YSI 58 - Manual (Page 5)

SECTION 1 INTRODUCTION
The YSI 58 Dissolved Oxygen Meter is intended for field or laboratory use for dissolved oxygen and
temperature measurement and water, but is also suitable for use in certain other fluids. See Section
4.4.2, Measuring Oxygen in Fluids Other than Water. The meter may be used with the YSI 5239
probe, 5905 probe or the 5700 Series probes.
Dissolved oxygen is displayed in mg/L (1 mg/L = 1 part per million) or in percent air saturation. The
air saturation feature is discussed in detail in the next section. Display sensitivity in the mg/L mode
may be selected to read in tenths to in hundredths of a mg/L.
Temperature is displayed in °C from -5 °C to +45 °C with 0.1°C resolution. The mg/L mode is
automatically temperature compensated for changes in solubility of oxygen in water and for
permeability of the probe membrane. A salinity compensation control allows direct determination for
mg/L of dissolved oxygen in oceanic or estuarine waters.
The probes use Clark-type membrane covered polargraphic 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.
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
effectively zero. Hence, the force causing the oxygen to diffuse 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.
PERCENT SATURATION
The percent saturation feature of this instrument allows quick determination of the degree of air
saturation occurring in fresh or saline water. This feature also allows measurement in fluids of
unknown oxygen solubility.
The percent saturation displayed in this mode is the saturation that would occur if the sample were
saturated with air under a normal barometric pressure of 1013 millibars (760 mm Hg or 29.92 inches
Hg). Results reported from such measurements should be noted as air saturation, corrected to standard
pressure.
This feature also makes possible a simple and quick calibration procedure that eliminates the need to
determine exact probe temperature or to calculate the barometric pressure effect on the calibration
value.