I have been watching my humidity over the last couple of weeks. I have compared it to every weather station within 20 miles of me and it appears that my readings are consistantly low. I have had 4 rain showers over this time period. Nights are so humid and wet that it appears that it rained overnight. Thru all this, humidity never hit 100%. As a matter of fact, it never hit 97% as I have it set to use 100% if it hits 97%.
So, I adjusted the slope reading to 7 from the standard 5, and ticked the “Use prop. offset” box. According to the output I should reach 97% when my humidity reaches 91%. That is about the highest reading I have seen from the sensor.
I am wondering if my sensor is broke and should be replaced, or, is it normal to adjust for the sensor via software to account for abnormalities like this?
Check my webcam during daylight, the sensor is in a shielded enclosure just below the rain bucket. It’s an over-hang from a roof beam, and height above ground is about 15’. It’s about 5’ to the side of the house.
I have weather stations located at 6.6 miles, 12.5 miles, 15.7 miles, and 16.1 miles. Most of our readings a very similar, ie. temp, wind, barometer. The humidity on all their stations hit 100% regularly, especially during rain showers. Searching my logs I cannot find a reading from my sensor over 93%, ever. This morning, condenstation was dripping off the enclosure shields and I was registering 93%.
Perhaps I need to adjust the slope to account for the shielded enclosure??
You cannot automatically discount the readings you are getting without first:
Understanding that at the extremes, humidity sensors get less accurate. WMII is supposed to be ±3% up to 90% and ±4% over 90%.
The location of the sensor (both distance above the ground and type of ground) can have an impact on the reading - ideal is 6 ft over an area of flat grassy land.
Checking the sensor versus some other humidity sensor in the exact same location.
I find with mine that it is dead on (within 2-3% of other area sensors) up to 85%. Then it just hits a wall where it won’t creep up past 90%. This is partly due to its accuracy in this range (poor) and partly due to its location… sheltered under an eve about 3 ft off the ground.
Since your weather station offers a calibration value, you can actually calibrate it using either the wet towel or wet salt methods of testing. It would require taking the sensor down (or going up on the roof) and some hours of offline time.
I will take a look and see if Davis has a calibration method I can do on it to see if it is reading correctly.
BTW, even though I set the slope, and the main WD display shows the Humidity at 95%, the graph only shows 87%. I wonder why it is graphing the non-slope value?
putting my WMR928N external temp/hum sensor in a Stevenson screen type shield has definitely reduced my maximum humidity readings. With my old makeshift shield that only shielded top and sides the max reading was 98% (sensor limit) for wet days - now in a proper shield I am lucky to see a value above 90% - I had thought about setting the offset to 10% or something but that would affect lower readings too, and a slope factor of 10 to get nearer to 98% would result in values way off the mark all round. Looks like I either have to take the sensor out of the shield again (which I don’t want to do because of temperature readings too high on sunny days) or just put up with the fact that on wet days my hum will be 90% or so. Just that when compared to other personal stations on wunderground my lower value sticks out like a sore thumb!
I’m thinking the shielded enclosures require a fan to read correctly. But setting the offset is cheaper
If I decide to buy a VPPro2Plus I will probably get the fan and see if that works any better. I will probably spend my money on a stardot webcam and see if I can fashion my own fan assembly…
