A measurement system has been developed to non-invasively detect plug formation in density pneumatic conveying. This plug formation detection system yields quantitative information that is validated by comparison to an electrical capacitance tomography (ECT) system. Both the plug detection system and the ECT system were applied to a horizontal pipeline (50 mm inner diameter) in density pneumatic conveying. The experimental conditions were varied for different heights of the initial particle dense layer and air volume flow rates. For all cases, the results obtained from the plug detector and ECT show similar patterns. This good agreement suggests that, where feasible, this plug detector can be used in place of the ECT system, which is advantageous since plug detection systems are often less expensive and faster than ECT systems. The factors influencing plug formation are discussed in detail. As a result, it can be seen that the height of the initial particle dense layer and the air volume flow velocity contribute to the nature of the plug formation rather than the solid-phase volume fraction in the dense layer. (C) Koninklijke Brill NV, Leiden and Society of Powder Technology, Japan, 2008