I have followed this forum for years, but never registered or even really considered posting. I have learned a great deal from following all of your comments and experiences over the years. So let me start by saying thank you. This particular topic, however, has brought me out of the shadows …
I have owned Davis equipment since 2005. I bought a VP2. In 2006, I moved to south Texas and upgraded to a 24hr FARS and this is where my frustration with the dew point readings of the VP2 began. My new sensor would never hit 100% humidity and the rest of the scale had a wet bias, but my VP2 from 2005 never had these issues. My 2006 VP2 with FARS had an SHT11 and my 2005 had the old "analog" sensor.
I am no expert obviously, just another guy with a borderline unhealthy obsession with this stuff, but bottom line: my opinion and experience strongly supports that despite the inherent limitation of the Sensirion sensors, there is something about Davis' manufacturing process that causes an offset toward a wet bias.
I have been obsessed with trying to fix and find the source of this bias as well as trying to find a sensor that consistently hits 100% RH. The SHT11 had these issues and disappointingly so did the SHT31. I have moved around a bunch, so I have tested these things from anywhere from Texas to Maine to DC and to the Pacific Northwest. The result has always been the same. I have spent hours over the years with my sensors within 10 feet from the HO-1088 and DTS1 of an ASOS. I have purchased a psychro-dyne for quality checking. I have had the sensors in the -20sF in Maine and Vermont and over 100F in DC and Texas. Here are the main points of what I have learned:
-Outgassing of silicon and glue offsets the Sensirion sensor. The offset seems irreversible. Silicon offsets to the high side. Some glues to the low side. (I discovered this trying to use Sensirion's own filters instead of Davis' filter.)
-The Davis filter may increase response time, but is not responsible for the wet bias.
-The Sensirion sensors do seem to have a slight wet bias as has been documented in some studies, but this wet bias is some how exacerbated by how Davis mounts the sensor.
-The wet bias for Sensirion sensors in prolonged periods near saturation as mentioned in their datasheet is a separate issue from the wet bias created by the Davis mounting.
-The SHT75 straight from the manufacturer without any filter is superior to the Davis mounted SHT31 in terms of dew point and humidity.
-The SHT75 will hit up to 99%, with random very short lived jumps to 100%.
-I miss the old analog sensor
I currently run my station with an SHT75 with no filter at all. I've been doing it for about 6mos. The first 5 of those months were in a stock Davis 24hr FARS. Now it is in the Davis 7714, which is a surprisingly good passive shield (much better than the VP2 stock passive shield) (sometimes a degree or two cooler than the FARS, sometimes a degree or two warmer, at least in the north latitudes of the Seattle area...) I am concerned with contamination and dust, but after trying to put a filter on it with silicon and other glues, I have ruined a fair number of SHT75s with seemingly irreversible offsets. So far I have not seen any evidence of poor performance for lack of a filter.
I run a calibration in Cumulus for the SHT75 (1.041x-3) and field check it with the psychro-dyne. It generally is very close. The psychro-dyne is surprisingly accurate compared side by side to ASOS. I set Cumulus to report 98% as 100% and the calibration I set ensures 99% is reported as 100% too.
This is the best I have been able to do given the limitations of these sensors. I really hope the efforts here will force Davis to look at their process as I'd be happy to have an accurate Davis stock sensor that is engineered to last.
Hope this helps...