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Ref. No. [UMCES] CBL 2016-011

ACT VS16-02


performance characteristics of commercially available DO sensors when applied in real world

applications in a diverse range of coastal environments.


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


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



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


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


C. Salinity was varied by addition of commercial