Topic: Building the SLOweather/Davis electronic psychrometer

[SLOweather]

The Sling Psychrometer thread rekindled an idea I've had for a while, and finally have almost enough stuff to try.

I have a Davis Soil Moisture/Temp/Leaf wetness station with 2 open temp probe jacks. Thursday I ordered 2 more temp probes from Ambient.

Yesterday, I dug out of the garage a old, unused WMII Temp/Humidity radiation shield, and disassembled it and modified the bottom 2 plates to mount a 12 VDC 3.5" computer fan.

The probes should be here Monday or Tuesday, so next weekend I'll figure out how to mount them, and take some pics. And I have a week to figure out how to get water into the radiation shield and what to use for a wick for the wet probe.

[Downlinerz2]

   Sounds like another interesting project .  It will be interesting to see the finished product.

[SLOweather]

Progress report...

The 2 Davis temp probes arrived this week. As posted in the sling psychrometer thread, I've had trouble finding tubular woven white shoelaces to use for a wick. What I didn't say over there is that the Davis probes are 3/8" diameter, so finding large enough laces is also a challenge.

My fall-back wick for now is the hem off an old white t-shirt. I cut one off last evening and stitched the tube tighter around the probe to make a snug fit. Then I submerged the tail of the wick in a couple inches of water in a glass, and laid the probe across the top to check the capillary  action of the knit fabric. In a few minutes it had wicked up around the probe.

The radiation shield is ready to go, so now all I have to do is mount the probes in it, figure out a water source, hook everything up, and see how well it works. 

[Mark / Ohio]

.....so now all I have to do is mount the probes in it, figure out a water source, hook everything up, and see how well it works. 

Do you get enough rain there to auto replenish a water source that way?  Maybe a small holding tank tapped into the house spouting?


I guess if all else fails a saddle valve on the water line, a stretch of tubing and a tip orifice limiting it to a drip an hour would do the trick.

[SLOweather]

Nope, dang it. We get no rain to speak of for 6 months or more every year. I've thought of tapping an emitter into a drip irrigation line.

Rain water would ideal, compared to our hard tap water. Using tap water, it won't take long for the wick to become encrusted with minerals. Rain water is really close to distilled. OTOH, if I locate the assembly on the house somewhere, I could use condensation off the metal roof to replenish it.

[SLOweather]

Update...

I put it all together last evening for a test run. After letting the wick saturate and probe temps stabilize, the dry probe was 64 °F, the wet probe was 59°F.

Using the calculator at http://www.srh.noaa.gov/epz/wxcalc/wetbulb.shtml gave an RH of 74.5%. The console at the time read 72%. This is well within the combined Davis stated accuracy of 3% for RH and 1% for temperature.

The larger issue is that, for the extra sensors, Davis only reports whole degrees F. For the stated conditions, a change in the wet probe temp of -1 °F makes a change in the calculated RH of almost 5%.

[W3DRM]

Way to go Chris! 

How about a picture for those of us who are imagination challenged... 

[SLOweather]

OK, Don. Here ya go. It was on my to-do list for today anyway.

Note... All I remember about the radiation shield is that it's a Davis product and I bought it years ago for the temp/rh sensor for a WMII and then never used it.

The test setup, with the assembled unit, the Davis moisture/wetness/temperature station, and a bike water bottle cage and a bottle of water. The probes are mounted to the underside of the 3rd plate from the top.



The bottom 2 solid plates with 3 and 5/8" holes sawed out of them and the fan mounted.



The probes mounted to a plastic bar which is in turn mounted to 2 of the molded posts for the original sensors, wires dressed and secured, and the T-shirt hem wick secured to the wet probe.



All but the fan plate stacked, to see how the probes project into the air stream.



The bottom plate with the fan mounted on it, and the terminal strip for fan power.

[Mark / Ohio]

Looking good! 

[Anthony]

Nice job. 

[NeverDie]

Looks like fun.  Did you ever do anything more with your experiment?

Oregon Scientific wireless temperature sensors (and probably Lacrosse and others) that come with wired probes have a resolution of 0.1 degrees, and are not very expensive.

Also, you could use condensate from an air conditioner as a free renewable source of nearly distilled water.

[DaleReid]

I'm just looking at the exposed evaporative area and wonder if you'd go through a lot less water if you ran a piece of aquarium air tubing down to the area where you really want the water, thereby eliminating many square inches of surface area for your precious non-mineral water to be lost to?  I'm not sure of mechanism for getting the drip rate right, though.  I was just thinking of the hundreds of miles of open topped aquaducts going through the desert and how much precious water is lost when it is 110 degrees and humidity is 5%!

Looks like fun. Tinkering is a blast if you finally get the thing to work as you want.

Could you run the tubing into the upper shield vent, coil it around in a spiral a couple times, then use a few wire stand offs to adjust it to drip right over the little fabric sock?  By the time the water from the reservoir was inside the tubing in the shield it should cool to ambient temperature and not artificially raise the temp of the wet thermometer.