Optical Dissolved-Oxygen - Measurement Specialties, Inc.

Eureka 2 Optical Dissolved-Oxygen Sensor (HDO)
This brings up the Austin-to-Denver example. If you calibrate DO in Austin and then take the instrument to Denver, the
concentration will be correct because a mg/l of oxygen is the same in both cities. However, the lower barometric pressure
in Denver means that saturated water there has less oxygen than does saturated water in Austin. So the instrument will
continue to calculate percent saturation based on the saturation level in Austin, not Denver. The percent saturation
reading will be wrong until you tell the instrument the barometric pressure of Denver. Then the instrument would lower its
expectations of oxygen pressure at saturation and provide the correct data.
Salinity - The saturation level of oxygen in water decreases with increasing salinity. If you have oxygen-saturated water at
a salinity of 5 (on the PSS) and slowly raise the salinity of the water to 30, oxygen will leave the water because salty water
can’t hold as much dissolved oxygen as can pure water because salt ions are clogging up the spaces in the watermolecule
matrix normally available for oxygen molecules. This phenomenon doesn’t matter to DO concentration (mg/l)
measurements. But it matters to DO saturation (% sat) measurements because % sat relies on knowledge of a water’s
DO saturation point, and that saturation point falls as salinity rises. Fortunately, Eureka instruments with conductivity
sensors calculate salinity, and compensate the DO readings accordingly.
The length of time that DO sensors operate well in the field is influenced by the drift characteristics of the sensor, and by
sensor fouling. Optical sensors are said to have much lower drift than Clark Cells, but there is little empirical evidence in
field deployments to quantify any difference in deployment times.
Fouling - Clark Cells are sensitive to passive foulants, i.e. material in the water that neither consumes nor produces
oxygen. When, for instance, silt accumulates on the Clark Cell membrane, the rate at which oxygen passes through the
membrane is reduced. Not knowing any better, the instrument reports an errantly low DO reading. Optical DO sensors,
however, are not so sensitive to passive foulants because they do not consume oxygen and so do not rely so much on
unimpeded oxygen movement.
However, optical DO sensors are more sensitive to active foulants, i.e. material in the water that produces and/or
consumes oxygen. When, for instance, photosynthetic algae accumulate on the optical sensor, the sensor tends to report
the DO associated with the algae colony, not the DO of the bulk water sample. This causes the DO reading to be errantly
high during the sunlight hours when the algae produces oxygen, and errantly low during the night hours when the algae
consumes oxygen.
Response time - When lowering a Manta 2 from the air into low-DO waters, the Clark Cell especially takes time to reach a
stable reading, particularly in cold waters. In most situations, the DO reading will be within specifications within 2 minutes,
though more accurate readings can be achieved by waiting at least 5 minutes. In monitoring mode, this is less of an
issue, since the sensors reach equilibration with the sample, and natural DO levels are unlikely to go through large, rapid
step changes.
Maintenance - Optical dissolved-oxygen sensor maintenance is nothing more than occasionally cleaning the sensing
surface (the red material; about a centimeter diameter) with a soft cloth and soapy water.
Clark Cell maintenance is little more than refilling its electrolyte and replacing the membrane. There should be no bubbles
in the electrolyte, and the membrane should be taut with no wrinkles or holes.
How does Eureka’s DO measurement compare with the competition?
Most Clark Cells are similar in performance, although Eureka seems to spend more time checking for flow sensitivity and
response time. YSI has a clever adaptation, called the Rapid-Pulse sensor, which largely eliminates the need for a
circulator – however, that sensor is reported to require considerable maintenance, and may present an error of up to -5%
or reading in still waters.
There is considerable discussion about the difference between “lifetime” and “intensity” types of optical Do sensors,
mostly driven by Hach (Hydrolab) because they were the first to market a lifetime sensor. There are some interesting
technical differences between the lifetime and intensity methods, but no one has presented a convincing case that one is
better than the other in field measurements. Eureka is the only manufacturer to offer both the lifetime sensor (we call ours
the “Hamilton”) and the intensity sensor (we call ours the “Insite”). Both appear to work quite well in the field, but the
Insite has the advantage of never needing its membrane cap replaced. All other optical DO sensors – Hydrolab, YSI, In-
Situ, and Eureka’s Hamilton – require that the cap be replaced annually, at a cost of between $200 and $300.
Eureka 2 HDO Sensor www.meas-spec.com September 2012
2113 Wells Branch, Suite 4400, Austin TX 78728 512-302-4333 WQ.sales@meas-spec.com