Some explanation to what I did:
I wanted a program to run unattended on the device, that will increase irrigation on hot days. I found the Penman-Monteith coefficient. It a number that is calculated by the amount of solar radiation, wind, temperature. you can get his number from a close by meteorological station.
For example here are such a website in Carolina: http://www.nc-climate.ncsu.edu/openwaterevap
Then you just need to make this simple calculation: Plant coefficient X area X PM coefficient = amount of litres to irrigate
Plant coefficient: 0.5 for grass and 0.7 for roses, what do you grow?
Area in square meters
Assume its a hot day here the PM may reach 6. Assume My plants coefficient is 0.6, and i have 10 square meters, I will water the garden with 6X0.6X10 = 36 litters that day.
I usually irrigate an hour after the sunset because then the air humanity rises, so water loss is minimal.Also the plant outer “skin” opens up to accept water.
Now if you water too much, you also end up hurting plants. So I started to seek a way to measure the exact amount of water I irrigate.