HI 9935 is a 2-in-1 pH and TDS controller engineered for dosage of fertilizer solutions in hydroponics.

HI 9935 measures pH from 0 to 14 and TDS from 0 to 1999 ppm. Two separate set points can be user-adjusted from 4 to 7 pH and 900 to 1800 ppm (mg/L). The relays are activated when the pH exceeds the set point or TDS falls below the desired value. Two pumps or electro-valves can be wired directly to the controller and be powered through the terminals. Independent proportional settings for pH and TDS can be adjusted from 0 to 90 seconds, 0 to 2.0 for pH and 0 to 400 ppm for TDS. A ground probe can be connected to the appropriate terminals to eliminate interference and prolong the electrode's life.

HI 9935 provides for an alarm relay which is activated in several circumstances. These include when the pH is below the set points in the operator-adjustable threshold of 0.5 to 2.5 pH, or similarly, TDS exceeding the set point by a value in the 50 to 450 ppm range. The alarm also goes off if the pH and/or TDS are not corrected within the operator-determined time frame of 1 to 10 minutes. Moreover, the alarm configuration is switchable from a normally-closed to a normally-open state or turned off during maintenance. The fertilization status can be ascertained from a distance through dosage and alarm LED's

HI 9935 automatically compensates for the temperature effect on TDS. It accepts pH electrodes with a BNC connector and a TDS probe with a DIN connector incorporating a temperature sensor.