YSI 55 - Manual (Page 14)

10
6. Principles Of Operation
The sensor consists of an acrylic body with a circular gold cathode embedded in the end. Inside the
gold ring there is a small chamber containing a porous silver anode. In operation, this chamber is
filled with a solution of KCl electrolyte containing a small amount of surfactant to improve wetting
action.
A thin 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
which 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.
It is important to recognize that oxygen dissolved in the sample is consumed during the test. It is
therefore essential that the sample be continuously stirred at the sensor tip. If stagnation occurs, your
readings will be artificially low. Stirring may be accomplished by mechanically moving the sample
around the probe tip, or by rapidly moving the probe through the sample. The rate of stirring should
be at least 1 foot per second.
6.1. Discussion Of Measurement Errors
There are three basic types of dissolved oxygen errors. Type 1 errors are related to limitations of
instrument design and tolerances of instrument components. These are primarily the meter linearity
and the resistor tolerances. Type 2 errors are due to basic probe accuracy tolerances, mainly
background signal, probe linearity, and variations in membrane temperature coefficient. Type 3
errors are related to the operator's ability to determine the conditions at the time of calibration. If
calibration is performed against more accurately known conditions, type 3 errors are appropriately
reduced.
Type 1 Errors
A. Meter linearity error: ±1% of full scale reading, or ±0.15 mg/L
B. Component and circuitry error: ±0.05 mg/L
Type 2 Errors
A. DO errors caused by temperature compensation for measurements at ±10°C from calibration
temperature: ±1% (0.08 mg/L at 25°C)
DO errors caused by temperature measurement errors: A maximum ±0.2°C temperature error is
equal to ±0.5% (0.04mg/L at 25°C).