The WPA1 holders enable the pH sensor to be removed, calibrated and cleaned without the need for expensive bypass lines and isolating valves. The retractable housing allows the sensor to be removed from the tank or pipe without interrupting the process. By turning the rear cover of the housing the sensor is retracted from service and an o-ring at the tip of the WPA1 seals the system. Only when the sensor is fully retracted and the housing is sealed, can the rear cover be removed allowing the sensor to be unscrewed from the housing. Sensors and cables to be ordered separately.