A downhole actuator apparatus that selectively maintains a pressure differential between two pressure regions in a well. The pump is coupled to a fluid source via an input line. One configuration includes a valve body and stopper with an elastic rib integrally formed with the valve body and the stopper having adequate length to maintain the stopper in a closed position absent external forces. Then, the collection quantity is calculated by adding the collection quantity increment value to the previous collection quantity calculated based on the pressure difference and the exhaust gas flow rate while the measuring accuracy is high.