As to the problems with the measurement of moisture using sensors you listed, sensors of their type have only surface layer of the ceramic plate as a measurement area.

It is easy to verify in the following experiment. Put a stack of sheets of paper or magazines that has a thickness of 1-2 cm on a ceramic sensor surface. This stack simulates the surface layer of the material. Then put your hand over the sensor and see how its readings change.

In order to avoid a mistake with amount of water, it is necessary to increase the volume of material analyzed. We performed several experiments: we took some samples of crushed stone and sand from different areas of the dispenser (this is easy to do: after opening the gate several samples of rolling sand and gravel were taken). Dosing unit volume is about 1 cubic meter. We have known that for sand and carbonate gravel moisture within this small volume can vary more than by 1% with an average moisture of 7.6% (moisture value was determined by drying).

As for the moisture meter FIZEPR-SW100, its measurement area includes material disposed between its probe and the hopper wall and also located in a cylindrical area about 20-30 cm in radius around the probe. The sensor with the flat-topped probe (nominal probe length - 24 cm) analyzes the area of about 20 liters. The direct probe (from one hopper wall to another) determines the average moisture at a significantly greater volume of about 1 cubic meter (depending on the size of the probe and the hopper).

The price of any version of the moisture meter for bulk materials is 68,971 RUB, including VAT.

Operating principle overview.

The probe (stick) is a half-wave TEM-wave resonator wherein the second conductor is the tank wall. The stick end is short-circuited because it is connected to the tank wall. Complex refractive index is determined using resonance frequency then permittivity and moisture (by calibrating tables) are calculated. The moisture meter controller includes calibrating tables for sand, carbonate rubble and granite gravel. The user can easily edit these tables if necessary. At that PC screen draws all data – diagrams of moisture dependence on the refractive index (or permittivity) for a temperatures set.

Moisture is determined by measuring permittivity in the metric VHF band. Moisture is measured regularly, about 1 time per second. The result is displayed as the average value of 5 measurements, but the user can optionally change the averaging base using the program for PC / Laptop.

Due to the selected range volume of the material can be approximately represented as the sphere with a radius of about 25-30 cm. Therefore the heterogeneity within this volume is averaged. For accurate measurements size of material fractions should be approximately 10-20 times smaller than the wavelength. We use wavelength of about 1 m and if necessary it can be increased up to 7.5 m.