Operational Guide
49
Dissolved
Oxygen
Meter
Configuration
Dissolved
Oxygen
Calibration
Salinity and Altitude Compensation
Temperature, altitude and salinity compensation are used for DO concentration
measurements (ppm or mg/L). When the water is colder, it can hold more
dissolved oxygen, when it is warmer it holds less oxygen. Compensation
for temperature-related solubility is done automatically using the built-in
temperature sensor within the DO probe and algorithms in edge®. When water
is measured at an altitude below sea level, oxygen solubility increases, but
above sea level the oxygen solubility decreases. To compensate for this during
calibration and measurement, the user must provide the approximate altitude
(in meters) in the SETUP menu. The settings are in 100 m increments; select
the value closest to the actual altitude. Some examples of altitudes around the
world follow:
Location
Meter
Feet
Sebkha paki Tah, Morocco
-55
-180
Lake Frome, Australia
-6
-20
Netherlands, coastal providence
-1 to -7
-3 to -23
Lake Michigan, USA
176
577
Lake Geneva; France, Switzerland
372
1220
Denver, CO USA
1609
5279
Mount Everest
8848
29029
The solubility of oxygen in water is also influenced by the amount of salt the
water contains. Seawater typically has a salinity of 35 g/L and the oxygen
solubility is 18 % less compared to fresh water at 25 °C. By entering the
approximate salinity value, the calibration and subsequent concentration
measurement will be compensated to display the correct oxygen concentration.
A 18 % error would result if the salinity value is not entered.
Note
: Salinity and Altitude have no effect on % oxygen solubility range.
When water is fresh, containing no sea water, the concentration of oxygen will
be at a maximum. The solubility of the oxygen dissolved in water is decreased
when water is brackish or seawater. The solubility of oxygen in water is
decreased when measurements are made at elevations above sea level.
Before proceeding with the calibration, make sure the probe is ready for
measurements (see page 46), i.e. the membrane cap is filled with electrolyte,
the probe is connected to the meter and properly polarized. For an accurate
calibration, it is recommended to wait at least 15 minutes to ensure conditioning
of the probe. Keep the protective cap on during polarization time and remove
it for calibration and measurements. Follow the calibration procedure. Calibrate
the probe frequently, especially if high accuracy is required. The probe can be
calibrated at 2 points: 100.0 % (slope calibration) and 0.0 % (zero calibration).
Initial Preparation
Prepare a fresh bottle of HI 7040 by following package directions. Use solution
within one month of preparation. Pour small quantities of HI 7040 Zero Oxygen
solution into a beaker. If used, remove the protective cap from the DO probe.
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