Model HI 983301N: Continuous reading TDS meter with a probe specifically made for horticulture and hydroponics applications. It compensates for temperature variations automatically. 

Hanna Instruments Water-resistant GroChek EC/TDS Indicator with 0.7 TDS conversion factor helps keep your nutrient solution accurate for a robust hydroponics garden.

The TDS GroChek meter is easy to clean and needs little maintenance. Comes with 6' cable to conveniently hang on the wall, 1500 ppm calibration solution (20 mL), calibration screwdriver and instructions.

TDS GroChek Specifications:

Range: 0 to 1990 ppm (mg/L)
Resolution: 10 ppm
Accuracy: ±2% F.S.
Calibration: manual at 1 point, through trimmer
Temperature Compensation: Automatic, 5 to 50°C (41 to 122°F) with ß=2%/°C
TDS Conversion Factor: 0.7
Probe: HI 7634 (included)
Power Supply: 12 Vdc power adapter (included)
Environment: 0 to 50°C (32 to 122°F); RH max 100%
Dimensions: 3.4" x 4.3" x 1.7"
Weight: 7.6 oz.

Watertight enclosure also prevents vapor and humidity infiltration. There's also a built-in alarm light that blinks when value deviates from desired range. UL-listed.

More info (PDF format)