Methodology - Optical Dissolved Oxygen Transmitter
Unlike conventional dissolved oxygen transmitters, the D-Opto optical dissolved oxygen transmitter's sensing element utilizes fluorescence to measure dissolved oxygen.  The sensing element in the optical dissolved oxygen transmitter consists of a fluorescing compound suspended stably in a robust optical “window”.  A light source at controlled wavelength briefly pulses the optical window.  This “excites” the fluorescent material, causing it to emit a specific wavelength of light.  The intensity of this fluorescence or light is determined by the amount of dissolved oxygen in the water in contact with the optical window.  A highly stable and accurate optical sensor circuit inside the D-Opto optical dissolved oxygen transmitter measures the emitted fluorescence.  As a result, the optical dissolved oxygen transmitter provides accurate dissolved oxygen measurements over long periods of time without the need for re-calibration.

Other Dissolved Oxygen Sensors
Traditional Galvanic and Polargraphic dissolved oxygen sensors have been used to measure dissolved oxygen in water.  Galvanic and polargraphic dissolved oxygen sensors utilize chemical electrical signatures based on a semi-permeable membrane covering electrodes that are immersed in an electrolyte.  These dissolved oxygen sensors have been poorly suited to long term reliable and accurate monitoring of lakes, streams, and rivers.  The semi-permeable membranes and electrolyte require frequent replacement.  The Galvanic and Polargraphic dissolved oxygen sensor electrodes are subject to deterioration and “poisoning”.  These dissolved oxygen sensors require constant water flow otherwise oxygen consumption occurs causing inaccurate readings.  Galvanic and Polargraphic dissolved oxygen sensors require frequent calibration resulting in high upkeep and labor costs.

Optical Dissolved Oxygen Transmitter Advantages
Unlike conventional Galvanic and Polargraphic dissolved oxygen sensors, D-Opto optical dissolved oxygen transmitters have no consumable components that require replacement, minimizing servicing requirments.  Neither do the optical dissolved oxygen transmitters consume oxygen.  Consequently the measurement of dissolved oxygen by the optical dissolved oxygen sensor is unaffected by water flow.  The D-Opto optical dissolved oxygen transmitters can even be deployed in stagnate groundwater bores.  Because of the copper anti-biofoul ring and the general construction of the optical dissolved oxygen transmitters, they are unaffected by pollutants and poisons and highly resilient to bio fouling.  The D-Opto optical dissolved oxygen transmitters have extremely stable electronics – a calibration interval of 1 year is typical.

The D-Opto optical dissolved oxygen transmitters can be used to monitor dissolved oxygen in almost any liquid, including wines, beer, and milk.  The optical dissolved oxygen sensors are not affected by color of the liquid.  Bubbles or aeration do not affect the optical dissolved oxygen transmitters measurements.  The optical dissolved oxygen sensors can also be mounted in process lines for quality assurance.

Global Water's PC300 Optical Dissolved Oxygen Controller can use the optical dissolved oxygen transmitter's 4-20mA output to control pumps or alarms.  In addition Global Water offers the GL500 optical dissolved oxygen recorder which adds recording capabilities to the 4-20mA optical dissolved oxygen transmitter.  The GL500 optical dissolved oxygen recorder connects to the optical dissolved oxygen sensor's 4-20mA output to record highly accurate dissolved oxygen data.

Applications

• Long term monitoring of dissolved oxygen in streams, rivers and lakes
• Scientific research projects, both long and short term projects
• Water quality monitoring around industrial outfall
• Homeland security
• Thermocline profiling
• Fish farm and aquaculture water quality monitoring
• Wastewater industry
• Food and wine industry

Close up of measurement window and copper biofoul ring.

A sensor being pulled out after 4 weeks in the drink…. Perfect condition.   These sensors are put in a 150mm (6 inch) pipe that is perforated.  This prevents growth of foul by removing photosynthesis as a growth influence.  This also protects the sensors for debris floating downstream.

This sensor was located downstream of a paper mill discharge.  Biofoul is always an issue with sensor deployments but the additional contaminates in this stream proved quite difficult.  The copper biofoul ring served its purpose and showed that cleaning was only needed every other trip.

Optical Dissolved Oxygen Transmitter Specifications

Output: SDI-12 or 4-20 mA (with converter)
Dissolved Oxygen
     Accuracy: 1% of reading or 0.02 ppm, which ever is greater
     Resolution: 0.01% Saturation, 0.001 ppm
     Range: 0.00 to 25.0 ppm
     Repeatability: 0.01 ppm
     Response time: 90% in less than 60 seconds
     Temperature: Compensated 0 to 50°C
     Sensor Drift: Less than 1% per year