YSI 5239 Dissolved Oxygen Probe - Manual (Page 9)

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3. Check the membrane to make sure there are no water
beads attached to it. Wipe to remove the water beads
with a soft tissue if necessary.
NOTE: Make sure that the probe has been in the
calibration bottle for more than 2 minutes before
calibrating so that both the thermistor and DO
sensor are at the same temperature.
4. Calibrate the system according to the instructions in
the operations manual for the particular YSI 50 series
meter you are using. Be sure to take into consideration
the local barometric pressure and adjust to the proper
calibration value. All YSI 50 series DO meters have a
table on the back which relates barometric pressures to
%-air saturation values.
Checking the Probe Zero
The probe zero (background current) is checked by immersing
the probe in a sodium sulfite solution (0.08M or 3g
Na
2
SO
3
/300 ml), or in an inert gas (e.g. nitrogen, argon). The
meter should read less than 1% dissolved oxygen in either of
these environments. If it does not, change the membrane or
clean the probe.
E RRONEOUS R EADINGS
Erratic readings will result from loose, wrinkled or fouled
membranes, or from bubbles larger than 1/8” in the electrolyte
solution. Erroneous readings may occur if the membrane is
coated with oxygen consuming bacteria or oxygen evolving
algae. Heavy residue may coat the membrane causing
incorrect readings. Frequent membrane changes will eliminate
this problem.