Analytical Instrumentation

Dissolved Oxygen Analyser Working Principle

Dissolved Oxygen Analyser Working Principle 3
  • A Dissolved Oxygen DO analyzer is also known as a dissolved oxygen meter.
  • This is a device used to measure the concentration of dissolved oxygen in a liquid, typically in water. 
  • The working principle of a dissolved oxygen analyzer is based on the electrochemical measurement of oxygen in the liquid.
  • A dissolved oxygen analyzer continuously observes the concentration of dissolved oxygen in the water.
  • It is commonly used in environmental monitoring, wastewater treatment, aquaculture, and various industrial processes where oxygen levels in a liquid are important. 

What is Dissolved Oxygen?

  • Dissolved oxygen is defined as the amount of net oxygen dissolved in water. 
  • The concentration of Dissolved oxygen is expressed as the amount of oxygen per unit volume.
  • The unit of dissolved oxygen is represented by mg/L. 
  • Biologically, oxygen is an essential element for the respiration of underwater or aquatic life and also acts as a chemical oxidizer. 
  • The solubility of oxygen in water is affected by water temperature, salinity of the water, barometric pressure of the water, etc.
  • The solubility of oxygen in water reduces for a rise in the temperature of water.
  • It is a critical parameter in aquatic ecosystems, as the availability of oxygen in water is essential for the survival of aquatic organisms.
  • Dissolved oxygen is crucial for various ecological processes and is a key indicator of water quality. 

Measurement of Dissolved Oxygen by the Membrane Electrode Method

  • The membrane electrode method determines a diffusion current or reduction current produced by the concentration of dissolved oxygen or partial pressure of oxygen.
  • This membrane electrode method is not affected by the pH value of water measured, Oxidation, reduction substances, color, or turbidity.
  • This measurement method offers better reproducibility.
  • When a sensor is immersed in water, an air layer forms on the Teflon membrane.
  • The partial pressure of oxygen or concentration of dissolved oxygen in the air layer is in equilibrium with the concentration of dissolved oxygen in the water. 
  • The membrane electrode method determines the concentration of oxygen in the gas phase and indirectly obtains the concentration of dissolved oxygen in water.

Classification of Membrane Electrode Method

These Membrane Electrode Methods differ only in the presence or absence of an externally applied voltage, but offer the same performance, features, and usage method and are classified into

  1. Galvanic Cell Method
Dissolved Oxygen Analyser Working Principle 2
  • The membrane has high permeability to oxygen and is constructed so that the electrodes and electrolytes are segregated from the water being determined.
  • The cathode is typically made of a noble metal like gold or platinum, and it is exposed to the liquid sample containing dissolved oxygen.
  • The anode is often made of a less noble metal, such as lead or lead-silver alloys. It is also in contact with an electrolyte solution.
  • An electrolyte solution, usually potassium chloride (KCl), is used to facilitate the electrochemical reaction between the cathode and anode.
  • Oxygen is allowed to flow through the membrane and is minimized on the working electrode.
  • The reduction of oxygen at the cathode generates an electrical current, as electrons are transferred from the cathode to the anode through the external circuit. This current is directly proportional to the concentration of dissolved oxygen in the liquid.     
  1. Polarographic Method
Dissolved Oxygen Analyser Working Principle 1
  • The heart of the analyzer is a Polarographic sensor. 
  • This Polarographic method determines the concentration of dissolved oxygen based on this current value.
  • This Polarographic Method resembles similar sensor construction compared to the galvanic cell method
  • Gold or platinum is used as a working electrode, and the counter electrode is silver-silver chloride.
  • When a voltage supply of about 0.5 volts to 0.8 Volts is applied between these working electrodes & counter electrodes, oxygen that has penetrated through the membrane commences a reduction reaction on the working electrode creating a Polarographic limiting current to flow proportional to the concentration of oxygen. 

