Technical Explanation of a Self-Regulating Hydration System: 
I've been learning more about this product as I've used it and experimented with making variations of the design. Here is an important point I think I have learned:
Drew has explained that the inner tank must be filled with a material to "absorb" water. Drew completed a series of tests, which resulted in his using compressed vermiculite in the inner tank. He noted that this material "absorbed" the greatest volume of water. I think, however, that "absorption" may be a misnomer, as perspective can become distorted when inner workings are concealed.
In my judgement, the material in the tank serves two distinct purposes:
1) The material in the tank holds or suspends water.
Depending on the material, water will be collected and suspended in a network of air pockets both in and between the particles. As water is absorbed by the hydrostone floor, any remaining water in the connected network will continue to flow through the hydrostone and external digging medium via capillary action.
2) The material takes up volume, creating a self-regulating system by limiting the maximum saturation ratio.
This is where things get tricky. 50 cc of vermiculite (4.6 g) compressed to a space of 35 cc will only hold about 15 g (15 ml) of water in the vermiculite before the water reaches the bottom of the container (a graduated cylinder) when filled from above. Prior to reaching this critical point, it is mostly suspended by the vermiculite. If water is continued to be added until the top is completely saturated, it will have a
maximum holding capacity of about 23 g (23 ml). At maximum capacity, compressed vermiculite and trapped air represent 34% of the volume, closely held water represents about 43%, and free water represents the remaining 23%.
Imagine for a moment that the 75 cc inner tank is not filled with any material. Adding 75 ml of water, bringing it to full capacity, would result in liquid slowly draining out through the hydrostone until an equilibrium of 100% saturation was reached between the inner and outer tanks. If the outside soil could not hold 75 ml before reaching 100% saturation, the formicarium would flood.
In a different scenario, the tank is filled with about 75 cc of compressed vermiculite. This 75 cc of vermiculite has a
saturation capacity—the amount of liquid it will hold before water starts to pool at the bottom—of 23 ml of water, and a
maximum holding capacity—the maximum amount of excess liquid that may be held by the vermiculite without draining away due to gravity—of 35 ml of water. Outside the tank, there is about 80 cubic centimeters of usable volume for sand or other digging medium. Depending on the medium used, this will absorb quite a bit of water, but always less than 80 ml.
#7 Small:
Total inner tank volume: 75 cc
Total outside tank digging volume, less hydrostone block: 80 cc
Inner tank saturation capacity (with compressed vermiculite): 23 ml
Maximum holding capacity (with compressed vermiculite): 35 ml
So what is happening when water is added to the formicarium? With no compressed vermiculite, 75 ml of water added to the 75 cc inner tank will result in all 75 ml being drained into the 80 cc outside tank. Eventually, the inside tank and outside tank will reach an equilibrium, but the outside will be completely flooded.
With compressed vermiculite inside the tank, the 75 cc of space only comfortably holds up to 23 ml of water, even though up to 35 ml may be filled before the inner tank begins to overflow. Because the vermiculite is "holding" the liquid, not all of the water drains away. For sustained, long-term use, the characteristic of water to be held by the compressed vermiculite or other holding media in the inner tank, until the outside digging media becomes dry and thirsty for more water, is what makes this system work. The outside saturation level is regulated by the limited saturation capacity and maximum holding capacity of the inside material.
What about a smaller tank and no media? While a reduced inner tank capacity of 25 cc or 35 cc would probably saturate the 80 cc of outside digging media similarly to the 75 cc tank filled with compressed vermiculite, it would not have the self-regulating feature of the compressed vermiculite. Unless the maximum holding capacity of the outside digging media was known, and the current capacity was also known, filling the smaller tank when the outside digging media is at or near maximum capacity could result in the outside media becoming oversaturated and flooded.
Edited by drtrmiller, July 18 2017 - 11:25 AM.
