The goal is a design that is robust, inexpensive, and uses materials you may already have.
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). The salt concentration remains unchanged as moisture leaves the salt pack because some salt precipitates out of the solution.
- 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 again the solution concentration remains unchanged.
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:
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
NaCl has a solubility of 359 g/L in water at room temperature, so I thought it best to exceed that by ~100% to ensure there is always undissolved salt present.
- 70g NaCl (non-iodized table salt)
- 17g Sucrose (table sugar)
- 100g (100mL) Distilled water (tap water is probably OK)
- Half-pint (8 fl. oz, 237mL) wide-mouth mason jar with lid band
- Tyvek envelope (free at USPS)
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.
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.
The completed salt pack was allowed to cool to room temperature before using. The jar doesn't look very full because I used a pint jar instead of a half-pint and didn't scale the recipe up.
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. The amount of water isn't so critical as long as some undissolved salt remains.
Finally, I'm sure that some of you will recognize some of this from another hobby, but please spare us the references.
- http://nvlpubs.nist.gov/nistpubs/jres/8 ... 89_A1b.pdf
- http://nvlpubs.nist.gov/nistpubs/jres/5 ... 19_A1b.pdf
- https://www.ncbi.nlm.nih.gov/pmc/articl ... 0-0019.pdf
- http://www.apsnyc.com/uploads/Demystify ... %20Gel.pdf