Experience in Field Shows Optical Sensing Technology Superior to Rapid
Pulse Polarographic DO Sensors for Long-Term Monitoring
Nearly 40 years ago, YSI, Inc. of Yellow Springs, Ohio commercialized the first
practical dissolved oxygen sensor using the Clark Polarographic electrode. Even
today, this technology is the global standard for dissolved oxygen measurement.
Two decades ago, YSI introduced its patented Rapid Pulse® stirring-independent
polarographic dissolved oxygen (DO) sensor. The company says that at the time,
it revolutionized the measurement of dissolved oxygen in long-term monitoring
applications.
Five years ago, YSI began marketing its first optical DO sensor that utilized
luminescent technology. Known as the ROX® probe, it reduces costs associated
with maintenance, especially when used for long-term monitoring. YSI says that
DO monitoring has been revolutionized once again due to this new technology.
The ROX™ luminescent dissolved oxygen sensor differs from polarographic-style DO
sensors, long the industry standard for DO measurement. The principal
distinguishing factors relate to the durability of the sensing element and the
stability of the signal. Both of these have been improved significantly in the
ROX sensor, according to YSI.
In the past, polarographic DO sensors, with delicate Teflon® membranes and
consumable electrolyte solutions, were often the limiting factors in deployment
times of multiparameter monitoring equipment due to sensor drift or the impacts
of biofouling.
The introduction of the ROX sensor changes DO sensors from having the shortest
deployment endurance to one of the longest, notes YSI. Maintenance intervals are
extended and sensor maintenance is reduced, saving time and money.
YSI says that with the ROX sensor, maintenance intervals stretch from two weeks
to six weeks or longer in heavy fouling environments.
The company provides a case study below that shows the advantages of the ROX
sensor over the Rapid Pulse polarograhic sensor. The monitoring was conducted by
a YSI customer, a national monitoring network, in brackish water in the
Mid-Atlantic area of the U.S. over a period of 14 days, at 15-minute intervals.
“This customer group manages a National Estuarine Research Reserve (NERR) in
Delaware. The data were collected in a mostly freshwater tributary that flows
into Delaware Bay. The goal of the deployment was to compare the ROX sensor with
the Rapid Pulse polarographic sensor in order to determine if switching to the
ROX technology would result in a lower cost of ownership through extending
deployment times (thus reducing the number of trips to the field) and reducing
the time spent on calibration and maintenance.
“The YSI sonde was deployed in warm (20-25 ˚C), shallow, productive waters. In
addition, the relatively high turbidity (~20 to >100 NTU) presents physical
challenges to polarographic membranes but pose no issues to the ROX membrane.
The site displayed tidally driven wide swings in DO concentration. The ROX QC
data points are seen as several readings before and after deployment and are
generated by placing the sonde in a saturated DO environment, such as an aerated
container, while allowing the sonde to continue to log data. The pre- and
post-calibration readings were 98.3 and 99.1% saturation respectively,
indicating no meaningful drift or biofouling effect during the two-week
deployment.”
In the chart below, YSI provides a cost comparison between the YSI Rapid Pulse™
and ROX™ DO sensors. It shows how the ROX optical sensor technology saves time
and money required to maintain and deploy in situ sensors when used for
long-term monitoring.