YSI 6000UPG - Manual (Page 106)

6-6
depth probe. For example, a change of 1 mm of Hg in barometric pressure will change the apparent
depth by approximately 0.045 ft (0.012 m).
6.6 TEMPERATURE
The 6000 UPG utilizes a thermistor of sintered metallic oxide which changes predictably in resistance
with temperature variation. The algorithm for conversion of resistance to temperature is built in to
the 6000
UPG
software, and accurate temperature readings in degrees Celsius, Kelvin, or Fahrenheit
are provided automatically. No calibration or maintenance of the temperature sensor is required.
6.7 DISSOLVED OXYGEN
The 6000 UPG employs 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 severely
limit the length of a remote deployment unless an external battery source is utilized) 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 associated with
the 6000
UPG since it needs no stirring to yield accurate readings for either spot sampling or
deployment applications. The 6000 UPG can accurately monitor DO for up to 90 days using only the
eight C-cells enclosed in the sonde. 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 which is not radically different from 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 6030 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 new
Rapid Pulse method for DO measurement as described in the following section.