I thought I’d share my experience here, this topic confirms my long-held suspicions of wet bias with Davis sensors (not just the SHT31). I've not read the entire thing though.
I'm in the UK, in a valley, where on many nights the humidity rises above 95% with radiation mist/fog quite common. My set up is a 24h FARS and an older passive ISS, running two SHT31 sensors (18 and 24 months old)
These two sensors both drifted at the top end within 6 months (and in opposite directions for a while), The first went from reaching 98-99% maximum to only 95-96% (thick fog). I thought it may be a dodgy sensor so bought the 2nd sensor, but that also drifted down while by summer 2017 the first sensor was back to reaching 98-99% lol.
Currently, they reach about 97-98%, with the 2nd sensor 1-2% wetter through the mid-range.
Concerning the mid-range, and yep.. they seem to have a wet bias. I upload to a website that allows an easy comparison between two stations. This is me compared to the nearby official Met Office station (5 miles away) yesterday (a cooler, fresher day than many this summer with a well-mixed environment until about 9pm).
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My dew point is typically 2-3C higher (x1.8 for difference in F) Of course these are only hourly observation points so brief fluctuations could skew the graph, however this is a typical story.
Humidity for August so far
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As you can see I'm usually approaching 10% more humid. We are at virtually the same elevation with similar surroundings and I can’t see any genuine reasons for this when the wind is in the direction that it was yesterday. It does seem with me at least the problem is not just with warm humid airmasses.
Not happy with that but what can I do.. I don't really have a controlled environment to recondition sensors (and if they have to be removed from the Davis board? that's a no-go for me anyway).
Nearby Davis stations do read similar to me but probably have the same issue.
Also, a brief history of the previous sensors I've used in case it's of interest:
My 1st SHT11 failed in August 2009, I think it had less of a wet bias, but only reached 95-96%.
The 2nd SHT11 had a significant wet bias in the mid-range, and reached 98-99%.
I had a SHT75 'Davis ready' sensor from a seller on Ebay. That possibly had little bias but it only reached about 95% so think I only used it in the old/backup ISS.
I also had a SHT15 (3rd party), and it reached 97-98%. It still had some degree of wet-bias.
So as you can see wet biased mid-range and no 100% readings seems to have been a general theme for me.
I still have the 15 and 75 lying around so am going to plug them into my backup ISS and see what they say compared to the SHT31.
I still like the SHT31 for temperature and prefer its humidity response to the 11's I used. If a stable 'corrected' sensor came out I'd buy that, although my records won't be consistent but they aren't anyway with those sensor changes and drift lol.