Extech PH90 Waterproof pH Meter for Liquids Semi-Solids Solids is designed to measure pH in liquids, semi-solids, and solids. The Extech PH90 does not break or clog like traditional glass bulb electrodes and features field replaceable flat surface pH electrodes, a PTS indication so you know when to replace the electrodes, a waterproof design, and more.