of course, but I’m still wondering how you’re seeing values like 17% inside your house, not to mention 6%.

I didn't. The right way to do it is to measure the dew point I guess. These IC based RH meters infer RH I think from changes in capacitance. All I know is in their algorithm they use a constant offset of -6 in their equation. When use the RH sensor in the bag, the RH comes out to -6% which we all know is not possible. However, when you read the datasheet, they mention that under process, voltage and temperature, that that output is possible. My luck I got a corner case part...

All of my cheapo RH meters (now I have 3 of them) are telling me it is very dry indoors. The dew point outdoors today is about 31F. Using a handy online calculator, the indoor RH is around 22% at a temperature of 71F. https://www.wpc.ncep.noaa.gov/html/dewrh.shtml. One RH meter is reading 24%, the other is reading 27%. The higher reading one usually reads higher. However, last night I put it closer to the bedroom (and adjacent bathroom) so I'd think it would read higher. The past week the dew points were around 11F. So the RH indoors has gotten very low.

RH depends on dew point and temperature. For a constant dew point, as the temperature increases, the RH decreases.

 

[WobblyHand]

What's happening is you are lowering the “boiling point” of the water.

Water in a vacuum transitions to gas at a much lower temperature under a vacuum, sometimes as low as room temp.

The same principle is used when servicing auto ac systems: put it under a vacuum and let it sit to remove the harmful water vapours from the system. Leave it sit long enough and it will start drying out the desiccant bag in the accumulator.

By putting the filament under vacuum, you are allowing the entrained water in the filament change state to gas and escape the plastic.

As a side bonus, as you remove the air from the bag, you are also removing whatever RH existed within that air.

Vacuum storing filament is just a “win-win” scenario….

Totally agree. Was simply physics in action, but surprisingly rather sudden. Wasn't expecting the RH to increase that much within seconds, but it did.

Now these bags are not exactly helium leak free, but one I tried been holding overnight. This convinces me that doing these vac bags and dessicant is the way to go. Simple desiccant in a bag is not removing the moisture from the filament very effectively. It may be ok for storing dry filament, but is not as effective removing absorbed moisture as evacuating the bag + desiccant.

 

[RJSakowski]

Silica gel is a great example of how equilibrium works. IMHO, it is a good reason not to use silica gel as a desiccant when packaging delicate equipment. The tiny packets quickly become saturated and are essentially constant humidity environments.

We used silica gel in a dedicator when I worked in a chem lab. It had cobalt chloride as an indicator and when it started to turn from blue to pink, we rejuvenated the silica gel in a lab oven. I don't recall the temperature we used but it wasn't excessive. Once rejuvenated, the silica gel was good for a couple of weeks of use again. Our desiccators were made of glass and had silicone grease on the seal to prevent moisture ingress.

Relative Humidity is a function of both temperature and air pressure. The gold standard for RH is a sling psychrometer which essentially measures the dew point of the ambient air. Electronic or mechanical hygrometers use surrogate means. IIRC. a single horsehair attached to a pointer was once used. The hair would lengthen with increasing humidity, moving the pointer. Modern mechanical hygrometers rely on a change in length of a strip die to moisture absorption. Electronic hygrometers utilize a change in resistance of a moisture sensitive material. Whether these devices track with a sling psychrometer over a full range of temperature and pressure is questionable.

 

[RJSakowski]

Our home breathes rather freely and the three RH meters read, 45%, 47%, and 50%. Accounting for the difference in temperature in the three locations, the RH agrees to within 1%. The dew point today is 30ºF and has been close to that for the past day. This would imply a 21% RH at 71ºF. However, our house is also capable of retaining moisture and outgassing as the dew point drops. This will be a completely different scenario in January.

 

[WobblyHand]

Silica gel is a great example of how equilibrium works. IMHO, it is a good reason not to use silica gel as a desiccant when packaging delicate equipment. The tiny packets quickly become saturated and are essentially constant humidity environments.

We used silica gel in a dedicator when I worked in a chem lab. It had cobalt chloride as an indicator and when it started to turn from blue to pink, we rejuvenated the silica gel in a lab oven. I don't recall the temperature we used but it wasn't excessive. Once rejuvenated, the silica gel was good for a couple of weeks of use again. Our desiccators were made of glass and had silicone grease on the seal to prevent moisture ingress.

