Answer: The set has a shelf life of up to 2 years. It should then be replaced.
Answer: You're not doing anything wrong. This only means that the water has a hardness of 0°dH (1 drop = 1°dH)
Answer: That's difficult to answer. Depending on the hardness of the water, you need different numbers of drops. 1 drop roughly corresponds to about 0.05 ml. This means you can get about 300 drops per bottle. If the water hardness is 20 °dH, you need 20 drops per measurement. As a rule, you can therefore carry out between 15 and 30 measurements (depending on water hardness) per bottle.
