A system for measuring the state of degradation of cooking oil in a deep fryer comprising: at least one fryer pot, comprising an oil inlet and an oil outlet; a loop of piping fluidly connected to said at least one fryer pot for selectively allowing flow of oil from the at least one fryer pot through the oil outlet into the loop and for selectively allowing the cooking oil to return to said at least one fryer pot through the oil inlet from the loop; a pump for urging the flow of cooking oil through the loop of piping and selectively to urge oil to return to the at least one fryer pot through the oil inlet, the loop further comprising a first valve (44, 744) that is positionable to a closed position to prevent oil flow from returning to the at least one fryer pot through the oil inlet, and is positionable to an open position to allow oil to return to the at least one fryer pot through the oil inlet, characterized by the loop further comprising a recirculation portion that extends from a discharge of the pump toward a suction of the pump, wherein the recirculation portion extends from a first portion that selectively receives oil flow from the discharge of the pump to a second portion that is fluidly connected with the suction of the pump, wherein either the first valve or a second valve is configured to selectively prevent or allow flow through the recirculation portion; a sensor disposed in the recirculation portion of the loop and adapted to measure an electrical property that is indicative of the quality of the cooking oil within the loop of piping, wherein the recirculation portion is configured to allow oil to flow past the sensor as urged by the pump with the first valve (44, 744) in the closed position. at least one fryer pot, comprising an oil inlet and an oil outlet; a loop of piping fluidly connected to said at least one fryer pot for selectively allowing flow of oil from the at least one fryer pot through the oil outlet into the loop and for selectively allowing the cooking oil to return to said at least one fryer pot through the oil inlet from the loop; a pump for urging the flow of cooking oil through the loop of piping and selectively to urge oil to return to the at least one fryer pot through the oil inlet, the loop further comprising a first valve (44, 744) that is positionable to a closed position to prevent oil flow from returning to the at least one fryer pot through the oil inlet, and is positionable to an open position to allow oil to return to the at least one fryer pot through the oil inlet, characterized by the loop further comprising a recirculation portion that extends from a discharge of the pump toward a suction of the pump, wherein the recirculation portion extends from a first portion that selectively receives oil flow from the discharge of the pump to a second portion that is fluidly connected with the suction of the pump, wherein either the first valve or a second valve is configured to selectively prevent or allow flow through the recirculation portion; a sensor disposed in the recirculation portion of the loop and adapted to measure an electrical property that is indicative of the quality of the cooking oil within the loop of piping, wherein the recirculation portion is configured to allow oil to flow past the sensor as urged by the pump with the first valve (44, 744) in the closed position.