YSI 58 - Manual (Page 15)

potential of the Ag/AgCl redox couple at the silver anode). As more and more silver chloride (an
insoluble solid and poor conductor of electricity) accumulates at the anode, it begins to block the
passage of current in the electrolytic cell. The silver anode must be cleaned periodically to prevent
this thick layer of silver chloride from reducing the sensitivity of the sensor.
THE FUNCTION OF THE ELECTROLYTE
There are two main functions for the electrolyte:
1. Supply the chloride (Cl
-
) to the anode/reference electrode for the counter reaction of the
oxygen reduction at the cathode.
2. Provide the ionic conduction of electricity inside the cell, especially in the thin layer between
the gold cathode and the membrane.
Under normal operating conditions, such as measuring oxygen around 100 %-air (8.27 mg/L) at 25°
C, the strength of the chloride concentration should last up to 500 hours before the chloride becomes a
limiting factor in the operation of the oxygen sensor. 500 hours translates into about 62.5 working
days at 8 hours per day operation.
4.2 TURNING THE INSTRUMENT ON
The YSI 58 may be used in a vertical, horizontal or tilted position. It may be carried or moved during
use without affecting its accuracy or stability of measurement.
1.
Connect the prepared probe to the PROBE receptacle and screw the retaining ring finger
tight.
2. Zero the instrument. Set the function switch to ZERO and adjust the display to read 00.0
with the O
2 ZERO control.
3. If using a stirrer, connect it now. Check the stirrer battery condition by turning the
STIRRER switch to its spring-loaded BATT CHK position. The warning LOBAT will
show on the display when approximately 5 hours of battery life remain.
4. Wait at least 15 minutes for the probe to stabilize. A wait is necessary whenever the meter
has been OFF or the probe has been disconnected.
4.3
CALIBRATION
To calibrate the YSI 58, the function switch is set to the percent saturation mode with the probe in
moist air; then the O2 CALIB control is adjusted to obtain a meter reading corresponding to the
calibration value for the local altitude. Charts for quickly determining the calibration values can be
found in Appendix F, Calibration Values Table.
This simple procedure accurately calibrates the meter for readings in both the mg/L and the percent
saturation modes. The instrument may be switched from one mode to the other without losing its
calibration. Other methods are also possible and are discussed in greater detail in Section 4.3,
Calibration.
Calibration consists of exposing the probe to a known oxygen concentration such as air at 100%
relative humidity or water of a known oxygen content, and then adjusting the 02 CALIB control so
the display shows a reading that matches the 0
2 concentration of the known sample.
YSI Incorporated YSI 58 DO Meter 11