A concentric cartridge for fluid mixing valve includes an inlet adapter configured to receive a first fluid and a second fluid, a pressure balance unit, a volume control plate rotatably coupled to the pressure balance unit and positioned between the inlet adapter and the pressure balance unit, a temperature control plate, and a mixing plate positioned between the pressure balance unit and the temperature control plate. The pressure balance unit and the volume control plate are rotatable relative to the inlet adapter and the temperature control plate is rotatable relative to the mixing plate. The mixing plate has an irregularly shaped opening configured to mix fluids with different temperatures such that the temperature of the mixed fluid increases linearly as a function of angular rotation of a temperature control handle.