Layer thickness in print agent concentration apparatus

In an example, a method includes providing a print agent comprising chargeable particles in a carrier fluid to a print agent concentration apparatus (600). The print agent may be passed between a conveyor (604) and an electrode (602), and a potential applied to cause the chargeable particles to be attracted to the conveyor and to form a concentrated layer of particles on the conveyor. A proportion of the carrier fluid may be separated from the concentrated layer of particles and a thickness of a remaining layer on the conveyor may be measured. It may be determined if the measured thickness of the layer meets predetermined criteria and, if not, an operational parameter of the print agent concentration apparatus may be adjusted.