Upon the advice of Mr. Denk I found a very interesting formula on a meteorology site. The author summed it up by saying that essentially evaporation rate varies exponentially with temperature. I should be able to use this as a basis for a ballpark compensation factor. I prefer to avoid mechanical solutions if possible, less maintenance. I, too, am a hobbyist. But growing up in a junk yard I learned to be self-sufficient as much as possible. We didn't have a lot of money so, if you wanted something you made it. If it broke you fixed it. Fortunately we usually had the necessary raw material on hand. My daughter and son in law are the horticultural professionals. Allen -----Original Message----- From: piclist-bounces@MIT.EDU [mailto:piclist-bounces@MIT.EDU] On Behalf Of Sergey Dryga Sent: Sunday, May 05, 2013 9:57 PM To: piclist@MIT.EDU Subject: Re: [OT] Greenhouse misting Allen Mulvey amulvey.com> writes: >=20 > Thank you for your comments. I'll try to reply to several > people all at once. >=20 > First, misting is not a part of irrigation. I am > experimenting with moisture sensors like the one below but > that is a totally different, and much simpler project. > Misting is primarily for new growth and cuttings. Their root > system is small if existent at all so they cannot > effectively process moisture from the soil. The misting > keeps the vegetation (above ground) from drying out. A > cutting with no root system can dry out very quickly, even > with moist soil.=20 >=20 > It would seem that relative humidity would be the most > significant factor but it is not. Temperature has a twofold > effect. First, as air is heated it is able to hold more > moisture so the relative humidity has a tendency to drop as > the air is heated. Secondly higher temperatures (more > energy) help bridge the latent heat of transition so warm > water evaporates more quickly than cold. A 60 percent > relative humidity at 40 degrees Fahrenheit has a very > different effect than 60 percent at 105 degrees. Both of > these are well within the operating parameters of a working > greenhouse. There is not a linear relationship. Barometric > pressure can also have an influence as partial pressures are > also involved. I was hoping to find a table which would map > the rate of evaporation, as closely as possible, to my > measurable variables. I had expected that some research > nursery somewhere must have already done this. >=20 > Allen >=20 > Moisture Sensor: > http://www.vegetronix.com/Products/VG400/ Thanks for explanation. As somebody suggested, look at hydorology. Also,=20 evaporation rate is a physico-chemical problem, this might help: http://en.wikipedia.org/wiki/Evaporation If the goal is to keep evaporation from cuttings low, than with RH close to 100% (but non-condensing) one can achieve that. This implies that temperature control is decent (no sudden drop in temperature that would lead to condensation). However, in real-world application this can be counterbalanced by just as rapid drop in RH, e.g. by blowing fresh air. =20 Please keep us updated on your progress/design decisions, it is an interesting topic. Especially for me, since I am doing similar things (although not on a commercial scale, just a hobby) for my wife (http://imagiknitter.blogspot.com/). Eventually I owuld like to have al kinds of sensors and reporting for growing plants.=20 Sergey Dryga http://beaglerobotics.com --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .