The article analyzes the issues of assessing the main parameters of products of the agro-industrial complex, where humidity is considered and controlled during its collection, during storage in warehouse conditions, as well as upon entry into grain processing plants, where it is subject to industrial processing. The measurement is based on the measurement method and the development of a prototype humidity control device and its metrological characteristics for grain products. The electrophysical characteristics and dielectric properties (complex permittivity and its components) of grains and granular materials in the high frequency range are investigated. On this basis, a functional scheme of a prototype humidity measuring device has been developed, which has been tested in production conditions ranging from grain acceptance, storage in warehouse conditions and industrial processing, graphs of the dependence of the main informative humidity parameter, as well as on an uninformative parameter, i.e., the degree of compaction affecting the measurement result, are presented. The mathematical description and information optimization of the measuring device are described, the relative errors that affect the metrological characteristics of the designed measuring devices are analyzed. The metrological characteristics of the developed prototype device for humidity control both in laboratory conditions and in technological processes of grain processing are discussed.