For flow visualization o-cresolphthalein was found to be superior to thymol blue as an electrode-activated pH indicator because its slower reverse reaction results in better colour retention; its poor solubility was overcome by using a 50:50 water-ethanol mixture.

The method revealed unknown flow patterns in plate heat-exchanger configurations with the corrugations on opposite walls abutting. The main flow is along the furrows on each wall. The interaction between these criss-crossing flows causes spiralling in the flow along a furrow. At angles between corrugations and the duct axis of at least up to 45°, the furrow flow is reflected only at the sidewalls of the duct, whereas at high angles, e.g. at 80°, there are intermediate ‘reflections’ at the nodes where the crests of opposite walls meet.