People...........this is an inexpensive, plastic meter, and it must be handled with gentle care.  It works just fine for testing my watering solution in my hydroponics garden.  Readings are very accurate (I'd have to guess within 2% most (90%) of the time (when tested with a more expensive laboratory model--I've worked in a chem lab for 40 years).  You have to keep it wet  or wet it for a few minutes before use, and the calibration MUST be checked often.....that's just the way it is with these types of meters. The TDS meters I use in the chem lab cost hundreds of dollars, are just a little more accurate, just as fragile, and require calibration often.