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 includes one membrane cap and one bottle of electrolyte fluid (60 mL).