YSI 5000 - Manual (Page 51)

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9. Principles of Operation
YSI dissolved oxygen probes use membrane-covered, Clark-type polarographic sensors with
built-in thermistors for temperature measurement and compensation. A thin permeable
membrane, stretched over the sensor, isolates the sensor elements from the environment, but
allows oxygen and certain other gases to enter. When a polarizing voltage is applied across the
sensor, oxygen that has passed through the membrane reacts at the cathode, causing a current to
flow. This current is read and interpreted by the YSI dissolved oxygen meter utilizing customized
electronics and software to assure the highest possible accuracy and reliability.
Oxygen diffuses through the membrane at a rate proportional to the pressure difference across it.
Since oxygen is rapidly consumed at the cathode, it can be assumed that the oxygen pressure
inside the membrane is zero. Hence, the amount of oxygen diffusing through the membrane is
proportional to the absolute pressure of oxygen outside the membrane. If the oxygen pressure
increases, more oxygen diffuses through the membrane and more current flows through the
sensor. A lower pressure results in less current.