With a tensiometer, can we measure the potential difference between two points in millimeter precision?
Millimeter scale is tough. Centimeter scale, I would say yes. Check out the TEROS 31 Tensiometer. The diameter of the shaft is 5 mm, and the water potential measurement with the TEROS 31 is extremely precise. You should be able to quantify a water potential gradient across about 1-2 cm with a pair of these. But to answer your question, I don’t know of any instrument small enough for mm scale measurement. There was a group working on a MEMs tensiometer that could maybe do that, but I don’t think it is on the market presently.
How does a tensiometer deal with partially saturated soils?
A tensiometer is able to measure positive pressures along with the water potential measurements. If you use the right tensiometer, you can actually have a very good measurement near saturation.
How can you measure kPa or MPa? And what tools can you use for container production?
kPa and MPa are really just a preference. You convert between the two by moving the decimal point. In containers, you can use tensiometers which are highly accurate in the wet range but not in the dry range. Matric potential sensors such as the TEROS 21 also work well. They aren’t as accurate as a tensiometer in the wet end, but they give you a better range and require less maintenance.