TDS (Total Dissolved Solids) measures how many milligrams of soluble solids dissolved in one liter of water. The measure is expressed in PPM (part per million). In general, the higher the TDS value, the more soluble solids dissolved in water, and the less clean the water is. Therefore, the TDS value can be used as one of the references for reflecting the cleanliness of water.  

Thanks to this product, it becomes possible to create your own measurement, collection and data transmission system. It is even possible to use this information to automate water treatment cycles. The TDS probe is waterproof; it can be immersed in water for long time measurement. However, the signal transmitter board is not waterproof. 

Specifications

• Operating Voltage: 3.3-5V. ( Protect power circuit) .
• TDS Measure range: 0 ~ 1000ppm
• TDS  Waterproof Probe length 1 meter 
• Module Size: 42 *
• Measurement Accuracy: ± 10% F.S. (25 ℃)
• Protect power circuit
• 32mm

 TDS sensor arduino Tutorial from DF Robot.