SLING PSYCHROMETER

[SlowModem]

I ran across this today in an almanac:

How to make a sling psychrometer  and  Heat Index and Humidity Tables.

[Downlinerz2]

   Cool!!!  Thanks for the post.  I am going to make one of those.  Probably knock myself out slinging it around for a few minutes (or someone else )!
    Mark

[SLOweather]

Yeah, I'm not really confident in their instructions to tape the thermometers to the board. Sounds like a recipe for thermometer missiles to me...

[SLOweather]

I've been thinking about this...

I have a VP2 with a sparsely populated soil moisture/leaf wetness station. With 2 more temp probes, a small water reservoir, a PC fan, and some wicking of some sort, I could make a Davis Psychrometer.

Hmmm, where's Ambient's number????

[wxtech]

I've made psychrometers.  Use a strip of wood or other material about 1 inch wide and 12 inches long.  Both thermometers extend beyond the bottom of the strip.  Offset one thermometer so that the glass bulb extends lower to allow you to stick the wick into a cup of water for wetting the wet bulb.  Offsetting one will allow you to wet one bulb keeping the shorter one dry.

Most glass thermometers have a glass bead on the backside at the top of the tube.  Drill 2 small (3/16") holes in the wood strip so that the beads of each thermometer fits into the holes.  This will provide security so that the glass thermometers don't fall from the strip.  Plastic cable ties or wire clamps may be used to hold the glass thermometers to the wood strip.
At the top of the strip, drill a hole for a machine screw with lockwasher and nut.  This will fasten a 3 inch lengh of jack chain from the handle to the strip.  I recommend using a wire or chain to attach the strip to the handle.  The handle can be a file handle or made from a dowell.
Fashon the wick from a cotton shoe lace.  White sport shoe laces are perfect for this.  Wash the shoe lace to remove conditioner and make it absorb water.  You'll need a length of about 2 inches of shoe lace.  The lace is a tube allowing it to fit over the glass bulb.  Tie the shoe lace to the wet bulb using string.

Pre-condition your water before wetting the wet bulb wick.  Take a cup of water outside and leave it (prevent freezing) so that the water is at outside temperature.  Otherwise you'll have to wait until the tap water comes to outside air temperature while you're slinging.

When slinging the psychrometer, sling in a counter-clockwise rotation as seen from the psychrometer operator.  This will tighten the screw in the handle instead of loosening it.  You don't want the psychrometer getting airborn because of rotating the handle screw out.

[casacota]

If you want an utility instead a table for psychrometer calculations, I've done this calculator (in Catalan) which considers also the absolute barometric pressure for the calculation: http://www.casacota.cat/2x2/psicrometric.pl The utility can calculate the abs. pressure knowing the SL barometer reading and the height where the observation is made. Also inverse conversions can be made (HR -> wet bulb), and outputs also dewpoint, vapor pressure, cloud base height, cloud base estimated temperature, and the expected cloud type. Sorry it is in Catalan and Metric units.

[Downlinerz2]

   You can also find a weather calculator for a bunch of weather measurements and conversions like wet bulb calcs for dewpoint and such at the following:
 
        www.srh.noaa.gov/epz/wxcalc/wxcalc.shtml

For a download of the calculator:

         www.srh.noaa.gov/epz/wxcalc/wxcalc2go.shtml

Mark

[SLOweather]

For anyone building one of these...

I've had trouble finding tubular shoelaces. The first ones I tried were solid flat woven. The second ones were round bootlaces. They had a core that could be pulled out leaving the woven tube, but were black, which is not optimal for this application.