What Does a DO Sensor Measure?

A dissolved oxygen sensor measures molecular oxygen (O₂) that is dissolved in water, not oxygen bubbles or gas pockets. The result is usually displayed in:

• mg/L (milligrams per liter)

• ppm (parts per million)

• Sometimes % saturation, which compares measured oxygen to the maximum possible oxygen at that temperature and pressure.

Two Main Types of Dissolved Oxygen Sensors

There are two dominant DO sensor technologies in use today:

1. Electrochemical sensors

2. Optical (luminescent) sensors

Each works on a different physical principle.

1. Electrochemical DO Sensors (Clark-Type)

How They Work

Electrochemical DO sensors measure oxygen by allowing dissolved oxygen to pass through a gas-permeable membrane into an internal electrolyte. Inside the sensor:

• Oxygen reacts at the cathode

• This reaction generates an electrical current

• The current is directly proportional to the oxygen concentration in the water

The sensor converts this electrical signal into a DO value.

Key Characteristics

• Oxygen is consumed during measurement

• Requires water flow or stirring for accurate readings

• Performance depends on membrane condition and electrolyte quality

Types of Electrochemical Sensors

• Galvanic: Self-powered, faster startup

• Polarographic: Requires warm-up time but offers high precision

Pros

• Lower upfront cost

• Widely used and well understood

Cons

• Requires regular calibration

• Membranes and electrolyte need maintenance

• Less stable at very low flow or very high DO levels

2. Optical (Luminescent) DO Sensors

How They Work

Optical DO sensors use a luminescent sensing layer that emits light when excited by an LED. Oxygen molecules interact with this light by quenching (reducing) the luminescence.

The sensor measures:

• The change in light intensity or decay time

• The degree of quenching caused by oxygen

More dissolved oxygen → more quenching → lower luminescence.

Key Characteristics

• Oxygen is not consumed

• No electrolyte

• Minimal dependence on water movement

Pros

• Very stable and accurate

• Low maintenance

• Ideal for continuous monitoring

• Performs well at low and high DO levels

Cons

• Higher initial cost

• Sensing caps need periodic replacement

What Can Affect DO Sensor Accuracy?

Several factors can influence readings:

• Biofouling (biofilm or algae on the sensor)

• Flow conditions (especially for electrochemical sensors)

• Calibration frequency

• Pressure and altitude

• Salinity (important in aquaculture)

Regular cleaning and calibration are essential for reliable data.

Measuring DO in High-Oxygen Systems

In systems using advanced oxygenation technologies such as nanobubbles or pure oxygen injection, DO levels can be higher and more stable than in traditional aeration systems.

In these cases:

• Optical sensors are often preferred

• Sensor range and response time become important

• Proper placement in the system is critical to avoid localized supersaturation artifacts

In Waboost we offer a range of sensors from our partner Aqualabo, see them here.

Why DO Sensors Are Critical for Control, Not Just Monitoring

DO sensors are not just passive measurement tools. In modern water systems, they are used to:

• Control oxygen injection rates

• Trigger alarms when oxygen drops

• Optimize energy use

• Prevent root stress or fish mortality

• Validate oxygen transfer efficiency

Accurate DO data allows operators to actively manage oxygen, rather than guessing.

Summary

Dissolved oxygen sensors work by detecting molecular oxygen dissolved in water, either through electrochemical reactions or optical luminescence quenching. While both technologies are widely used, optical DO sensors are increasingly preferred for high-performance, low-maintenance, and continuous monitoring applications.

For any system where oxygen directly impacts biology, dissolved oxygen sensors are an essential foundation for data-driven water management.