The problem you're having has less to do with the RBA's than it does with the juice you're using. Certain juices-especially fruit-flavored or "sweet" ones- tend to have issues with fully vaporizing their contents, leaving behind some of the flavoring/sweetener components- and, while I have no evidence to support this, I believe that these "residuals" are also chemically altered via the heat from the atomizer coil during the liquids' phase change from liquid to gas, becoming heavier and more complex molecules.
What happens next depends on what kind of a system you use: In the old days when everything was top-feed, you would barely notice these residuals until it was time to clean or change your atomizer, because they would collect in the atomizer itself, condense, and-having nowhere to go- they would stay there, eventually affecting the taste and leading to a "dirty" or "spent" coil. Nowadays, we have bottom-feed designs, often with clear tanks, and what we're seeing- provided my hypothesis is somewhat accurate- is that when these heavier molecules are created at the coil, some of them are being either displaced by lighter, unaltered molecules of the original juice traveling up the wick, OR the heavier molecules are "sinking" back down the wick into the tank- or a combination of the two processes. It's impossible [for me] to know for sure because the whole process of wicking is very dynamic, and I haven't brushed up on my knowledge of chemistry, physics or fluid dynamics for years. Either way, you end up seeing the results of the process(es) at the bottom of your tank after vaping a substantial part of the unaltered juice.
I've noticed this process happening in silica-wick bottom-fed tanks, SS Genesis-style tanks, and even in bottom-feed box mods like my Vmod, so it isn't specific to SS, or any wick material for that matter- even my Vmod, with no wick at all, gets dark "sludge" buildup in the bottom of the feed bottle after heavy use. I recommend avoiding vaping it when it gets too dark, as we really have no idea what these heavier molecules are, nor what their effects on our health might be.
Again, a quick disclaimer: This topic needs hard research in order for my hypotheses to be labelled "the truth". Even then, the processes happening are so complex that we may never really know the exact mechanisms for how this is occuring- but I am fairly certain it can be proven, provided someone has the right credentials and the necessary time, resources and willpower to do so.
Before someone tries to say that it's "sugar" that's being "burnt" or "caramelizing"- while it may be a similar process that is occuring, e-liquid shouldn't contain sugar, if it is being manufactured correctly. Artificial sweeteners are used in place of sugar to provide sweetness in e-liquids.
Vape On!
