Milwaukee MC410 TDS Monitor.
The MC Monitors are designed to continuously monitor pH, EC or TDS values directly in your reservoir. Each unit features a user selectable set-point. An LED visual alarm is activated and flashes when the pH, EC or TDS level rises either Above or Below (user selected) that set-point. All these instruments run on a 12V adapter so there's no need to worry about failed batteries and each unit comes with mounting kit, probe, probe holder and starter calibration solution.
TDS - PPM Monitor