YSI 550 DO, Руководство по эксплуатации

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YSI Incorporated YSI 550 DO Handheld Instrument 15
SECTION 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.
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.
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 instrument
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
•
Instrument linearity error: ±1% of full scale reading, or ±0.15 mg/L
•
Component and circuitry error: ±0.05 mg/L
TYPE 2 ERRORS
•
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).