Tech-Tip: Dissolved Oxygen Sensor Verification

This SBE 43 dissolved oxygen sensor was deployed with a damaged membrane, resulting in the erratic output. The “Ruptured Membrane” figure above shows more data from this sensor when deployed in a shallow test cast.

A postmortem analysis of this sensor showed that, at some point between final calibration and deployment by the user, something ruptured the sensor’s membrane. Membrane damage is rare, but occasionally happens in one three ways:

1. Freezing. If the SBE 43 is exposed to extreme cold for an extended period of time, the internal electrolyte can freeze and expand, rupturing the membrane. Accordingly, if an SBE 43 is stored in a loop of tubing filled with DI water, this water can freeze and damage the membrane as well.
2. Physical shock to the sensor. A powerful enough strike to the SBE 43 can rupture the membrane. Sea-Bird takes precautions to secure all equipment before shipping.
3. Physical contact with the membrane. While cleaning the SBE 43 with a surfactant is advised, Sea-Bird explicitly recommends against contacting the membrane with any object. Improper care can result in damage such as the “wrinkled membrane” shown above

The SBE 43 measures oxygen by counting oxygen molecules that diffuse across the membrane, transferring from the ocean to the sensor’s internal electrode. If this membrane is damaged or severely fouled, the flux of oxygen molecules to the electrode becomes unpredictable and inconsistent with the sensor’s calibration.