Humidity Control: DIY Salt Packs

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pedant
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Tue Dec 19, 2017 2:59 pm

Here I describe the background (including theory of operation) of salt packs and give a design suitable for pumidor humidity control.
The goal is a design that is robust, inexpensive, and uses materials you may already have.

Background

Salt packs, sold under the brand name Boveda among others, are humidity control devices that employ a salt solution to maintain a water vapor equilibrium in a sealed environment.

In practice, an excess of salt is used for a few reasons:
  • It ensures that the solution is always saturated and is therefore at fixed concentration at a given temperature. This provides a fixed relative humidity (RH) equilibrium.
  • This fixed humidity point is ultimately not affected by reasonable moisture sinks (like dry puerh cakes). As moisture leaves the salt pack, some salt precipitates out of the solution, and it will stay at the same concentration.
  • It provides two-way humidity control: if the RH goes above the salt pack's humidity point, the solution behaves hygroscopically and draws moisture out of the air. Some of the excess salt dissolves, and the solution concentration remains unchanged.
Commercial salt packs are composed of a saturated salt solution (with excess salt) that has been thickened to improve handling characteristics using xanthan gum or similar. This gel is disposed within an impulse-sealed membrane pouch. This pouch is typically a polyethylene-paper laminate and is engineered so that the polymeric film is sufficiently thin to be permeable to water vapor while also being liquid impermeable. The paper backing provides puncture resistance.1

Different salts give different humidity points, and this can be exploited in salt pack design. For example, saturated NaCl solution gives ~75% RH at standard temperature and pressure (STP), and NH4Cl gives ~84% RH.2 It is possible to use salt mixtures to design packs at arbitrary humidity points, but the interactions are complex and sometimes counterintuitive. For example, a buffered mixture of the two aforementioned salts is used in a 69% RH salt pack -- note that this is below both salts' individual humidity points.3 Instead of using salt mixtures, pack design is simplified by adding a non-electrolyte to a salt with a higher-than-desired humidity point. A non-electrolyte, such as sucrose, reliably decreases the pack's humidity point.4

Another consideration is a salt's humidity point over the temperature range of interest. NaCl is ideal in that it has a particularly flat RH-T curve5:
Image

Design

The most obvious question is which humidity point to design for. I arbitrarily chose 70% RH. The general consensus online seems to be that 60-80% RH is acceptable, but mold growth in sealed pumidors is a serious concern. One article suggests6:
  • ≤60% RH: molds don't grow
  • 70% RH: molds don't grow much aside from xerophiles (e.g. P. chrysogenum germinates at 73% RH on leather and paper)
  • ≥80% RH: molds grow readily
I found experimentally that a mixture of 6:25 sucrose:NaCl by mass gives about 70% RH at room temperature. A saturated solution of this mixture could be held in any container, but I chose a mason jar. A Tyvek membrane was added to give it splash resistance in case the container gets bumped.

NaCl has a solubility of 359 g/L in water at room temperature, so I thought it best to exceed that by ~100%.

Materials
  • 70g NaCl (non-iodized table salt)
  • 17g Sucrose (table sugar)
  • 100g (100mL) Distilled water (tap water is probably OK)
  • Tyvek Envelope (free at USPS)
Total jar contents: 187g

A note on salt: I've recently noticed that some brands of salt have an odor. I believe that this smell is adsorbed onto the crystals and comes from packaging, transportation, or storage. If your salt pack is smelly, let it air out for 2-7 days (stirring daily) until the odor dissipates. If that fails, try another brand. See later in this topic for related discussion.

Construction

100mL water heated to 65°C (150°F) and 17g sugar were combined in a mason jar and mixed until the sugar dissolved. The sugar was intentionally added before salt (before saturation) to hopefully get a more consistent sugar:salt dissolved ratio.
70g salt was then added and swirled for a bit. Not all of the salt dissolved.

Next, a piece of Tyvek larger than the jar diameter was cut from an envelope, placed on the jar opening, and the lid band was screwed onto it. Excess Tyvek (exposed all the way around the lid band) was trimmed a bit.

Image
image credit: eBay fungi_supply

The completed salt pack was allowed to cool to room temperature before using.

Conclusion and Notes

Note that this design can be scaled up or down, but the surface area of the container opening controls how quickly it can operate. I suggest using multiple salt packs for this reason.

Because commercial salt packs are pouch-based, they have a much higher surface area to volume ratio and therefore increase humidity faster than this DIY one. However, this isn't a big deal if you aren't constantly opening up your pumidor. Your cakes will eventually hydrate and reach an equilibrium with the salt packs. Once this happens, their own moisture content gives buffering capacity that helps the salt packs rapidly restore humidity after opening and closing the pumidor. FYI, this is how silica gel works7, and the effect works best if your pumidor is stuffed with tea. ;)

An advantage of the DIY pack is that you can tell at a glance if it needs to be refilled. To refill, just top it up with warm water, stir it well, let it cool down, and put it back in the pumidor.

Finally, I'm sure that some of you will recognize some of this from another hobby, but please spare us the references. :lol:

References
  1. https://bovedainc.com/about-us/patents/
  2. http://nvlpubs.nist.gov/nistpubs/jres/8 ... 89_A1b.pdf
  3. https://bovedainc.com/safety-data-sheets/
  4. https://patents.google.com/patent/US5037459A
  5. http://nvlpubs.nist.gov/nistpubs/jres/5 ... 19_A1b.pdf
  6. https://www.ncbi.nlm.nih.gov/pmc/articl ... 0-0019.pdf
  7. http://www.apsnyc.com/uploads/Demystify ... %20Gel.pdf
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pedant
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Tue Dec 19, 2017 3:11 pm

for further reading, here's a nice article by John_B:
http://teaintheancientworld.blogspot.co ... ative.html
Atlas
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Tue Dec 19, 2017 3:18 pm

Awesome write-up!

