WPI APOLLO 4000, Instruction Manual

Page: 55 / 74

APOLLO 4000
OXYGEN DETECTION
World Precision instruments d-7
note that this value corresponds to that given in table a.
For accurate results the temperature of the water sample and the fluid being tested
should be identical. they should be continuously stirred using a magnetic stirrer.
When measuring fluid samples for dissolved oxygen, periodically rinse the
exterior of the probe with distilled water, blot the membrane dry and recheck the
electrode’s calibration as described above.
Creating a Calibration Curve
to create a calibration curve, the user measures the current (nA) of the sensor
at various concentrations of oxygen. the calibration could be either a two-point
or three-point calibration, including measurements at 0% oxygen, 00% oxygen
or air (2% oxygen) with the corresponding correction for water vapor in case of
aqueous measurements (see
Table 3 ).
Note:
For absolute measurements in % and ppm units, zeroing function should
not be used after the initial zeroing.
After calibration the user can select to show the oxygen concentration in % or
ppm units. the conversion factor %-to-nA or ppm-to-nA (sensitivity of the probe)
is determined from the slope of the calibration curve and must be typed into the
corresponding field (see
Fig. B15 ). only after selecting ppm or % unit will oxygen
concentration be displayed.
Calibration Method for O
2
Measurements in Living Tissue
or Blood
the APollo 4000 and iso-oXY-2 probe may be used extensively in applications
involving o
2
measurements
in vitro
or
in vivo
in living tissue or fluids such as
blood. You may still use the calibration procedure in this manual for these
measurements since a membrane-covered amperometric oxygen electrode will
always measure oxygen activity, not concentration. Although it is normal to think in
terms of dissolved oxygen concentration, it is actually more appropriate to define
oxygen in solution in terms of activity, since this is the “effective concentration”.
For example, in distilled water the activity coefficient,
γ
c
, is close to unity; but
in solutions with high salt concentration the activity coefficient is different from
unity and concentration and activity of dissolved oxygen are no longer equal: the
oxygen concentration falling with salt concentration increase, while activity remains
constant. For a membrane-covered oxygen electrode this is an important effect