The Hanna Instruments HI 9931-1 is a Wall Mounted EC Controller with Proportional Fertilizer Dosing for Hydroponics Applications. The Hanna Instruments HI 9931-1 also provides for an alarm relay which is activated when the measurements exceed the set point by a user selectable margin from 0.5 to 2.5 mS/ cm. The alarm also triggers if, due to a malfunction, the continuous dosing time exceeds the operator adjustable period of 1 to 10 minutes. The alarm can be configured in either normally closed or open position and turned off during maintenance. The Hanna Instruments HI 9931 also provides an isolated output signal which is user selectable between 0-20 or 4-20 mA. The Hanna Instruments HI 9931' s four-ring conductivity probes ending in a DIN connector can be quickly attached to the HI 9931. Readings are automatically compensated for the effects of temperature in the 0 to 50° C ( 32 to 122° F) range. For quick and easy wiring, the Hanna Instruments HI 9931 comes with extractable terminal modules. Several LED' s show whether the set point or alarm relays have been activated.