6 / 59
Ref. No. [UMCES] CBL 2016-011
ACT VS16-02
6
performance characteristics of commercially available DO sensors when applied in real world
applications in a diverse range of coastal environments.
INSTRUMENT TECHNOLOGY TESTED
The Precision Measurement Engineering (PME) miniDOT uses an optical method of
determining oxygen concentration. This method takes advantage of the oxygen ‘quenching’ effect
that is observed of certain types of fluorescent materials. Photons of excitation light entering these
materials transfer energy, excite, electrons of the molecules that make up the materials. These
excited electrons briefly occupy higher energy levels but ultimately return to their normal, rest,
state. There are at least two paths by which these electrons can lose their excited energy and return
to rest. One path is to emit photons. Another path is to lose the energy by transfer to an oxygen
molecule. This non-emissive energy loss is the quenching effect.
The presence of oxygen provides a non-emissive path and thereby reduces the intensity of
emission. Also, by providing this path oxygen reduces the average length of time that electrons
remain in the excited state. So there are two approaches to measuring the amount of oxygen
present: measure the intensity of emission or measure the lifetime of emission.
Optical oxygen sensors typically use emission lifetime to sense oxygen. This is because the
lifetime is not as sensitive to various problems that can affect the emission intensity. For example
as the excitation light source ages it can emit less excitation light. This doesn’t affect the lifetime
but does affect the emission light intensity, less excitation light resulting in less emitted
light. There are a host of other circuit and physical problems that likewise can affect emitted light
intensity.
The miniDOT makes both lifetime and intensity measurement at each measurement time.
The DO value recorded by the miniDOT is determined from the emission lifetime. However,
miniDOT software also computes the DO determined from the emission intensity. miniDOT
records both DO and also the ratio (DO determined from lifetime / DO determined from intensity)
at every measurement. Patent Pending. This DO ratio should ideally be 1 and in practice is usually
very close to 1.0. If this ratio deviates significantly from 1.0 it can indicate various measurement
problems. This unique feature provides a quality estimate for each DO measurement miniDOT
records.
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
coils distributed within the tank. Four RBR temperature loggers were deployed within the tank to
verify actual temperature to better than 0.02
o
C. Salinity was varied by addition of commercial