HM Digital Hydromaster 500 - Continuous pH/TDS/EC/Temp meterHM 500 HydroMaster: EC TDS, pH, Temp Continuous Monitor The HM 500 is the most versatile continuous monitor indicating EC TDS, pH, and Temp providing accurate, stable readings the user can rely on. With its rechargeable battery this unit is portable or can be wall mounted.. To make is even more user friendly, it comes equipped with touch screen, high and low alarm set points, and recorded readings. It is ideal for hydroponics and other applications