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Core Professional Terminology of Optical Dissolved Oxygen Analyzers: A Complete Explanation from Principle to Units
In water quality monitoring, wastewater treatment, aquaculture, and environmental science research, dissolved oxygen (DO) is one of the most critical parameters. The optical fluorescence method has become the mainstream DO measurement technology due to its advantages of zero oxygen consumption, strong anti-interference capability, and long maintenance intervals, gradually replacing traditional electrochemical methods.
However, many professionals are often confused by key concepts in practical applications: What is the fundamental difference between mg/L and % saturation? What does the instrument actually measure directly? How does fluorescence quenching occur? What roles do partial pressure of oxygen and Henry’s Law play?
This article systematically explains these professional terms and, combined with the technical characteristics of Nexisense optical dissolved oxygen sensors, helps readers build a clear and accurate understanding framework.
Fluorescence Quenching Principle — The Physical Basis of Optical DO Measurement
The core of optical dissolved oxygen measurement lies in the phenomenon of dynamic fluorescence quenching.
The sensor probe is coated with an oxygen-sensitive fluorescent material, typically a ruthenium complex such as Ru(dpp)₃²⁺. When excited by light of a specific wavelength (usually blue or green light at approximately 450–470 nm), the fluorescent molecules transition from the ground state to an excited state and then emit red light (approximately 600–650 nm) as they return to the ground state.
When dissolved oxygen is present in the water, oxygen molecules collide with the excited fluorescent molecules and undergo collisional energy transfer. This causes part of the excited-state energy to be released via non-radiative pathways, resulting in a shortened fluorescence lifetime and reduced fluorescence intensity. This phenomenon is known as oxygen quenching.
This process follows the Stern–Volmer equation:
F₀ / F = 1 + Ksv × [O₂]
Where:
F₀: Fluorescence intensity or lifetime in oxygen-free conditions
F: Fluorescence intensity or lifetime in the presence of oxygen
Ksv: Stern–Volmer quenching constant (related to temperature and fluorophore type)
[O₂]: Oxygen concentration (essentially a function of oxygen partial pressure)
Modern optical DO analyzers typically use phase modulation or fluorescence lifetime measurement techniques. By detecting the phase shift between excitation and emission light, or the fluorescence decay time (usually in the microsecond range), the oxygen concentration can be calculated. Because this method is highly sensitive to oxygen partial pressure, the raw output of the instrument is often oxygen partial pressure or % saturation.
Two Major Measurement Units: Physical Meaning and Application of mg/L and % Saturation
mg/L — Absolute Concentration Unit
mg/L represents the actual mass of oxygen dissolved in one liter of water (mg O₂ per liter of H₂O), equivalent to ppm (parts per million by mass). This is the most commonly used unit in environmental standards, water quality reports, and ecological assessments.
It directly reflects the absolute availability of oxygen in water and is convenient for comparison with biological oxygen demand (BOD), chemical oxygen demand (COD), and the oxygen requirements of aquatic organisms.
% Saturation — Relative Degree of Saturation
% saturation is defined as:
Actual dissolved oxygen concentration / theoretical saturated dissolved oxygen concentration under given conditions × 100%
The “given conditions” mainly include temperature, atmospheric pressure (or altitude), and salinity (or conductivity). This parameter indicates whether the water body is in equilibrium with atmospheric oxygen and the degree of deviation from equilibrium.
At 100% saturation, the water is at equilibrium; values greater than 100% indicate supersaturation (commonly observed during daytime with intense algal photosynthesis); values below 100% indicate undersaturation (often found in polluted waters with high organic oxygen demand).
Nexisense optical dissolved oxygen sensors directly respond to the partial pressure of oxygen (pO₂) through fluorescence quenching. Therefore, the primary measured value is usually % saturation (based on standard atmospheric pressure, current temperature, and zero salinity), which is then converted in real time to mg/L using built-in algorithms such as the USGS solubility tables or the Benson–Krause equation.
Key Influencing Factors and Compensation Mechanisms: Henry’s Law and Environmental Corrections
The equilibrium concentration of dissolved oxygen follows Henry’s Law:
[O₂] = kH × pO₂
Where:
[O₂]: Oxygen concentration in water
kH: Henry’s constant (strongly temperature-dependent and decreases as temperature increases)
pO₂: Partial pressure of oxygen in the gas phase (atmospheric pressure × oxygen volume fraction ≈ 0.2095)
This explains three major influencing factors:
Temperature: Higher temperature → lower kH → lower saturation concentration
Atmospheric pressure / altitude: Lower pressure → lower pO₂ → lower saturation concentration
Salinity: Higher salinity reduces oxygen solubility; seawater saturation is approximately 20% lower than freshwater
Nexisense sensors integrate high-precision temperature measurement and support atmospheric pressure input or automatic compensation (some models include built-in pressure sensors), ensuring accurate mg/L outputs in high-altitude, marine, and variable-temperature environments.
Technical Highlights of Nexisense Optical Dissolved Oxygen Sensors
Nexisense optical DO sensors combine mature fluorescence quenching technology, ruthenium-based fluorescent coatings, and phase detection algorithms to achieve:
Response time ≤ 30 seconds
No oxygen consumption, no membrane or electrolyte replacement
Strong resistance to sulfides, chlorine, and other chemical interferences
IP68 protection for long-term immersion
RS485 MODBUS protocol for remote data integration
These features make the sensors particularly suitable for continuous surface water monitoring, precise oxygen control in aeration tanks, and oxygen management in aquaculture.
Frequently Asked Questions (FAQ)
Why do optical DO instruments often display % saturation directly?
Because fluorescence quenching responds directly to oxygen partial pressure, and % saturation is a normalized representation of oxygen partial pressure, making it convenient for comparison across different environments.
How can mg/L and % saturation be converted?
DO (mg/L) = % saturation × saturated DO (mg/L). The saturated DO value is obtained from tables or algorithms based on temperature, pressure, and salinity.
Do water quality standards need adjustment at high altitudes?
It is recommended to prioritize % saturation for ecological assessment (e.g., ≥80% indicates good status), or to correct mg/L limits based on local atmospheric pressure.
What are the main differences between fluorescence quenching and membrane electrode methods?
Optical methods do not consume oxygen, have no membrane aging, offer stronger anti-interference performance, and require less maintenance (fluorescent caps typically last 2–3 years).
Conclusion: Mastering Terminology for Accurate Interpretation of DO Data
From fluorescence quenching and the Stern–Volmer equation, to oxygen partial pressure and Henry’s Law, and finally to the dual representation of mg/L and % saturation—these professional concepts form the theoretical foundation of modern dissolved oxygen monitoring. Only by understanding them thoroughly can accurate scientific judgments be made in complex aquatic environments.
Nexisense optical dissolved oxygen sensors transform these principles into high-precision, reliable measurements through robust optical technology and intelligent compensation algorithms, providing strong support for water ecosystem protection and industrial process optimization. We hope this article helps you interpret every DO dataset more professionally and work together toward sustainable water resource management.
