What is Sorption curve: water activity and moisture content Meter working principle ( one of many methods) Hair hygrometer to measure relative activity Diy using digital hydrometer Aqualab: Role of Water activity in coffee , ie molding, or mycotoxins Reference: Moisture content of soil ASTM-D2216 YouTube Rotronic webinar Royalcoffee Mems capacitive sensor Hacking capacitive soil moisture sensor based on TLC555 Yet another instructable based on LM393 Arduino forum Aquamatic 5300 using 150 MHz Researchgate frequency selection UGMA , design criteria , compliant testers Stripline cell , 149 MHz Jircas.jp , 10k to 10 MHz