Refractometer Digital Coffee Concentration Meter with BRIX/TDS Dual Scale Display, Temperature Compensation Function, Multifunction Portable Refractometer for the Food Industry and Agricultural Fruit

I purchased 2 units last year, and a 3rd unit this year and none of them are consistent. We purchased them for our coffee training school and it's impossible for them to be calibrated to each other, or even to have them set to 0.00 with battery water.I hope the seller can recommend a way for us to fix these units, as I can't find a way to contact them anywhere online...

works great