YSI Model 52, Operation Manual

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YSI, Incorporated Model 52 5
SECTION 3 CALIBRATION
Calibration is accomplished by exposing the probe to a known concentration of oxygen
and then adjusting the calibration value to match. Since the oxygen content of our
atmosphere varies predictably according to relative humidity and atmospheric pressure,
we recommend that you calibrate in air, see Section 3.3, Calibration in Air.
Daily calibration is usually appropriate. Calibration can be disturbed by a physical shock,
touching the membrane, fouling of the membrane or drying out of the electrolyte. Check
calibration after each series of measurements, and in time you will develop a realistic
schedule for recalibration.
3.1 PERCENT AND MG/L
100 percent air saturation corrected to standard pressure is the dissolved oxygen value of
water saturated with air at 760 mm Hg (1013 millibars or 29.92 inches Hg). The mg/L
values describe the concentration of oxygen. Since calibrations in mg/L do not necessarily
yield correct percent readings, the percent readings are disabled after calibrations in mg/L.
Calibrations in percent are correct for both percent and mg/L readings.
3.2 DETERMINING CALIBRATION VALUE
Typically, local barometric pressure is not equal to "standard pressure." Therefore, unless
you are calibrating with a known sample determined by a Winkler titration, you should
determine the proper calibration point for the local barometric pressure. This corresponds
to a reading from a mercury barometer. Do not use the pressure reported by the Weather
Bureau that has been adjusted to sea level. The calibration value for a given barometric
pressure can be read from the chart on the back of the meter, or the one in Appendix B. of
this manual.
The chart also lists calibration values according to altitude. These values are only
approximate because they are derived from the average pressure at the given altitudes,
and do not take pressure variations into account. The typical error resulting from
calibration to altitude alone will be ±4% of your reading. Use altitude values only when
you cannot determine the true barometric pressure.