A few tidbits/questions, since I've been messing around with it over the last couple of weeks.

Is there any need for concern about the sucrose growing anything undesirable? (Not that I have any reason to think there is, just a question that springs to mind.

An alternative to the sucrose is doping with other commonly-available salts, per HERE. Morton's Lite salt, coincidentally, works out to be ~71%.

A quick and dirty way to get (close to) maximal saturation is to add water, mixing thoroughly, until you see a small amount of water at the bottom of your mixing container. Dump that small amount of water and add an arbitrary amount of dry salt, and mix again.

Being that my pumidor is opened regularly, there's a need (want) to recover humidity as quickly as possible after opening the door. A small (think 30mm 5V) fan is sufficient to double the rate of recovery, in the single test I ran.

Given that the salt packs are mildly annoying to recharge, I've been using a small cup of water with a small wicking filter, placed near the intake of the fan - this has done a great job of reducing the rate of depletion of moisture in the pack, and the size of the filter can be altered to determine (ie limit) the rate of evaporation to a level that the salt pack can cope with.
Last edited by Atlas on Tue Dec 19, 2017 3:31 pm, edited 1 time in total.
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pedant
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Tue Dec 19, 2017 3:30 pm

Atlas wrote:
Tue Dec 19, 2017 3:18 pm
Awesome write-up!

An alternative to the sucrose is doping with other commonly-available salts, per HERE.
Yeah, that is done. I went sucrose because it's about as commonly available as it gets (you probably already have it). It's also non toxic and has more predictable effects than salt mixtures as I explained. Fewer experiments I had to do to get there.
Atlas
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Tue Dec 19, 2017 3:33 pm

Whoops, my mistake, I somehow managed to miss that whole paragraph. :oops:
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Victoria
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Tue Dec 19, 2017 5:18 pm

Impressive article and your references are really useful too. Still wrapping my head around the science of humidity control, so just trying what you propose will be interesting, and fun, plus I’ll learn a little more in the process. Thank you.
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pedant
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Wed Dec 20, 2017 12:31 am

Atlas wrote:
Tue Dec 19, 2017 3:18 pm
Is there any need for concern about the sucrose growing anything undesirable?
some halophiles could grow i guess, but it's not something i'd worry about
Atlas wrote:
Tue Dec 19, 2017 3:18 pm
Morton's Lite salt, coincidentally, works out to be ~71%.
looks like a mix of NaCl and KCl. you tested it and got ~71%?
Atlas wrote:
Tue Dec 19, 2017 3:18 pm
Being that my pumidor is opened regularly, there's a need (want) to recover humidity as quickly as possible after opening the door. A small (think 30mm 5V) fan is sufficient to double the rate of recovery, in the single test I ran.
yes, fans make a huge difference
Atlas
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Wed Dec 20, 2017 1:34 am

pedant wrote:
Wed Dec 20, 2017 12:31 am
looks like a mix of NaCl and KCl. you tested it and got ~71%?
Yeah, which probably links back to your whole "sucrose is easier" point, because the EqRH of KCl is higher, so it seems unintuitive.

This was with a sensor (BME280) that tested within half a percent of 75% when calibrated with moistened NaCl in a ziplock bag, so it should be reasonably accurate.
.m.
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Wed Dec 20, 2017 6:44 am

This is incredibly interesting. Thanks for sharing it. I was just contemplating getting some boveda packs. Not anymore. Time to play. :P
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VoirenTea
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Wed Dec 20, 2017 2:55 pm

Does that lid have a plastic or glass top in it, or is it open on top of the Tyvek? I was assuming open, but the website isn't clear. I think I've only seen glass lid + wire clasp versions of Mason jars.
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pedant
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Wed Dec 20, 2017 3:08 pm

There is just a metal band/hoop that goes on the threads and holds the tyvek sheet. The top of the jar is only tyvek.

It's a two-piece lid system, but you're only using the band.

Image
Mchrlund1
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Wed Dec 27, 2017 2:06 am

Great idea - thank you!

Just a question for clarification: are you using just the membrane from the envelope, ie cutting the Tyvek out of the envelope?
Diocha5757
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Wed Dec 27, 2017 6:27 pm

Hi...i am using 18gr sucrose and 75gr nacl....is 110ml water good ratio.....

Thanks
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pedant
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Wed Dec 27, 2017 9:44 pm

Mchrlund1 wrote:
Wed Dec 27, 2017 2:06 am
Great idea - thank you!

Just a question for clarification: are you using just the membrane from the envelope, ie cutting the Tyvek out of the envelope?
some of the USPS mailers are made of tyvek. i.e. a piece envelope itself is the membrane. pretty much any part of it is suitable.
Diocha5757 wrote:
Wed Dec 27, 2017 6:27 pm
Hi...i am using 18gr sucrose and 75gr nacl....is 110ml water good ratio.....

Thanks
yes
Mchrlund1
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Thu Dec 28, 2017 12:59 am

pedant wrote:
Wed Dec 27, 2017 9:44 pm
Mchrlund1 wrote:
Wed Dec 27, 2017 2:06 am
Great idea - thank you!

Just a question for clarification: are you using just the membrane from the envelope, ie cutting the Tyvek out of the envelope?
some of the USPS mailers are made of tyvek. i.e. a piece envelope itself is the membrane. pretty much any part of it is suitable.
Diocha5757 wrote:
Wed Dec 27, 2017 6:27 pm
Hi...i am using 18gr sucrose and 75gr nacl....is 110ml water good ratio.....

Thanks
yes
I see, thank you! Just ordered some Tyvek envelopes, looking forward to trying this.
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