i got the ph pen (ph51 model) and when i recieved it, it came with a small card that says,
***Attention***
The instructions for the PH51 - PH52 - PH53 instruct you to "remove the cap and immerse the unit in MA9015 storage soloution for 2 hours." If you do not have MA9015 storage solution you may use bottle drinking water or RO water instead. You should also store the unit in an upright position with bottled water or RO water in the cap."
hope that helps.