This is how they work but, first the basics on how your battery loses water.
There are two basic functions that cause water loss, electrolysis and evaporation.
Electrolysis occurs when the current going through the electrolyte (acid + water) solution splits the water molecules up into pure hydrogen and pure oxygen gas. This accounts for about 40%-50% of the batteries water loss. The second way your battery water is depleted is through heat which causes evaporation of the liquid electrolyte. This heat occurs when the battery is working to power your motor coach or when the ambient temperature in your house battery compartment is too warm or a combination of both. Evaporation accounts for the remaining 50%-60% of the water loss.
ORIGINAL OEM VENTS....
Standard vents that come as original equipment on your battery are typically a very basic cap with a direct path from the interior of the battery to the outside environment with nothing to prevent either evaporation or gas from escaping. A direct line of sight from the inside to the outside.
THIS IS HOW THEY WORK...
The Water Misers have three internal barriers built into the vent that prevents nearly all of the evaporating electrolyte from escaping. Initially, the evaporating electrolyte must pass through a chamber filled with specially fabricated polypropylene co-polymer beads that attract the liquid eventually forming droplets that accumulate and are redirected back into the battery. Secondly the evaporate must pass through a compressed mesh network of filter fibers that further capture and electrolyte that tries to escape from the battery. The internal design itself of the Water Misers was developed to create a large surface area relative to the amount of escaping gas. Condensation forms on the interior, is accumulates, and drips back into the battery by way of the pointed electrolyte probe on the inside of the vent
These three elements combined prevent the water loss, corrosion and acid build up related to your house batteries.
I can say all this with pretty high confidence in its accuracy because I actually run Flow Systems, the company who manufactures the Water Miser. Coincidentally, I ran across this post when searching for info on battery boxes for a friend who just purchased a used motor coach.
I hope this helps explain how the work. If you have any questions about the Water Miser battery vent caps, batteries themselves or charging, let me know I would be happy to help in any way I can.