Thank you for your comments. I'll try to reply to several people all at once. 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 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. Allen Moisture Sensor: http://www.vegetronix.com/Products/VG400/ -----Original Message----- From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of Sergey Dryga Sent: Sunday, May 05, 2013 1:43 AM To: piclist@mit.edu Subject: Re: [OT] Greenhouse misting Allen Mulvey amulvey.com> writes: >=20 > My daughter and son in law operate a perennial nursery. > Several years ago I built a misting system for one of their > greenhouses. It works fine - all analog, no smarts. A > misting duration of about two seconds every twenty minutes > or so usually does the job. If the weather changes someone > needs to manually alter the interval. I would like to make > another, microcontroller, version which monitors the > temperature and relative humidity and makes appropriate > changes. I know there are a lot of other factors involved > but they have standardized on soil, pot size, etc. so most > of those are relatively constant. It is gross changes in > temperature and relative humidity that seem to be most > important. There is no wind in the greenhouse. >=20 > Are there any algorithms or charts available which would be > helpful in computing the necessary changes? >=20 > Allen >=20 >=20 I see several approaches: 1. Go all scientific about it, measure as many parameters as possible, such as T, RH, amount of light (direct sunlight and cloud), soil moisture, soil pH, etc. Then build (or use an existing algorithm likely used by big greenhouse operators. Should be published somewhere in trade journals) algorithm to control misting.=20 2. Measure only T and RH as you intend, and then have some algorithm to control misting. Record data over time and see what works best. 3. Record (as in write down, video record etc, and formalize) procedure how people adjust misting time/interval. Very often people will use factors not "obvious", such as how comfortable they feel. Which might translate in a combination of T, RH, light intensity, time of day... If what is done now is satisfactory and the desire is to automate, approach #3 might be the easiest. =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 .