Relative Humidity is a function of both temperature and air pressure. The gold standard for RH is a sling psychrometer which essentially measures the dew point of the ambient air. Electronic or mechanical hygrometers use surrogate means. IIRC. a single horsehair attached to a pointer was once used. The hair would lengthen with increasing humidity, moving the pointer. Modern mechanical hygrometers rely on a change in length of a strip die to moisture absorption. Electronic hygrometers utilize a change in resistance of a moisture sensitive material. Whether these devices track with a sling psychrometer over a full range of temperature and pressure is questionable.

When used appropriately silica gel is ok. First thing is to have enough of it. That way one isn't asking 1 gm of material to do all the heavy lifting when 100 gm is more appropriate. I bought a pile of bulk silica gel. My intention was to keep 3d printing filament dry. I got a bad spool of filament, when I was printing with it, I could hear it spitting and crackling due to all the absorbed water that was being flashed to steam inside the extruder. I thought that putting the spool in a bag and using lots of silica gel might dry it out. It does, but not quickly. The other thing is to only operate on a limited volume. Drying a few liters works with desiccant, won't do a whole room.

Evacuating the bag and using desiccant seems to be more effective than just desiccant in the bag. I'm not talking high vacuum, but something to encourage the absorbed water to out gas.

My father used to have a sling psychrometer. I wonder if I have it... Remember vaguely him spinning that.

Silica gel is pretty easy to renew. I spread out some in an electric toaster oven pan, and heat it. Don't remember the temperature, but it wasn't that hot. About 30 minutes and it was good for quite a while. I used to keep the bulk silica gel in sealed glass mason jars. Color never changed over several years. Dark blue - as that was the cobalt indicator type. The new stuff is orange, turning green when done. I haven't tried drying that yet.

 

[great white]

Totally agree. Was simply physics in action, but surprisingly rather sudden. Wasn't expecting the RH to increase that much within seconds, but it did.

Now these bags are not exactly helium leak free, but one I tried been holding overnight. This convinces me that doing these vac bags and dessicant is the way to go. Simple desiccant in a bag is not removing the moisture from the filament very effectively. It may be ok for storing dry filament, but is not as effective removing absorbed moisture as evacuating the bag + desiccant.

Yep, happens quick. As the pressure drops, so does the boiling point. If you’re below the boiling point, moisture transitions from liquid to gas almost immediately.

As another example, the higher in altitude you go, the shorter the time required to boil water. Was pretty cool to throw on a pot in the aircraft at altitude on to boil and see how quickly it boiled. Had to watch it closely or you could boil the hot cup dry in short order…

 

[great white]

When used appropriately silica gel is ok. First thing is to have enough of it. That way one isn't asking 1 gm of material to do all the heavy lifting when 100 gm is more appropriate. I bought a pile of bulk silica gel. My intention was to keep 3d printing filament dry. I got a bad spool of filament, when I was printing with it, I could hear it spitting and crackling due to all the absorbed water that was being flashed to steam inside the extruder. I thought that putting the spool in a bag and using lots of silica gel might dry it out. It does, but not quickly. The other thing is to only operate on a limited volume. Drying a few liters works with desiccant, won't do a whole room.

Evacuating the bag and using desiccant seems to be more effective than just desiccant in the bag. I'm not talking high vacuum, but something to encourage the absorbed water to out gas.

My father used to have a sling psychrometer. I wonder if I have it... Remember vaguely him spinning that.

Silica gel is pretty easy to renew. I spread out some in an electric toaster oven pan, and heat it. Don't remember the temperature, but it wasn't that hot. About 30 minutes and it was good for quite a while. I used to keep the bulk silica gel in sealed glass mason jars. Color never changed over several years. Dark blue - as that was the cobalt indicator type. The new stuff is orange, turning green when done. I haven't tried drying that yet.

Probably worth keeping in mind poor quality filament can often contain “air bubbles” from poor manufacturing processes. These “bubbles” can often mimic the effect of water in filament by popping, hissing and skipping (ie:voids) when printing….