YSI 556 MPS - Manual (Page 107)

Storage Section 12
YSI Incorporated YSI 556 MPS Page 101
2. Seal the empty port with the provided port plug.
3. Place the sensor in the storage vessel (plastic boot or bottle)
which was on the sensor at delivery. The vessel should
contain a solution which is 2 molar in potassium chloride.
NOTE: Make certain that the vessel is sealed to prevent
evaporation of the storage solution.