Water quality test pens (TDS pens) should be registered in categories 9 and 16 in the trademark category.

Specifically, Category 9 mainly includes measuring instruments, including water meters, thermometers, etc., and the water quality test pen can be regarded as a measuring instrument. Class 16 mainly includes paper products, including pens, folders, etc., while water quality test pens can be considered a type of pen. Therefore, the water quality test pen (TDS pen) should be registered in both Class 9 and Class 16 trademark categories.