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Ref. No. [UMCES] CBL 2016-016
ACT VS16-07
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breakthrough optical DO sensor technology that In-Situ, Inc. first brought to the environmental
market in early 2004, the RDO Sensors provide higher quality data and more robust performance
than traditional methods.
Optical sensor technology uses LED lights, a lumiphore and a detector. A source of light
excites a lumiphore which luminesces, emitting a light of a different wavelength. Typically,
luminescence lifetime methods are used to determine DO concentration. However, optical
technologies differ in measurement method. Luminescence lifetime can be measured using either
the time domain method or a frequency domain method.
1.
Time Domain Method – Uses a pulsed measurement method to measure a single or an
average of a series of exponential decay events. This method is susceptible to drift and
interference from stray light.
2.
Frequency Domain Method – Measures the phase shift between the entire signal and
reference wave forms across a number of cycles. This method, which is used by the RDO
Sensor, delivers the highest accuracy across the widest operating range.
When the RDO Sensor initiates a reading, a blue LED emits blue light, which excites the
lumiphore molecules. Excited lumiphore molecules emit red light, which is detected by a
photodiode. Oxygen molecules quench the excited lumiphore molecules and prevent the emission
of red light. This process is called “dynamic luminescence quenching.” The RDO Sensor
measures a phase shift between the red returned light and a red reference light. DO concentration
and red returned light are inversely proportional. Optical electronics calculate DO concentration
and report results in mg/L. DO determination by luminescence quenching has a linear response
over a broad range of concentrations and offers a high degree of accuracy and stability.
In addition, the RDO technology improves deployments by providing the following:
•
The sensor operates with no drift over long-term deployments. And, unlike membrane-
based sensors, the RDO Sensor excels in hypoxic conditions. No sample flow or stirring is
required.
•
The sensor face requires periodic cleaning. No hydration, conditioning, or special storage is
required. Membranes and electrolyte/filling solution are eliminated.
•
The abrasion-resistant sensing element withstands fouling, high sediment loads, and rapid
flow rates. The lumiphore is not affected by photo bleaching or stray light. In addition,
unlike membrane-based sensors, the RDO Sensor is unaffected by sulfides, sulfates,
hydrogen sulfide, carbon dioxide, ammonia, pH, chloride, and other interferences.
The RDO Sensor Cap includes pre-loaded calibration coefficients, serial number, and
manufacture date for traceability and simplified setup.
PERFORMANCE EVALUTION TEST PLAN
Laboratory Tests
Laboratory tests of accuracy, precision, response time, and stability were conducted at
Moss Landing Marine Lab. All tests were run under ambient pressure (logged hourly from a
barometer at the laboratory) and involved the comparison of dissolved oxygen concentration
reported by the instrument versus Winkler titration values of water samples taken from the test