The Membrane Cap at the tip of the PINPOINT® II Oxygen Probe should be replaced when calibration or oxygen readings are not stable. The clear portion of the membrane is a Teflon polymer which will resist tear and stretching much longer than other materials. Typical life of a daily used Membrane Cap is 9-15 months. The Membrane Cap at the end of the probe should be changed when necessary to ensure optimum performance. This kit comes complete with two membrane caps and a bottle of electrolyte fluid (60 mL).