Spare dissolved oxygen screwcap membranes designed especially for Hanna’s portable polarographic dissolved oxygen probes. The caps are made from durable ABS plastic with polytetrafluoroethylene (PTFE) oxygen permeable membranes. Each cap is easily filled with electrolyte and installed on the probe. The membranes separate the probe’s cathode and anode from the water being measured. Oxygen diffuses across the membrane and interacts with the polarographic system to produce a current proportional to oxygen concentration. Each cap is easily filled with HI-7041 electrolyte and screwed onto the probe.