The HI76407A/P are spare dissolved oxygen screwcap membranes designed especially for Hanna’s portable polarographic dissolved oxygen probes. The caps are constructed of durable ABS plastic with polytetrafluoroethylene (PTFE) oxygen permeable membranes. The membranes separate the probes 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 HI7041 electrolyte and screwed onto the probe.