novel rain predicter

[Dippy]

Well, I think you'd have to add a complex alarm system to remind about topping up wet bulb reservoir.

How does it work?

 

[Jeremy Leach]

Evaporation on the wet-bulb lowers it's temperature. Comparing wet to dry and using a look-up you can calc the relative humidity .... don't ask me to explain any further though ! Small little lookup table might be all that's needed to give rough idea.

 

[Dippy]

Oh, a lot of variables in there then. I think I'll stick to an expanding g-string.

 

[Peter M]

Tables

Hmm tables ... useful for all sorts of things! Oh you mean tables of figures n stuff, orh yeah, now I'm with ya's... got lost in the string theory happenin' here?

wet and dry bulb table.. here's a link to one
http://www.dpi.nsw.gov.au/agriculture/horticulture/greenhouse/structures/evap-cooling

And another with a bit more explanation for those that want to know
http://www.bom.gov.au/lam/Students_Teachers/Worksheet21.shtml

this is a measure of Relative humidity as air can hold different amounts of moisture depending on the temperature of the air... thats a tad more complex, hence the variation of the differences at various temperatures. (yeah yeah whatever).

 

[papaof2]

Seems like everything has been provided except code to run the fan for a couple of minutes before reading the temperature difference of wet/dry DS18B20's ;-)

John

 

[Jeremy Leach]

ok, so we have a constantly revolving platform with ds18B20s attached with the little man or girl raising an umbrella etc ....? Ok, some ideas aren't good ones !

 

[BCJKiwi]

@papa'
If the air flow is boosted then the temp drop on the wet bulb will be grossly exaggerated!
Should be in still air but not a restricted air space.

 

[sagacity_61]

does any know how those mantleshelf weather huts worked. A little man came out in raincoat if rain coming or a girl dressed for sunny day emerged if fine.

Also what were they called - i've googled with no luck.

I'm wondering if principle could be applied to novel picaxe weather device.

Try human hair to measure humidity - much easier to get than cat guts.
(although the downside is that then there are more cats to kill native fauna)
http://www.science.gc.ca/Lesson_Plans/Lesson_Plans:_Monitoring_the_Atmosphere-WS8E2607CB-1_En.htm

 

[papaof2]

wet bulb temperature

@papa'
If the air flow is boosted then the temp drop on the wet bulb will be grossly exaggerated!
Should be in still air but not a restricted air space.

Subect to debate ;-) - the "sling psychrometer"
http://asd-www.larc.nasa.gov/SCOOL/psychrometer.html uses moving air to aid evaporation.

John

 

[BCJKiwi]

Hmmm, was working from first principles - just seemed logical that the faster the air flow the higher the evaporation rate. But if air 100% saturated it could not take up more moisture however fast the air was flowing (within reason)!
Live and learn as they say.