Quote:
Originally Posted by MiLKMD
Two points from a very informative earlier thread that I want clarification:
How does PG not absorb water? PG is hygroscopic and by definition will absorb excess water out of the atmosphere until equilibrium is reached. Part of what makes PG/water solution so effective is the vapor pressure lowering (of water) effect of PG. At various concentrations around 50% the vapor pressure of water is kept around 70% RH and vapor pressure of PG almost negligible at room temperature (<1%). The SAP acts as a medium to hold the solution similar to foam but one could just as simply leave a cup of PG in a humidor and achieve the same effect as beads. Is it the SAP itself that prevents absorption? As stated above, the advantage of PG/water in SAP is the amount of water that the crystals have. In environments in which the atmospheric RH is significantly less than the humidor and there is some leak, SAP has advantages over beads in providing larger reservoirs of water to release.
I have seen this statement frequently on the forums, that KL will not release water. Why is this so? KL is silica gel and all silica gels are hygroscopic. I will concede the point that plain gel (which is probably what KL is composed) has an M(H) value much lower than treated Heartfelt beads at 70% RH. But nevertheless KL will release as well as absorb water. The key point is finding a buffering agent with the highest M(H) value at the desired RH. Artsorb for example has a very high buffering capacity at RH > 70%. I read with great interest Shilala's experiment with KL and treated beads the amount of each needed. In essence he has derived experimentally rather than mathematically a comparison of M(H) of various substances at 70% RH. However it is still possible to use KL or any silica gel for that matter to maintain a set RH via pre-conditioning. The only question, as Shilala so eloquently answered in his experiment, is how much of each agent is needed and at what price/performance ratio.
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Propylene glycol is hygroscopic, but only so far as it's volumetric nature allows it to gather water, and that volume is very minimal volume per volume.
There's charts and stuff, I'm not going to blather. Well, that's not completely true. I'm going to blather, just not about PG so much.
To answer your question, when combined with water or SAP, it's hygroscopic properties are instantly defeated, because you've just created a saturated solution. Saturated meaning, well, saturated. As in "I can no longer gather water because I'm saturated, thus I, propylene gylcol, am no longer hygroscopic until I get this water out of me (at least to a point where the water has been reduced below it's capacity to hold water hygroscopically)".
Some instances for illustration...
1.) Mixed with water or SAP, PG will only release water at "a rate equal to 70% RH". That's cool, because in a saturated solution you can control the release of water, holding it at 70% (because of the vapor pressure variance you mentioned earlier).
So in this instance, you've got a great opportunity to use it where a lot of water is needed to maintain a humidor.
It's an excellent choice for very dry places, or for leaky humidors in very dry places.
If your humidor sits in an area where the ambient RH is higher than 70%, the humidor is going to assume the higher ambient over time, whether it be through frequent opening or an inherently leaky humi.
That's the "downfall". PG is hygroscopic, yes. It's not when it's saturated.
Even when it's not saturated, it's ability to soak up water is nominal at best.
2.) Used in beads...
Beads are a dessicant. They gather water. They'll gather water until they reach an equilibrium with their ambient.
If part of the beads are treated with a PG/water solution, those beads will release water until they've achieved 70%RH.
The untreated beads in the bead product will try to gather water until the cows come home.
This is tough to explain, because there's more than one thing at play.
You have dry beads available to adsorb water, and you have PG/water soaked beads trying to give up water.
This all adds up to a medium that will release water AND adsorb water, but it has little to nothing to do with the hygrospic nature of Propylene glycol, because we've already defeated that by saturating it. The only reason I even qualified with the word "little" is because some beads could exist in the mixture that only have PG in them, and there's a very insignifigant chance that would play some tiny role.
In essence, you have beads "battling it out", with a control medium that is the Propylene glycol.
That allows a nice control, by both gathering and releasing water at a particular RH.
The only way to screw this up is to overwater the beads. Once the beads are saturated, you've just turned them into the same thing as a pg/water/sap medium.
Inversely, we can keep the beads totally dry and they will simply adsorb water until they've reached equilibrium, which would be the 70% that the PG dictates. Then they'll need to be dried out to continue adsorbing water.
3.) My beads...
There's no Propylene Glycol, salts, or anything. They simply work on equilibrium. They have a much higher affinity for gathering and releasing water than do silica beads, and their construction allows for a far greater resevoir of water to be made available for use in a humidor.
In a carefully sealed humidor, they are far and away the fastest and easiest way to control humidity. They also scavenge and hold free ammonia.
They simply rely on a bunch of math and physical principles to maintain RH in a humidor.
There's one thing that no beads or SAP can defeat, and that's a leaky humidor. Whether it be because it's warped, or it's a wine cooler and the drain isn't plugged, or because it's a generic piece of chinese handiwork.
A guy (or girl) is far better off with a rubbermaid container or a ziplock bag than a leaky humi.
Problem is, most are leaky.
Good thing is, most can be fixed.
Hope this helps!!!
Scott