6 / 59
Ref. No. [UMCES] CBL 2016-013
ACT VS16-04
6
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 Basic sensor,
delivers the highest accuracy across the widest operating range.
When the RDO Basic 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 Basic 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 Basic technology improves deployments by providing the following:
•
The sensor has virtually no drift over the life of the sensor cap. And, unlike membrane-
based sensors, the RDO Basic sensor excels in hypoxic conditions. No sample flow or
stirring is required.
•
The sensor face requires periodic cleaning, but 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 Basic sensor is unaffected by sulfides, sulfates,
hydrogen sulfide, carbon dioxide, ammonia, pH, chloride, and other interferences.
The HOBO U26 DO Logger is part of the HOBO family of data loggers which feature optical
communication for reliable offload in harsh field conditions and easy-to-use HOBOware software
for logger setup, data analysis and data export.
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
baths. All tests were run in thermally controlled tanks at specific temperature, salinity, and DO
concentrations. Tanks were well mixed with four submersible Aquatic Ecosystem Model 5 pumps
with flow rates of 25 L/min. Temperatures were controlled to within approximately 0.2
o
C of set
point using Thermo Digital One Neslab RTE 17 circulating thermostats flowing through closed