The best dissolved oxygen sensor for teaching with water quality test. I have used several of these sensors 5 times a day, once a week for over two months. They last for years. The first lasted about 7 years before needing to be replaced. Calibration is rarely required.
Portable digital multimeter with sound continuity and thermocouple (electric tester) LTX DT-838
7 Review
Efficiently Test Your Antifreeze And Battery Acid With A 3-In-1 Refractometer
18 Review
π― Honey Refractometer with Automatic Temperature Compensation (ATC) - Tiaoyeer Moisture, Brix, and Baume Refractometer for Honey, 58-90% Brix Scale Range - Honey Moisture Tester
31 Review
Digital PH Meter Tester - Tiaoyeer Water Quality Tester With ATC, 0.01PH High Accuracy, 0.00-14.00 Measurement Range, 0.01 Resolution For Drinking Water, Aquarium, Swimming Pools, Hydroponic
8 Review