Thermo Fisher Scientific CyberScan DO 600 - Manual (Page 124)

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3.2.5 Electrolyte Solution
The electrolyte solution in your probe’s cap will deplete on usage and
will need to be replaced periodically.
Your new DO probe comes with accessories of one replacement
electrolyte solution and a spare membrane cap. The replacement
electrolyte comes premixed and ready to use. To order more
electrolyte solution, see “Accessories” section.
-END OF D.O. SECTION-
Note: Membranes can only be used once. When a membrane cap is
screwed onto the probe, the membrane is stretched by the cathode. If the
same O-ring and the membrane is used a second time it will not fit perfectly
onto the cathode. This will result in erratic readings.