I was wondering if anyone had just used tap water or stored dry because Aprea says it can be stored with solution, a bit of tap water, or dry. Just never use distilled or deionized water for storage.
Do not use water!
For a probe routinely used in the pilot plant (gel filled) I use pH 4 buffer,
For a probe which is not used but hiding/ forgotten in a cabinet storage solution.
I am guessing your Aprea pH probe is stored above a sponge with pH 4 buffer. Some gel filled probes allow you to keep them in a 100% humidity container.
The thin glass bulb has been conditioned at the factory and loses conditioning/ becomes less responsive, if it gets dry. It can also lose conditioning if stored in tap water.