From my experience with any of these sensors whether it be the sht 15, 75, or 31 if it doesn't have a wet bias in the low to mid RH it will usually not read above 98%. If it does reach the 99% to 100% it tends to read too wet a lower values. Very rare does it seem like I find a sensor that does both. My preference is to have better accuracy in the mid ranges than to have a sensor that reads 100%, but that"s just me.
Yes, my 31 which is normally accurate in the mid ranges (when it hasn't spent extended time above 80% and gets out of calibration for awhile), will no longer read above 95% and even then only briefly. Practically speaking, 94% is the top end. Originally, I believe 97% was the top with maybe flashes of 98% in extended foggy periods. This is a sensor first installed at the end of November last year. Not even a year old!
Feeling pretty frustrated right now because we had some wet weather recently and the sensor got out of calibration. This afternoon humidity was in the 70s and the sensor wouldn't recalibrate. Last time it took almost 48 hours to return to normal. More rain is on the way tonight thru Wednesday. So... thinking to myself... do I yank the -8% offset I put in this afternoon (after testing with my Kestrel)? That's probably the only course of action I have or it will run dry when it's raining and immediately afterward.
It's getting quite ridiculous the lengths we all have to go to so we can have even somewhat accurate humidity readings.
UPDATE 11PM: Once the sun set and humidity started climbing into the 80s, the biases in the 31 converged. So I was able to safely remove the offset... at least until humidity drops below 80% again.