Dissolved Oxygen Rox Optical - YSI 6 series User Manual

Principles of Operation Section 5
(4) For short term storage (2 weeks or less), keep the probe moist when not in use, either by immersing in
water or by placing a damp sponge in the calibration vessel. For longer-term storage, remove the probe
from the sonde and store it in water with a membrane and electrolyte in place. If the membrane appears to
be damaged or has dried out, be sure to replace it prior to calibration and deployment.
(65 Before you install a new membrane, make sure that the O-ring groove and the probe tip are clean and
smooth. If the KCl electrolyte solution leaks from the probe surface during monitoring studies, the
accuracy of the readings will be compromised.
5.9 DISSOLVED OXYGEN – 6150 ROX OPTICAL
In general, optical dissolved oxygen sensors from a variety of manufacturers are based on the well-
documented principle that dissolved oxygen quenches both the intensity and the lifetime of the
luminescence associated with carefully-chosen chemical dyes. The 6150 sensor operates by shining a blue
light of the proper wavelength on this luminescent dye which is immobilized in a matrix and formed into a
disk about 0.5 inches in diameter. This dye-containing disk will be evident on inspection of the sensor
face. The blue light causes the immobilized dye to luminesce and the lifetime of this dye luminescence is
measured via a photodiode in the probe. To increase the accuracy and stability of the technique, the dye is
also irradiated with red light during part of the measurement cycle to act as a reference in the determination
of the luminescence lifetime.
When there is no oxygen present, the lifetime of the signal is maximal; as oxygen is introduced to the
membrane surface of the sensor, the lifetime becomes shorter. Thus, the lifetime of the luminescence is
inversely proportional to the amount of oxygen present and the relationship between the oxygen pressure
outside the sensor and the lifetime can be quantified by the Stern-Volmer equation. For most lifetime-
based optical DO sensors (including the YSI 6150), this Stern-Volmer relationship (((Tzero/T) – 1)) versus
O pressure) is not strictly linear (particularly at higher oxygen pressures) and the data must be processed
2
using analysis by polynomial non-linear regression rather than the simple linear regression used for most
polarographic oxygen sensors. Fortunately, the non-linearity does not change significantly with time so
that, as long as each sensor is characterized with regard to its response to changing oxygen pressure, the
curvature in the relationship does not affect the ability of the sensor to accurately measure oxygen for an
extended period of time. The 6150 probe is warranted for 2 years and sensor modules are warranted for 1
year by YSI.
Each YSI sensor module (the assembly which is attached to the face of the probe by three screws) is
factory-calibrated over a range of 0-100 percent oxygen to quantify the relationship of its luminescence
lifetime as a function of oxygen pressure. The Stern-Vollmer parameters from this data are then fit to a
3 2
third order regression equation (ax + bx + cx) and values of a, b, and c determined. These coefficients,
along with the luminescence lifetime at zero oxygen pressure (Tzero), are provided to the user in coded
form with each sensor membrane module or probe/sensor module combination. If you install a
replacement sensor membrane assembly (YSI 6155) on your existing probe, you will be required to enter
these coded constants into the sonde as described in the instructions which come with the 6155 prior to the
use of the sensor. If you have purchased a probe/membrane combination, i.e. a new 6150 Optical DO
sensor, the constants are already stored in your probe and will automatically be transferred to your sonde
when the sensor is installed.
A schematic representation of the 6150 ROX Optical DO sensor is shown in the picture below:
YSI Incorporated Environmental Monitoring Systems Manual 5-10