After a couple of PM's on this, let me address the issue of "best practice". So where does the best practice come from about not aspirating humidity sensors. It comes from us field techs. It's frustrating when I tell people in a PM what we are seeing in the field and they come back with, "but the sensor manufacturer says this" or "the manufacturers document says that". I like to use the car analogy here. Who would you trust more in getting information about minor problems being experienced with a particular type of GM car? GM or the local independent mechanic that works on their cars everyday? Us meteorological field techs are the independent mechanics.
Just like the wet bias I discovered over a year ago with the SHT-31. Did Davis know about it? Nope! A year later someone on this forum finally told them about it. My test proved that the field techs are aware of many sensor issues that the manufacturers aren't. Not everything gets back to the manufacturers and even if it does they may not think it's a big enough problem to do anything about it. From our experiences, we develop our own best practices when it comes to sensors.
So now back to the humidity sensor best practice and where it comes from. We recycle humidity sensors on average every 18 months. We lab test the sensors when they are new right before they are put in service. They are also spot checked right after being put in service and tested in more detail every 90 days thereafter. That test data gets logged by sensor serial number. That sensor is spot checked again at the 18 month recycle time before it is pulled out of service. We lab test it again right after it is pulled out of service. What we are seeing is that we have a much higher number of sensors that we pull out of FARS installations that are right on the edge of the spec or are already slightly out of spec versus those we pull out of passive shields. Unfortunately none of us field techs has taken the time to try and fully understand why this is happening. Those nearly out of spec or out of spec sensors all show a wet bias. They often fail other sensor metrics we test as well. Getting to travel all over the country to work on different installations, it's a much bigger problem in humid regions.
Again this is not happening to ALL humidity sensors in a FARS. We pull many humidity sensors that show no ill effects after 18 months of being in a FARS. However, based on what we are seeing across the board, we recommend client configurations of temperature sensor in FARS and humidity sensor in an open passive shield (non-gill plate) to maintain optimum performance and calibration. FARS is great for humidity response time and a few other positives but what good is that if we check the sensor during the 18 month in service period and it's out of spec or fails one of the other metrics we test.
Just an FYI, I don't do any work with Davis sensors. My SHT-31 tests I did were done on the side and for my own curiosity.
So then I get asked, "What should I do since I have a combined temperature and humidity sensor?" I say put it in a FARS for temperature accuracy and hope your humidity sensor holds up. If not, replace the sensor. If you don't have the means to properly test the humidity side of the sensor and you are concerned about its accuracy long term, just get on a schedule to replace it every 12 to 18 months.
Again, for the record, I'm not saying the FARS is causing the problem with the Davis sensors in any way shape or form. It's just one possibility that needs to be investigated.