Emerson Rosemount Analytical RDO, Manual

Page: 23 / 32

23
MODEL RDO CALIBRATION
SECTION 6.0
CALIBRATION
6.1. CALIBRATION OPTIONS
There are three ways to calibrate the RDO probe:
• Saturation: Both a two-point (100% and 0% satura-
tion) and one-point (100% saturation) calibration are
available. 100% saturation refers to water com-
pletely saturated with atmospheric oxygen. 0%
refers to water containing no dissolved oxygen.
The solubility of atmospheric oxygen in water
depends on the barometric pressure, temperature,
and humidity. If these are known, the concentration
of dissolved oxygen in air-saturated water can be
readily calculated, making it a useful calibration
standard. In practice, however, air-saturated water
is almost never used. Air is used instead. Air works
because, according to Henry’s Law, the the concen-
tration of oxygen in air-saturated water (expressed
in mg/L) is in eqilibrium with the partial pressure of
oxygen in the air used to saturate the water.
Therefore, whether the probe is in air or in air-satu-
rated water, the fluorescence quenching will be
exactly the same.
During calibration at 100% saturation, the micro-
processor uses the temperature measured by the
probe and the barometric pressure measured by the
analyzer to calculate the equilibrium solubility of
oxygen in water. Because the calculation assumes
the air is completely saturated with water, the cali-
bration must be done with the probe in air having
100% humidity.
Calibrating at 0% saturation requires water contain-
ing no dissolved oxygen. A good 0% saturation stan-
dard is water containing about 5% sodium sulfite
(Na
2
SO
3
). Add about one teaspoonful of sodium sul-
fite crystals to a cup of water.
• Concentration: If the probe is installed in a waste-
water aeration basin, it is often inconvenient to
remove it for saturation calibration. In this case, con-
centration calibration, in which the probe is calibrat-
ed against a referee instrument, is more suitable.
• Default: Default restores the default calibration.
Although a new probe can be used as received from
the factory, it is recommended that a two-point satura-
tion calibration be done when the probe is first placed
in service.
6.2 ONE-POINT CALIBRATION (100%
SATURATION)
1. Remove storage cap from top of calibration cham-
ber and replace it with the calibration cap (cap with
the vent hole).
2. Fill the calibration chamber to the lower fill line with
approximately 10 mL water. The water temperature
should be as close to the sample temperature as
possible.
3. Remove the sensor from the process liquid. If the
sensor is fouled or dirty, clean it following the proce-
dure in section 7.2. Gently dry the probe and sens-
ing foil with a soft cloth, making sure there is no
water on the body of the sensor or on the sensing
foil.
The sensing foil must remain dry during
100% saturation calibration.
4. Place the probe in the calibration chamber keeping
the sensing foil about 1 inch (25 mm) above the sur-
face of the water.
Figure 6-1. Calibration and storage caps.
Figure 6-2. Fill the calibration chamber to the
lower line with water.