Thank you very much rw950431: as that pdf you suggested links to wick suppliers and I am wondering if my shoelace wick is causing a problem by being too thick. It also did not occur to me that the food industry used this kind of method to track RH, so I can do more digging there.
Peter_n: My primary interest in this method has to do with the fact that the caves are above 90% humidity much of the time, and atmospheric pressure changes above the caves can push them into condensing environments quite often. Most of the capacitive RH sensors I found list very long recovery times (with "drying out" procedures) if the sensor is exposed to 100% humidity and I wanted something that would gracefully bounce between 90-100% without issue. Also the accuracy of the more affordable capacitve sensors fluctuate quite a bit in that range, so there is even a chance that using a couple of DS18s might actually provide better data. But that's still a big if because while the DS18b20's discriminate down to 0.0625 C my current wet bulb depressions will be quite small above 90% as well. I have not tried averaging yet, but given the lag from the thermal mass of the waterproof DS18s, I don't expect to see much change there. My previous experience with them is that they are pretty stable sensors.
WRT Screens, just getting to the deployment site often involves squeezing through some pretty small spaces, so I would need something much more portable than a full-on Stevenson. Ideally something that fits into a small kit bag wrapped around my shoulder. It also has to be rugged enough to take some knocking about, as that also goes with the territory.