Xylem WQ-FDO OPTICAL DISSOLVED OXYGEN TRANSMITTER - Manual (Page 3)

Global Water
800-876-1172
• globalw.com
I. Overview
The WQ-FDO Optical DO Sensor is an instrument designed for measuring
dissolved oxygen in water. The optical DO sensors were developed to meet the
requirements ranging from surface water monitoring programs to harsh waste
water applications. The WQ-FDO has been specifically designed to meet the
demanding requirements of the environmental monitoring and scientific research
sectors, providing long term, accurate and reliable dissolved oxygen measurement.
The sensor has low power requirements and a 4-20 mA output making it ideal for
incorporation into remote environmental monitoring installations.
The WQ-FDO Optical DO Sensor has many advantages over traditional DO
sensors. Unlike conventional Galvanic and Polarographic DO sensors, optical
sensors have no consumable cathodes or anodes that require replacement,
minimizing maintenance requirements. The sensor does not consume oxygen
either. Consequently, the measurement of DO by the sensor is unaffected by water
flow. The WQ-FDO can even be deployed in stagnate groundwater bores. The
sensors have extremely stable electronics – a service interval of 1 year is typical.
The measuring and reference path of the optical components are identically
designed inside the sensor. Natural aging processes of the sensor’s optical
components can therefore be compensated by the reference path and accordingly
compensated in the measuring path. As a result, the sensor provides accurate DO
measurements over long periods of time without the need for re-calibration.
Service of the sensor usually consists of replacing the cap and membrane
assembly. The cap has a memory chip installed which contains recalibration
information that is read by the sensor, making recalibration unnecessary after this
service.
The sensor’s optical measuring technique provides accurate readings over a wide
range of conditions. Because the concentration and saturation levels of dissolved
oxygen vary with temperature, barometric pressure and conductivity (or salinity);
the level of oxygen is reported by the sensor as partial pressure, allowing the
conversion to concentration and saturation to be calculated externally using these
other parameters. An in-line interface module converts the digital partial pressure
and temperature data into two separate 4-20mA signals for monitoring with data
loggers and PLC devices.