YSI ADV6600 - Manual (Page 117)

Section 9. Principles of Operation
ADV6600
Y S I
Environmental
Page 107
detectors of DO are highly flow-dependent and therefore require external stirring of the medium
being evaluated. This stirring must be supplied either by an auxiliary stirrer (which can consume
much of the battery reserve in a portable system) or by manually agitating the sonde when carrying
out spot sampling applications (which can be inconvenient). These disadvantages are overcome by
the Rapid Pulse dissolved oxygen technology that is associated with the ADV6600 because it needs
no stirring to yield accurate readings. In addition, because of the nature of the technology, some
effects of fouling of the sensor are minimized.
The Rapid Pulse system utilizes a Clark-type sensor that is similar to other membrane-covered
steady-state dissolved oxygen probes. The system still measures the current associated with the
reduction of oxygen which diffuses through a Teflon membrane, and this current is still proportional
to the partial pressure (not the concentration) of oxygen in the solution being evaluated. The
membrane isolates the electrodes necessary for this reduction from the external media, encloses the
thin layer of electrolyte required for current flow, and prevents other non-gaseous, electrochemically
active species from interfering with the measurement. However, as the user will note from
examination of the probe, the sensor consists of three electrodes (a cathode, anode, and reference
electrode) while a steady state Clark probe usually has only two electrodes (a cathode and a
combined anode-reference electrode). In addition, the geometry of the sensor is novel, consisting of
a thin linear gold cathode placed between two silver rectangles which serve as anode and reference
electrodes. These sensor changes were required to implement the Rapid Pulse method for DO
measurement as described in the following section.
9-9.1. Method of Operation
Standard Clark dissolved oxygen sensors, which are marketed by YSI and other manufacturers, are
continuously polarized at a voltage sufficiently negative to cause oxygen to be reduced to hydroxide
ion at the cathode and silver metal to be oxidized to silver chloride at the anode. The oxygen
diffuses through the Teflon membrane. The current associated with this process is proportional to
the oxygen present in the solution outside the membrane. However, as this electrochemical reaction
proceeds, oxygen is consumed (or depleted) in the medium, resulting in a decrease in measured
current (and apparent oxygen content) if the external solution is not stirred rapidly. To minimize
this oxygen depletion, the probe electrodes in the YSI Rapid Pulse system are rapidly and
reproducibly polarized (on) and depolarized (off) during a measurement sequence. The Rapid Pulse
system thus measures the charge or coulombs (current summed over a specific time period)
associated with the reduction of oxygen during a carefully controlled time interval. The coulombs
due to charging of the cathode (capacitance), but not to reduction of oxygen, are subtracted during
integration after the cathode has been turned off. The net charge, like the steady state current in a
standard system, is proportional to the oxygen partial pressure in the medium. Because oxygen is
only being reduced 1/100th of the total measurement time, even if the probe is pulsed in this manner
continuously, oxygen consumption outside the membrane is kept to a minimum, and the stirring
dependence of the system is greatly reduced.
One key to the practicality of Rapid Pulse oxygen system is the fact that the “on time” is very short.
This allows the “off time” to also be relatively short and still maintain the off to on ratio of 100