BLUELAB pH pen should be stored in either KCI solution or plain tap water.
The literature that generally accompanies the pen says KCI, Tap or buffer4 but having contacted BLUELAB when I purchased mine they have since retracted the buffer4 info.
After 5months of use and correct storage/calibration my BLUELAB pen crapped out. It got sent back to manufacturer where they cleaned it and re calibrated it and its working fine again now and holding a steady reading.
When it crapped out I had an "ER" message appear on screen.
J