Eureka Manta Manual Page 30 of 67
active compound fluoresces – that is, it absorbs energy in
the form of blue light and then emits energy as red light.
In each measurement cycle, the blue light is first turned on,
and then turned off. The red-light receiver measures the
time it takes, after the blue light is turned off, for the
fluorescence to die off. This value is proportional to
dissolved oxygen.
The sensor output is corrected for the temperature and salinity of the water.
Eureka is an advocate of the “air-saturated water” calibration method – that’s different from the “water-
saturated air” calibration commonly used in the past. Here are the steps to air-saturated water calibration:
1 Make sure your instrument’s Barometric Pressure setting is accurate. (D.18)
2 Put a half-liter of tap water in a liter jar, put on the lid and shake the jar vigorously for one minute.
Take the lid off the jar and let the water stand for about five minutes to let the air bubbles float out.
3 Screw your calibration cup onto the Manta housing. With the sensors pointed upward, fill the
calibration cup until your aerated water covers the DO cap by a centimeter or so.
4 W
ait a few minutes for the temperature to equilibrate.
5 Follow the Manta Control Software calibration instructions – remember that you are calibrating %
sat, not mg/l, so select % sat from the list.
What’s the real story on optical DO sensitivity to fouling?
Glad you asked. Several years ago, there was rumor floating around that optical DO sensors
were not affected by fouling. The rumor was only half true.
Suppose you put an optical DO sensor in a river. If you’re just downstream of a rendering
plant or oil patch, your sensor might become coated with grease or oil. Unless that coating
is impermeable to oxygen, your sensor will still give accurate readings (though it may be slow
to respond to changes in oxygen). That’s because the coating is not oxygen-active, i.e. it
doesn’t produce or consume oxygen.
On the other hand, if your sensor picks up an oxygen-active coating, for instance of
photosynthetic algae. The algae’s respiration can cause the sensor to report exaggerated
swings in diurnal oxygen pressure because the algae have their own micro-environment of
oxygen pressure – and the optical DO sensor thinks that the oxygen pressure immediately
adjacent its membrane is representative of the rest of the world.
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