MesuLab Dissolved Oxygen Meter

MesuLab Dissolved Oxygen Meter  

A dissolved oxygen meter is an instrument used to measure the oxygen content dissolved in an aqueous solution. Oxygen dissolves in water through surrounding air, air movement, and photosynthesis. It is widely used to monitor processes where oxygen levels impact reaction rates, process efficiency, or environmental conditions, such as in aquaculture, biological reactions, environmental testing (lakes, streams, oceans), water/wastewater treatment, and wine production.

Features  
Dissolved oxygen meters measure the oxygen content in aqueous solutions. Oxygen dissolves in water through air exposure, water movement, and photosynthesis.

Oxygen is consumed in water via respiration and decomposition processes and is replenished primarily through air exchange and photosynthesis. The oxygen content in water is highly temperature-dependent, with warmer water holding less oxygen than cooler water. However, excessively high dissolved oxygen levels can be harmful to aquatic plants and animals.

Dissolved oxygen electrodes can measure the oxygen content in aqueous samples both in the field and in the laboratory. Since dissolved oxygen is a key indicator of water quality, these electrodes are widely used in various applications, especially in aquaculture, photosynthesis and respiration studies, and on-site measurements. When assessing the ability of streams and lakes to support aquatic life, biochemical oxygen demand (BOD) tests are conducted. These tests measure the oxygen consumption during the decomposition of organic matter in water samples and establish the relationship between dissolved oxygen concentration and water temperature.

Dissolved oxygen concentration is typically expressed in mg/L (milligrams of oxygen per liter of water) or ppm (parts per million). Some instruments compare calculated oxygen content with observed concentrations to determine the percent saturation (% O₂ sat.).

There are two primary methods for determining dissolved oxygen: polarographic and galvanic. Polarographic electrodes require an applied voltage to polarize the electrode. Since the voltage may take 15 minutes to stabilize, these electrodes typically require a warm-up period to ensure proper polarization. Galvanic electrodes consist of two different metals that spontaneously generate a voltage without external polarization. As the voltage is self-generated, galvanic electrodes do not require a warm-up period.

Environmental Impact
Adequate dissolved oxygen is essential for good water quality, as all life forms depend on oxygen. Natural water purification processes require sufficient oxygen levels to support aerobic organisms. If dissolved oxygen falls below 5.0 mg/L, aquatic life may struggle to survive, and levels below 1-2 mg/L for several hours can lead to mass mortality.

Applications  
Dissolved oxygen electrodes are used to monitor processes where oxygen levels affect reaction rates, process efficiency, or environmental conditions, including aquaculture, biological reactions, environmental testing (lakes, streams, oceans), water/wastewater treatment, and wine production.

Temperature Compensation
For standard dissolved oxygen measurements, temperature affects both the solubility and diffusion rate of oxygen, making temperature compensation necessary.

Salinity Correction
The presence of dissolved salts limits the amount of oxygen that can dissolve in water. The relationship between oxygen concentration and partial pressure varies with the salinity of each sample. Most instrument manufacturers provide manual salinity adjustment to correct for changes due to ionic concentration differences.

Biochemical Oxygen Demand (BOD)
BOD testing is commonly used in wastewater treatment plants to measure the amount of oxygen consumed by microorganisms during the decomposition of organic matter. This test helps determine the efficiency of water treatment or the amount of residual pollution. The relative oxygen demand of wastewater, effluent, and sewage is determined by measuring the dissolved oxygen at the beginning (T1) and end (T2) of a specific incubation period. BOD is calculated as follows:
BOD (mg/L) = (T1 – T2) × VF / V
where VF is the final sample volume and V is the initial sample volume.

Troubleshooting

When using polarographic electrodes, allow a warm-up period of 15–30 minutes before calibration or measurement.

Ensure no air bubbles are trapped in the electrolyte of the membrane. ASI membrane caps are designed to eliminate air from the liquid chamber during assembly.

Remove any bubbles from the membrane surface, as they may be misinterpreted as oxygen-saturated samples.

Calibrate the electrode at a temperature close to that of the sample, even when using instruments with automatic temperature compensation.

Calibrate the electrode in air, using air as the 100% saturated dissolved oxygen reference.

Stir the solution during measurement to prevent oxygen depletion at the probe surface due to electrode consumption.

Replace the membrane if it is damaged.

Measurement Principle
Most dissolved oxygen meters use a membrane electrode as a transducer to convert dissolved oxygen concentration (actually oxygen partial pressure) into an electrical signal. This signal is amplified, adjusted (including salinity and temperature compensation), and converted for digital display.

There are two types of membrane electrodes:

Polarographic Electrode:

Cathode: Gold (Au) or platinum (Pt) ring

Anode: Silver-silver chloride (or mercury-mercurous chloride)

Electrolyte: Potassium chloride solution

The cathode is covered with an oxygen-permeable membrane made of materials such as polytetrafluoroethylene (PTFE), polyvinyl chloride (PVC), polyethylene (PE), or silicone rubber.

A polarization voltage of 0.5–1.5 V (typically 0.7 V) is applied between the electrodes.

When dissolved oxygen permeates the membrane and reaches the cathode, the following reactions occur:

Cathode reduction: O₂ + 2H₂O + 4e⁻ → 4OH⁻

Anode oxidation: 4Cl⁻ + 4Ag – 4e⁻ → 4AgCl

The diffusion current (i∞) generated is proportional to the dissolved oxygen concentration:
i∞ = nFA(Pm/L)Cs
where:

n: number of electrons transferred

F: Faraday constant (96,500 coulombs)

A: cathode surface area (cm²)

Pm: membrane permeability coefficient (cm²/s)

L: membrane thickness (cm)

Cs: dissolved oxygen concentration (ppm)

With a fixed electrode structure and membrane, constants can be combined as K = nFA(Pm/L), simplifying to:
i∞ = KCs

Thus, measuring the diffusion current allows determination of dissolved oxygen concentration. Temperature, salinity, and pressure compensation techniques are employed to ensure accuracy.

Galvanic Electrode:

When oxygen molecules permeate the membrane and reach the cathode’s three-phase interface, the following reactions occur:

Silver cathode reduction: O₂ + 2H₂O + 4e⁻ → 4OH⁻

Lead anode oxidation: 2Pb + 2KOH + 4OH⁻ – 4e⁻ → 2KHPbO₂ + 2H₂O

Oxygen is reduced to hydroxide ions at the cathode, while the lead anode is corroded by potassium hydroxide, generating lead acid potassium.

The current generated in the external circuit is proportional to the dissolved oxygen concentration.

Classification of Dissolved Oxygen Meters

By Portability: Portable, benchtop, and pen-type meters.

By Application: Laboratory meters and industrial online meters.

By Technology Level: Economy, smart, and precision meters; or analog (pointer-style) and digital (numeric display) meters.

Pen-type meters are typically single-range instruments with a narrow measurement range, designed for simplicity and specialized use.

Portable and benchtop meters offer a wider measurement range and are commonly used. Portable meters are battery-powered for field use, while laboratory meters provide high precision and advanced features.

Industrial dissolved oxygen meters prioritize stability, reliability, environmental adaptability, and anti-interference capabilities. They often include analog output, digital communication, alarm functions, and control features.