Optical dissolved oxygen (DO) sensors work by utilizing fluorescence quenching, a process where oxygen molecules interfere with the fluorescence emitted by a specific chemical dye. Here's a breakdown:
Fluorescence Quenching Explained
The core principle relies on a special fluorophore, a substance that emits light (fluoresces) when excited by a specific wavelength of light. The intensity and lifetime of this fluorescence are affected by the presence of oxygen. Specifically, oxygen molecules act as quenchers:
- Excitation: A blue light (or other suitable wavelength) is shone onto a fluorescent dye immobilized on a sensor surface.
- Fluorescence: The dye absorbs this light and emits light at a longer wavelength (e.g., green or red).
- Quenching: Oxygen molecules diffuse into the dye and collide with the excited fluorophore molecules. These collisions transfer energy from the fluorophore to the oxygen.
- Reduced Fluorescence: This energy transfer reduces the intensity and shortens the lifetime of the fluorescence emitted by the dye.
The Relationship Between Oxygen and Fluorescence
Crucially, the amount of fluorescence quenching is directly related to the concentration of dissolved oxygen. A higher concentration of dissolved oxygen leads to more quenching, resulting in a weaker and shorter-lived fluorescent signal. Conversely, a lower concentration of DO leads to less quenching and a stronger, longer-lived signal.
How the Sensor Measures DO
The optical DO sensor measures either the intensity or the lifetime of the fluorescence emitted by the dye. This measurement is then correlated to a dissolved oxygen concentration using a calibration curve.
- Intensity-based sensors: Measure the absolute intensity of the emitted light. However, these can be affected by photobleaching and drift.
- Lifetime-based sensors: Measure the time it takes for the fluorescence to decay. These are generally considered more robust and less susceptible to interference.
Advantages of Optical DO Sensors
Compared to traditional polarographic DO sensors, optical DO sensors offer several advantages:
- No oxygen consumption: Optical sensors don't consume oxygen during measurement, eliminating the need for sample flow. Polarographic sensors do consume oxygen at the electrode, requiring sample flow to ensure accurate measurements.
- Lower maintenance: Optical sensors generally require less frequent calibration and maintenance.
- Faster response time: Optical sensors typically have a faster response time than polarographic sensors.
- Less sensitive to fouling: While fouling can still affect optical sensors, they are generally less susceptible than membrane-based polarographic sensors.
Summary
Optical dissolved oxygen sensors offer a reliable and convenient method for measuring DO levels. By exploiting the phenomenon of fluorescence quenching, they provide accurate readings with minimal maintenance and oxygen consumption.