Calibration of the Dissolved Oxygen Analyzer

The calibration of the dissolved oxygen analyzer is conducted for the situations shown below

  • When a new Dissolved Oxygen Analyser is installed
  • When the membrane assembly or the electrolyte solution is replaced.
  • When the sensor has been removed and fixed during maintenance
  • When a measuring error after cleaning crosses the desired value from the reference method

The following method describes the calibration of the dissolved oxygen analyzer.

Air calibration:

  • The air calibration method is generally satisfactory 
  • This Performs span calibration in ambient air. 
  • This is the most common and easiest way to calibrate the analyzer.
  • The calibration table shows an experimental value at a relative humidity of 70%. 
  • It is essential to wash and then wipe off the remaining water droplets from the sensor membrane gently with soft paper for any contamination deposits. 

Span calibration in air-saturated water:

  • This method is only required if it is necessary to calibrate the Dissolved Oxygen analyzer to get higher accuracy.
  • This method follows ISO      5814.

Zero calibration in sodium sulphite solution:

  • This calibration method requires more time but this method is optional.
  • Real solution calibration: Measurement is conducted by using an instrument based on the water.
  • The calibration is executed in accordance with the value obtained.

Dissolved Oxygen Analyzer principle methods

  • The primary principle behind a Dissolved Oxygen Analyzer involves electrochemical or optical methods.
  • Usually, the choice between the electrochemical method and the optical method depends on factors like the specific application, the required accuracy, and the environmental conditions in which the analyzer will be used
  • The readings for both methods are given in units such as milligrams per liter (mg/L), parts per million (ppm), or percentage of saturation. 
  • To obtain accurate measurements these analyzers require periodic calibration and maintenance

Electrochemical Method:

  • The electrochemical method is the most common and widely used for measuring dissolved oxygen. 
  • It involves a membrane-covered electrode system that reacts with the dissolved oxygen in the water.
  • The analyzer consists of a DO probe or sensor, which typically contains two electrodes called a working electrode and a reference electrode. 
  • The working electrode is covered with a gas-permeable membrane, usually made of Teflon, which permits oxygen to diffuse through it keeping the electrolyte inside.
  • A voltage is applied between the working electrode and the reference electrode, causing a reduction-oxidation (redox) reaction at the working electrode.
    • Reduction: O2 + 2H2O + 4e- → 4OH-
  • This reaction is proportional to the amount of oxygen that diffuses through the membrane.
  • The current generated by this reaction is measured and used to calculate the dissolved oxygen concentration in the water. 
  • The analyzer often compensates for temperature and atmospheric pressure to provide accurate readings.
  • Dissolved oxygen measurements are often temperature-dependent. The analyzer includes temperature sensors or compensates for temperature changes to adjust the dissolved oxygen readings to a reference temperature (usually 25°C or 20°C). This compensation is essential for accurate results.
  • The electrochemical method generally provides a rapid response time, allowing for real-time or near-real-time monitoring of dissolved oxygen levels. This is particularly useful in dynamic aquatic environments.

Optical Method:

  • This optical method is not widely used to measure dissolved oxygen
  • It is based on the principle of oxygen quenching the luminescence of a fluorescent dye.
  • A sensor or probe contains a fluorescent dye that is sensitive to oxygen concentration. 
  • The dye is energized by a light source, and its luminescence is determined through a photo-detector.
  • When oxygen molecules come into contact with the dye, they quench the fluorescence of the dye, and the extent of quenching is proportional to the dissolved oxygen concentration.
  • The analyzer measures the change in luminescence and converts it into a dissolved oxygen concentration reading.

Advantages of Dissolved Oxygen Analyzer

  1. Accuracy 
  2. Speed
  3. Portability
  4. Real-time monitoring
  5. Process control
  6. Environmental monitoring
  7. Efficiency
  8. Data logging

Disadvantages of Dissolved Oxygen Analyzer 

  1. High Cost
  2. Calibration 
  3. Maintenance Requirements.
  4. Interference
  5. Sensor fouling

Applications of Dissolved Oxygen Analyzer

A dissolved oxygen analyzer is commonly used in 

  1. Environmental Monitoring, 
  2. Wastewater Treatment, 
  3. Aquaculture,
  4. Various Industrial Processes.
  5. Brewing and beverage industry
  6. Biotechnology and pharmaceutical manufacturing
  7. Power, Oil and gas industry
  8. Food and beverage industry

Frequently Asked Questions 

What is the principle of a Polarographic Dissolved Oxygen Analyzer?

