Hanna Instruments HI 9146 - Manual (Page 24)

24
IMPORTANT
In order to have accurate and stable measurements, it is important
that the membrane surface is in perfect condition. This semipermeable
membrane isolates the sensor elements from the environment but allows
oxygen to enter. If any dirt is observed on the membrane, rinse
carefully with distilled or deionized water. If any imperfections still
exist, or any damage is evident (such as wrinkles or tears-holes), the
membrane should be replaced.
Make sure that the O-Ring sits properly in the membrane cap.