Xylem 6-Series Multiparameter - Manual (Page 245)

Principles of Operation Section 5
YSI Incorporated Environmental Monitoring Systems Manual 5-7
MEASUREMENT AND CALIBRATION PRECAUTIONS
(1) Be certain that the sonde is not immersed in water during the calibration procedure unless you know the
exact distance between the sensor and the water surface. Calibration (zeroing) in air is usually the
recommended method.
(2) Remember that the depth sensors for the sonde are not vented. In practical terms, this means that
changes in barometric pressure after the sensor is calibrated will appear as changes in depth. This effect is
significant, particularly for the 0-30 ft option of the depth probe. For example, a change of 1 mm of Hg in
barometric pressure will change the apparent depth by approximately 0.045 feet (0.012 m). As noted
above, this error is eliminated for level sensors because they are vented to the atmosphere.
5.7 TEMPERATURE
The sondes utilize a thermistor of sintered metallic oxide that changes predictably in resistance with
temperature variation. The algorithm for conversion of resistance to temperature is built into the sonde
software, and accurate temperature readings in degrees Celsius, Kelvin, or Fahrenheit are provided
automatically. No calibration or maintenance of the temperature sensor is required.
5.8 DISSOLVED OXYGEN – 6562 RAPID PULSE
POLAROGRAPHIC
The sondes employ the patented YSI Rapid Pulse system for the measurement of dissolved oxygen (DO).
Use of this technology provides major advantages for the monitoring of DO without significantly
compromising the accuracy of sampling applications. Standard electrochemical 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 sonde 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 6562 probe, the sensor consists of
three electrodes (a cathode, anode, and reference electrode) while steady state Clark probes usually have
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.
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