A Polarographic Dissolved Oxygen Analyzer operates on the principle of oxygen reduction at a polarographic electrode. Polarographic Dissolved Oxygen Analyzer makes use of an oxygen-permeable membrane that allows a chemical reduction reaction to generate an electrical signal to seize the DO concentration value.

State the law of dissolved oxygen in water

Henry’s Law of Dissolved Oxygen in water states that the amount of dissolved oxygen in water is directly proportional to the partial pressure of oxygen in the surrounding air. In this process, MnSO4 reacts with an alkali to give a white precipitate of Manganese hydroxide. The further precipitate of O2 gets oxidized into a brown-colored higher hydroxide. Upon acidification, this higher hydroxide liberates iodine equivalent to the amount of fixed oxygen present, demonstrating the relationship between dissolved oxygen and chemical reactions.          

What is the purpose of the Polarographic Dissolved Oxygen Analyzer in industrial processes?

A Polarographic Dissolved Oxygen Analyzer allows 

  1. Real-time measurement of dissolved oxygen at a defined set point. 
  2. Detection of a leakage in the process that leads to oxygen infiltration.

What is the Input to the Dissolved Oxygen Analyzer?

Input to Dissolved Oxygen Analyzer

  1. 12V DC – MODBUS:     This input specifies that the analyzer can be powered with a 12-volt direct current (DC) power supply and communicates using the MODBUS protocol, which is a commonly used communication protocol in industrial automation and control systems.
  2. 24V DC – 4-20mA & MODBUS:This input specifies an alternative power supply option, with the analyzer powered by a 24-volt DC source. Additionally, it communicates using both the 4-20mA analog signal (commonly used for process control applications) and the MODBUS protocol.

What is the output of the Dissolved Oxygen Analyzer?

  1. The output of the Dissolved Oxygen Analyzer is a Millivolt
  2. For easy PLC integration, a 4-20mA analog signal or MODBUS/RS485 digital signal is scaled to the sensor range. 

What is the unit of dissolved oxygen?

Milligrams per liter

What is 100% Dissolved Oxygen?

100% Dissolved Oxygen is defined as

In an equilibrium for a given temperature, when water holds all Dissolved Oxygen it is said to be saturated with oxygen.

How do I use a laboratory-grade DO probe?

  • A laboratory-grade DO probe consists of a Poly Tetra Fluoro Ethylene membrane, with a cathode and anode immersed in an electrolyte.
  • Laboratory-grade DO probes can be immersed fully in freshwater or salt water up to the SMA connector.

How do I use an Industrial Grade DO probe?

  • An Industrial Grade DO probe consists of a silicone membrane instead of a Poly Tetra Fluoro Ethylene membrane.
  • For industrial processes, we can link a DO probe to a DO circuit and an RTD circuit via a carrier board and to a DO transmitter.
  • Industrial DO probes can be immersed fully in fresh or salt water up to the tinned lead connector.

Do industrial-grade and Laboratory Grade DO probe works on the same principle 

Yes, the working principle of an Industrial Grade DO probe is the same as Laboratory Grade DO probe

What are the factors that affect dissolved oxygen in water?

When it is required to determine dissolved oxygen the following are the factors that affect dissolved oxygen in water like 

  • Temperature
  • Salinity
  • Barometric pressure
  • Humidity

Sundareswaran Iyalunaidu

With over 24 years of dedicated experience, I am a seasoned professional specializing in the commissioning, maintenance, and installation of Electrical, Instrumentation and Control systems. My expertise extends across a spectrum of industries, including Power stations, Oil and Gas, Aluminium, Utilities, Steel and Continuous process industries. Tweet me @sundareshinfohe

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