The second generation In-Situ Rugged Dissolved Oxygen (RDO^®) sensor measures dissolved oxygen (DO) using the principle of “dynamic luminescence quenching.” The RDO sensor advances lifetime-based optical fluorescence technology to provide an extremely stable, accurate, low-maintenance DO sensor. Sensor optics include a lens, blue LED and filter, red LED and filter, and a photodetector or photodiode (see Figure 1). When the blue LED emits light, it causes the lumiphore molecules embedded in the gas-permeable sensing foil to emit red photons. The In-Situ RDO sensor measures the “phase” (or delay) of the returned signal compared to the excitation signal, and is thus based on the “lifetime” rather than the “intensity” of the luminescence. The presence of oxygen in the foil quenches the luminescence and causes a phase shift in returned signal, detected by the photodiode (see Figure 2). The phase difference between the blue excitation light and the return red light is measured, and the result is used to quantify DO.

Figure 1: In-Situ RDO sensor design. All of the optics and electronics are solid-state with no moving parts.

Figure 2: Lumiphore excitation process

Comparing sensor optics
Optical DO sensors can measure dissolved oxygen via three mechanisms (Figure 3):

1. Magnitude – Measures the peak height of luminescence. Accuracy degrades over time as the lumiphore degrades due to photobleaching by ambient light.
2. Time Domain – Measures the decay rate of the luminescence, but the signal to noise ratio can limit the sensor range.
3. Phase Domain – Phase domain measures the phase difference based on the entire signal and references wave forms across a population of pulses. This method delivers the highest accuracy and widest operating range. The In-Situ RDO sensor uses this method.

Figure 3: Mechanisms for measuring dissolved oxygen with an optical DO sensor

Eliminating problems associated with electrochemical DO sensors
Dissolved oxygen (DO) is one of the most important parameters monitored when evaluating water quality, aquatic biology, and related processes. Until development of optical DO technology, the ability to accurately monitor DO levels over long periods of time was limited. Electrochemical sensors (Clark, Galvanic) require sample stirring and are functionally limited by the durability of their membrane and electrode, while galvanic diffusion types offer characteristically slow response.

Optical DO technology eliminates the need for:

• Complex sensor storage and conditioning
• Flow and stirring
• Frequent membrane and fill solution replacement
• Frequent calibration

Electrochemical sensors, no matter the level of biofouling, require a site visit every two weeks or more often for maintenance and recalibration. Environmental professionals can deploy the optical RDO sensor for an entire monitoring season, spanning months, without calibration, and can obtain accurate results in even high fouling environments, without the need for stirring. Though DO sensors are subject to interference from active fouling, the optical RDO sensor can typically be cleaned and redeployed without recalibration - even in high fouling sites. Electrochemical sensors require frequent recalibration and replacement of the membrane and filling solution.

Comparing methods

Variable	Galvanic	Polarographic Steady State	Polarographic Pulsed	Optical Life
Flow Dependence	Low	High	Low	None
Response Time (to 90%)	Slow	Medium	Medium	Fast
Range (0-200%) •Low end (0-1 ppm) •High end (20 ppm)	Yes * **	Yes * **	YES * **	Yes ** **
Long-Term Stability	*	*	*	**
Frequency of Maintenance	High	High	High	Low

** Indicates better performance than *
State of Technology in the Development and Application of Dissolved Oxygen Sensors, Alliance for Coastal Technologies (ACT) Workshop Proceedings, Savannah, Georgia, January 2004. University of Maryland Technical Report Series No. TS-444-04-CBL, p. 11.

Taking advantage of second generation technology
Water quality professionals will benefit from In-Situ Inc.’s aggressive investment in new product development. By improving upon the breakthrough optical DO sensor technology that In-Situ Inc. first brought to market in early 2004, our next generation RDO sensors provide higher quality data and more robust performance at a significantly lower cost than currently available optical DO sensors. The superior performance of optical DO technology combined with the lower total cost of ownership will essentially render the membrane-based Clark cell electrode obsolete. The RDO sensor maintains monitoring integrity, provides accurate data, and reduces data loss because it is much less susceptible to sensor drift, sensor failure, and battery failure.