Apera Instruments GroStar GS4-E Replacement pH/EC Combo ElectrodeGroStar GS4 E replacement pH EC combo electrode Measures pH, conductivity (EC and PPM) and temperature Combo probe built with lithium glass membrane and titanium alloy to ensure high accuracy and durability Premium probe technologies and materials ensure high accuracy and durability with minimal maintenance Double junction structure uses blue gel electrolyte without the air bubble 1 year warranty