Abstract: The present invention is directed to precise, accurate mixing two or more chemicals in a metered vessel to achieve a precise, accurate concentration of the chemicals. More particularly, the chemicals are mixed in a vessel having constricted vent tubes with level sensors to precisely and accurately determine the volume of chemical introduced into the vessel. Most specifically, the vessel is closed so that the introduction of the chemicals increases the vessel pressure to enhance the mixing of the chemicals, decrease turbulence and enhance the accuracy of the level sensors.