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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. If your Optical DO sensor is reading zero, do not replace the cap. See Why is my Go Direct Dissolved Oxygen Probe reading zero? first.
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 (
Go Direct® Optical Dissolved Oxygen Replacement Cap (