(TDS) Total Dissolved Solids meter for testing water purity, etc.  Typically to determine ‘hardness’ and suitability of water for dilution of high-strength spirit / effectiveness of reverse-osmosis equipment. 

Dual range: measures from 0-999 ppm, with a resolution of 1 ppm. From 1,000 to 9,900 ppm, the resolution is 10 ppm. Also temperature and ‘hold’ functions.  Handy 'pen-style'  with protective cap over probe, water-resistant design.