  Current non-intrusive measurement techniques for characterising the solids
flow in gas-solids suspensions are limited by the low temporal or low spatial
resolution of the sample volume, or in the case of optical methods, by a short
range of sight. In this work, a sub-millimetre wave range-Doppler radar is
developed and validated for non-intrusive sensing of solids concentrations in a
gas-solids particle system with known characteristics. The radar system
combines favourable features, such as the ability to see through at optical
frequencies opaque materials, to measure the local solids velocity and the
reflected radar power with a spatial resolution of a few cubic centimetres over
distances of a few metres. This paper introduces a method to relate the
received radar signal power to the solids volumetric concentrations (cv) of
different particulate materials. The experimental set-up provides a steady
stream of free-falling solids that consist of glass spheres, bronze spheres or
natural sand grains with known particle size distributions and with particle
diameters in the range of 50-300 $\mu$m. Thus, the values of cv found using the
radar measurements are validated using the values of cv retrieved from closure
of the mass balance derived from the measured mass flow rate of the solids
stream and the solids velocity. The results show that the radar system provides
reliable measurements of cv, with a mean relative error of approximately 25%
for all the tested materials, particle sizes and mass flow rates, yielding
values of cv ranging from 0.2x10$^{-4}$ m$^{3}$/ m$^{3}$ up to 40x10$^{-4}$
m$^{3}$/ m$^{3}$ and solids velocities within the range of 0-4.5 m/s. This
demonstrates the ability of the radar technology to diagnose the solids flow in
gas-solids suspensions using a unique combination of penetration length,
accuracy, and spatial and velocity resolution.
