Abstract: A pH sensor comprising a sleeve assembly having at least first and second spaces, a partition separating the two spaces from each other with a water tight seal, and a barrel including a flange, a reference electrode accommodated in the first space and including a liquid junction, a glass electrode having a sensor unit disposed adjacent to the liquid junction, liquid grounding means mounted adjacent to the liquid junction, and a cable disposed in the second space with a watertight seal, the cable being electrically connected to the reference electrode, glass electrode and liquid grounding means. The pH sensor can be used for continuous flow, immersion or submersion types of installations, and can be fabricated inexpensively and is easily maintained. The sensor unit of the pH sensor can be easily cleaned by a cleaning unit which includes a rotatable shaft to which is connected a vane and a brush at one end.