TDS is directly related to quality of water i.e., the lower a TDS figure, the more pure the water. As an example, reverse osmosis purified water will have a TDS between 0 and 10, whereas tap water will vary between 20 and 300.

Displays TDS levels of the feed water and product water with in & out electrode probes.
Highly efficient and accurate due to its advanced microprocessor technology.
Auto-Off function conserves battery power. The unit shuts off automatically after 10 minutes of non-use.
Dual Range 0-9990 ppm (From 0 - 999 ppm the resolution is 1 ppm. From 1000 to 9990 ppm, the resolution is 10 ppm, indicated by a blinking 'x10' image.)
Meters are calibrated with a 342 ppm NaCl solution. Meters can be recalibrated with a mini-screwdriver.
Comes equipped with John Guest Quick-Connect T-fittings.
Installs quickly and easily.
Accuracy: ±2%
Cable Length: 24.5" (including sensor)
Power source: 2 x 1.5V button cell batteries (included)
Battery life: approx. 1000 hours of continuous use
Size: 7.6 x 2 x 4.7 cm (3 x .8 x 1.9 in.)
Weight: 79.4 g (2.8 oz.)