YSI Pro-BOD - Manual (Page 10)

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The average replacement interval is two to four weeks. Probes in
constant or heavy use may require more frequent membrane
changes.
Interferences
Hydrogen sulfide, sulfur dioxide, halogens, carbon monoxide,
chlorine, nitric oxide, and nitrous oxide can cause the probe to
give erroneous readings. If you suspect erroneous readings, it
may be necessary to determine if these are the cause.
Acids
Avoid any environment that contains substances such as
concentrated acids, caustics, and strong solvents that may attack
the probe. Probe materials that may be damaged by these
substances include FEP Teflon or Polyethylene, EPR rubber, ABS
plastic, and stainless steel.
Erroneous Readings
Erroneous readings may occur if the membrane is coated with
oxygen consuming bacteria or oxygen evolving algae. Heavy
residue may coat the membrane causing incorrect readings.
Frequent membrane changes will eliminate this problem.
Storage
When the probe is not in use, store the probe in a BOD bottle
containing at least 1 inch of water. For long term storage, remove
the membrane cap, rinse the probe tip with deionized water, and
install a dry membrane cap (without electrolyte solution).