In this comment, we provide a critique of the article published by R Wajman et al (2006 Meas. Sci. Technol. 17 2113-8) with regard to 3D electrical capacitance tomography. Moreover, we further examine the sensor design and image reconstruction employed by Wajman et al, and we provide criteria for a robust direct 3D imaging system. References to pioneering and related work on 3D electrical capacitance direct imaging are also included.