Non-invasive interrogation of systems and processes involving dry particulate materials is particularly challenging. X-rays and gamma rays, for example, offer the prospect of truly non-invasive monitoring of material inside metal pipe work or metal containment as typically found in industrial practice, but require extensive calibration and are (usually) subject to licensing regulations. Exploitation of inductance and capacitance is limited by constraints on the properties of the pipe work or process vessel holding the material which must be modified to be partly, and usually significantly, non-metallic.

Interrogation by acoustic waves is not a universal panacea to these difficulties, and while especially attractive and effective in dilute flow systems, can also be used in some situations with bulk solids.

We will review and reflect on a number of different applications of acoustic sensing, including flow monitoring in pneumatic conveying and in gravity flows in down-comers. We will also discuss recent work relevant to the direct measurement of bulk density and fill level.