YSI 550A
Y S I
Environmental
Page 5
PRINCIPLES OF OPERATION
The sensor consists of a silver body as the anode and a circular gold cathode embedded in the end. In
operation, this end of the sensor is filled with a solution of electrolyte containing a small amount of
surfactant to improve wetting action.
A thin semi-permeable membrane, stretched over the sensor, isolates the electrodes from the
environment, while allowing gases to enter. When a polarizing voltage is applied to the sensor
electrodes oxygen that has passed through the membrane reacts at the cathode causing a current to flow.
The membrane passes oxygen 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 force causing the oxygen to diffuse through the membrane is proportional to the partial
pressure of oxygen outside the membrane. As the oxygen partial pressure varies, so does the oxygen
diffusion through the membrane. This causes the probe current to change proportionally.
PREPARING THE PROBE
MEMBRANE CAP INSTALLATION
The YSI 550A DO probe is shipped with a dry, protective membrane. Before using the instrument for
the first time, remove the protective cap and replace it with a new one following these instructions:
1. Remove the probe sensor guard to access the probe tip.
2. Unscrew and remove the old membrane cap and discard.
3. Thoroughly rinse the sensor tip with distilled or DI water.
4. Fill a new membrane cap with O
2
probe solution that has been prepared according to the directions
on the bottle. Be very careful not to touch the membrane surface. Lightly tap the side of the
membrane cap to release bubbles that may be trapped.
5. Thread the membrane cap onto the probe. It is normal for a small amount of electrolyte to
overflow.
6. Replace the probe sensor guard.
MEMBRANE MAINTENANCE
Additional membrane changes will be required over time. The average replacement interval is 4 to 8
weeks, although they may last longer if kept clean. To clean the membrane, use a lint-free cloth, such
as a Kimwipe, and rubbing alcohol to gently remove the contamination. In harsh environments, such
as wastewater, membrane replacements may be required every 2 to 4 weeks.
