Method and device for ensuring high yields during spraying of a settable slurry composition. The slurry is conveyed through a length of hose to a nozzle spray-orifice, and a set accelerator is introduced into the slurry at a distance "D" from the nozzle spray-orifice. The level of accelerator is monitored using a sensor located in the slurry path at a point after the set accelerator is injected. A preferred sensor comprises two pairs of conductivity sensors each having an annular body with a bore corresponding to the diameter of the hose or nozzle bore. Voltage of the slurry can be correlated to yields obtained when a salt-based accelerator fluid is injected into the slurry, such that levels of accelerator can be finely adjusted, and consistently high yields of spray coatings can be achieved.