Dissolved oxygen (DO) and water quality
Dissolved oxygen is a key measure of water quality relied upon in various applications. In industrial water treatment, dissolved oxygen levels can be an indicator of water quality issues that lead to corrosion of equipment. In aquaculture, fish transport, and aquarium applications, dissolved oxygen is monitored to ensure that aquatic species have enough oxygen in their habitat to survive, grow, and reproduce. In municipal water treatment facilities, dissolved oxygen in wastewater is monitored during aeration water treatment processes.
Measuring dissolved oxygen concentration
The concentration of dissolved oxygen in water can be sampled or monitored continuously using a dissolved oxygen sensor. How does a dissolved oxygen probe work? The answer to this question depends on the type of dissolved oxygen sensor used. Commercially available dissolved oxygen sensors typically fall into 3 categories:
- Galvanic dissolved oxygen sensors
- Polarographic dissolved oxygen sensors
- Optical dissolved oxygen sensors
Each type of dissolved oxygen sensor has a slightly different working principle. Therefore, each dissolved oxygen sensor type has advantages and disadvantages depending on the water measurement application where it will be used.
Electrochemical dissolved oxygen sensor working principle:
Both galvanic DO sensors and polarographic DO sensors are types of electrochemical dissolved oxygen sensors. In an electrochemical DO sensor, dissolved oxygen diffuses from the sample across an oxygen permeable membrane and into the sensor. Once inside the sensor, the oxygen undergoes a chemical reduction reaction, which produces an electrical signal. This signal can be read by a dissolved oxygen instrument.
Polarographic vs. galvanic DO sensors:
The difference between a galvanic DO sensor and a polarographic DO sensor is that a polarographic DO sensor requires a constant voltage to be applied to it. It must be polarized. By contrast, a galvanic DO sensor is self-polarizing due to the material properties of the anode (zinc or lead) and cathode (silver). This means that while galvanic DO sensors can be used immediately after calibration, polarographic sensors require a 5-15 minute warm up time.
Optical dissolved oxygen sensor working principle:
An optical dissolved oxygen sensor does not have an anode or cathode, and oxygen is not reduced during measurement. Instead, the sensor cap contains a luminescent dye, which glows red when exposed to blue light. Oxygen interferes with the luminescent properties of the dye, an effect called “quenching.” A photodiode compares the “quenched” luminescence to a reference reading, allowing the calculation of dissolved oxygen concentration in water.
Optical vs. galvanic DO sensors:
Both optical dissolved oxygen measurement and galvanic dissolved oxygen measurement have advantages and advantages. The good news is that both technologies offer a similar level of accuracy when measuring dissolved oxygen concentration. This holds true across a wide range of measurement values: field tests have shown similar results for optical and galvanic DO sensors from ~1mg/L up to 14mg/L.
One point of differentiation between optical and galvanic DO sensors is that galvanic DO sensors exhibit flow dependence. This means that a minimum inflow velocity (2 inches/sec for Sensorex models) is required to maintain measurement accuracy. Optical DO sensors require no minimum inflow velocity.
Some sample constituents may interfere with measurement accuracy. Hydrogen sulfide, for example, a compound found in wastewater, lake bottoms, and wetlands, can permeate the galvanic sensor membrane. An optical dissolved oxygen sensor would make a better choice in these environments, as these sensors are not susceptible to interference by H2S.
One advantage of galvanic DO sensors over optical DO sensors is that galvanic DO sensors have a faster response time. Galvanic DO sensors respond 2-5x faster than optical DO sensors depending on the membrane material. This limitation of optical DO sensors is more cumbersome in applications where a high number of sample measurements will be taken. Response time is typically not a limiting factor when choosing a DO sensor for continuous monitoring applications.
Comparison of polarographic, galvanic, and optical DO sensors:
The table below summarizes advantages and disadvantages of the 3 primary methods for measuring dissolved oxygen concentration in water: