Topic: Split Temperature and RH Sensors

[aweatherguy]

I'm posting this under "other hardware" because it could apply to any brand  of hardware; I'm using OS hardware in this case.

I keep losing RH sensors. Lost another one a couple of weeks ago. I've got a fan-aspirated solar radiation shield (home-built) with a decent amount of airflow. Even though it is situated over grass, it still manages to pick up a fair amount of dust and dirt, which is hurled mercilessly at my wireless sensor. Eventually, the RH sensor picks up enough dust/dirt that it stops giving accurate measurements.

I've read where you can clean the sensor off and put some hair gel or something on it, but I'm skeptical as to how accurate the resulting measurements will be. So until now, I've just been replacing the sensor with a new one, and I'm starting to get tired of the expense.

Now I've come up with a solution that should fix the problem. It happened in part because  I've been playing around with replacing the sensor's thermistor to improve accuracy. You can read about that in this post here:

http://www.wxforum.net/index.php?topic=17353.0

Although this is cool, it degrades the sensor's RH accuracy and I wound up purchasing some temperature only sensors (THN132N) and modifying them instead. They actually work a tiny bit better for that purpose anyway.

So that led me to where I placed a second wireless sensor in the fan-aspirated shield to collect RH data since the (improved-accuracy) temperature sensor does not measure humidity.

Then it dawned on me -- the RH sensor does not need to be in the fancy-pants aspirated shield -- it can be nearby in a low-tech naturally-aspirated shield. Both sensors are seeing the same air with the same amount of moisture (i.e. same dew point), even though one sensor may be a few degrees warmer than the other. With this change, the RH sensor won't get pelted with dust and dirt and should have a long, happy life.

The only problem is that I'm reporting temperature (e.g. to internet web sites) from the fan aspirated sensor, but the RH sensor is measuring RH at a different temperature. The dew point from the RH sensor is correct, but I need to adjust the RH number to be correct at the temperature in the fan-aspirated shield. Not a huge problem if you write your own weather logging software . Or you could just live with the RH errors created by the small temperature difference between the two sensors.

I don't know how many folks will find this useful as it might require some RH adjustments that aren't easy to do in many cases. Perhaps it will give others some ideas...

[SlowModem]

I'm posting this under "other hardware" because it could apply to any brand  of hardware; I'm using OS hardware in this case

What type of sensor are you using?  968?  122?

[aweatherguy]

I've used a variety...122NX, 801, 810. In each case, eventually, the dust/dirt always finds its way to the RH sensor. I've tried wrapping the sensors in a couple layers of cotton or other cloth as a filter but that slows down the response time.

[Old Tele man]

Have you thought about using the paper unwrapped from a car engine filter? It's designed to pass a LOT of air and simultaneously catch very small particles.

[aweatherguy]

No, I had not thought of that. Those filters are pleated with a kind of Z-fold so you get tons of surface area and each small patch does not need to pass huge amounts of air. Don't know how much surface area I'd need for that to work. One idea would be to get a circular patch of that material and place it over the inlet to the radiation shield...it would be interesting to see how much air flow I got with that setup. I wonder if there would be a problem if it got wet during rains.

Thanks for the idea and I will put that in my back pocket if I decide to move the sensor back to the fan-aspirated shield.

I went out and got one of those naturally aspirated units from Ambient -- SRS1000LX so I'll go with that for a while to see what happens. This will have the side benefit that I can compare the effectiveness of the Ambient unit to the fan-aspirated shield. I'll need to wait until summer to get a good feel for how it works under a variety of conditions. Today it was sunny with a high of 57 and light to calm winds. The largest difference between the two sensors was about 1.0C...but the sun does not get all that high this time of year.