All optical dissolved oxygen probes work on the same principle and all must have the sensing cap containing the luminophore replaced periodically due to a event known as photo-bleaching.
The Optical DO Probe and Go Direct Optical DO cap is warranted to be free from defects for a period of twenty four (24) months from the date of purchase; it is possible that you may get somewhat longer use than the warranty period. If you start to notice a reduced or slow response, it is probably time to replace the cap.
RELATED VERNIER PRODUCTS
Optical DO Probe Replacement Cap (ODO-CAP)
Go Direct® Optical Dissolved Oxygen Replacement Cap (GDX-ODO-CAP)