Xylem 6-Series Multiparameter - Manual (Page 350)

ROX Optical DO Sensor Appendix M
YSI Incorporated Environmental Monitoring Systems Operations Manual M- 1
APPENDIX M ROX OPTICAL DO SENSOR
This appendix is in the format of “frequently asked questions”, is designed to allow users to optimize the
performance and the trouble-shooting of problems for your YSI 6150 ROX Optical dissolved oxygen probe
by supplementing the discussion of optical dissolved oxygen measurement that is provided in the other
sections of this manual (Getting Started, Basic Operation, Principles of Operation, and Maintenance).
How does the ROX Optical DO Sensor work?
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
2
pressure) is not strictly linear (particularly at higher oxygen pressures) and the data must be processed
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.
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-Volmer parameters from this data are then fit to a
third order regression equation (ax
3 + bx 2 + 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.
What are the key advantages of the ROX sensor over membrane-covered polarographic sensors?
The ROX dissolved oxygen sensor has three key advantages over the YSI Rapid Pulse sensor:
1.
The set-up and maintenance of the ROX sensor is much easier since there is no membrane or
electrolyte to be changed by the user.
2.
Testing indicates that the ROX sensor is significantly less susceptible to field drift.