I meant what is it do they actually measure? Turns out it's capacitance of the sample, with some compensation for temperature being made, so presumably they also have a thermometric function in-built. The devices my potential clients currently use cost about US$ 1,000, and are only typically found at grain collection centres (we're dealing with small-scale farmers). So I guess the challenge is not so much to build a smartphone app, or to even get the moisture readings into a phone, but to build a cheap moisture measuring device.