A liquid level sensing system for an appliance comprising: - a liquid reservoir having a volume adapted to hold a liquid and including a liquid port; - a reservoir receptacle adapted to releasably receive the liquid reservoir and to communicate the liquid between the appliance and the liquid reservoir through the liquid port when the liquid reservoir is received within the reservoir receptacle along an axis; wherein the liquid reservoir further includes a window receiving light along the axis and reflecting light from an interior surface of the window adapted to contact the liquid; wherein the reservoir receptacle further includes an optical sensor assembly affixed with respect to the reservoir receptacle to transmit light along the axis into the window and to receive and measure light reflected along the axis from the window, wherein the liquid reservoir further includes a first coupler half positioned at a rear wall of the liquid reservoir configured to releasably connect with a second coupler half in the rear wall of the receptacle configured to join with the first coupler half, wherein the reservoir receptacle receives the liquid reservoir along a horizontal axis and the transmitted light and received and measured light travel in a direction aligned along the axis; the first coupler half comprising the liquid port and the second coupler half provide a liquid-tight conduit when connected with each other when the liquid reservoir is received within the reservoir receptacle, wherein the reservoir receptacle and the liquid reservoir provide interfitting registration surfaces (48, 50) positioning a height of a predetermined volume of the liquid within the liquid reservoir with respect to the optical sensor when the liquid reservoir is received within the reservoir receptacle. - a liquid reservoir having a volume adapted to hold a liquid and including a liquid port; - a reservoir receptacle adapted to releasably receive the liquid reservoir and to communicate the liquid between the appliance and the liquid reservoir through the liquid port when the liquid reservoir is received within the reservoir receptacle along an axis; wherein the reservoir receptacle further includes an optical sensor assembly affixed with respect to the reservoir receptacle to transmit light along the axis into the window and to receive and measure light reflected along the axis from the window, wherein the liquid reservoir further includes a first coupler half positioned at a rear wall of the liquid reservoir configured to releasably connect with a second coupler half in the rear wall of the receptacle configured to join with the first coupler half, wherein the reservoir receptacle receives the liquid reservoir along a horizontal axis and the transmitted light and received and measured light travel in a direction aligned along the axis; the first coupler half comprising the liquid port and the second coupler half provide a liquid-tight conduit when connected with each other when the liquid reservoir is received within the reservoir receptacle, wherein the reservoir receptacle and the liquid reservoir provide interfitting registration surfaces (48, 50) positioning a height of a predetermined volume of the liquid within the liquid reservoir with respect to the optical sensor when the liquid reservoir is received within the reservoir receptacle.