Oxygen is essential for almost all forms of life. Aquatic animals, plants and most bacteria need it for respiration (getting energy from food), as well as for some chemical reactions. Dissolved Oxygen is the term used for the measurement of the amount of oxygen dissolved in a unit volume of water. Dissolved oxygen is typically expressed in milligrams of oxygen per liter of water (mg/L) or parts per million (ppm). Concentration of dissolved oxygen is also measured in units of % saturation which allows direct comparison between results from samples with different salinity and temperature values.
Typical applications where measurements of dissolved oxygen are required include
- Water treatment plants
- Sewage treatment works
- Effluent activated sludge process
- River monitoring
- Fish farming
- Biotechnical processes e.g. Fermentation
Dissolved oxygen can be measured using a suitable oxygen probe and meter. Although dissolved oxygen is usually displayed as mg/L or ppm, dissolved oxygen probes do not measure the actual amount of oxygen in water, but instead measure partial pressure of oxygen in water. Oxygen pressure is dependent on both salinity and temperature.
For oxygen measurements in air refer to the Respiratory Gas Analysis application page.
For non-invasive arterial oxygen saturation measurements refer to the Oximetry/Pulse Oximetry application page.
There are two fundamental techniques for measuring dissolved oxygen- Polarographic and Galvanic. Both probes use an electrode system consisting of a positive (cathode) and negative (anode) electrode contained within a salt-bridge (cylindrical body filled with electrolyte solution). The end of the electrode is covered with a permeable membrane (usually Teflon or polyethylene) across which the dissolved oxygen molecules diffuse and react with the cathode to produce a current. Polarographic probes require an excitation voltage as the difference in potential between the cathode and anode is less than 0.5 volts. This includes Clark polarographic types. A galvanic probe requires no excitation voltage as the difference in potential between the cathode and anode is greater than 0.5 volts.
Both Clark and galvanic oxygen electrodes depend on the diffusion of oxygen from the sample solution into the electrode. Consequently, their response times are slow (usually several seconds at best) and only slow sampling rates are required within the data acquisition system (4/s or slower). The meter and electrode should be purchased from the same manufacturer so that they will be matched for sensitivity. It should be ensured that the meter has an analog output (± 10 V) that is compatible with the PowerLab unit.
The LabChart Advantage:
(may require additional Modules and Extensions)
- Units Conversion for easy calibration into appropriate units such as ppm, mg/L, µM, etc
- The Multipoint Calibration Extension performs linear and non-linear calibration on transducers, sensors, pH electrodes and other equipment
- Comments can be added in real time, viewed or moved at a later stage
- Fast data extraction, analysis and export (e.g. csv. or txt.) to other applications using Timed Add to Data Pad and Multiple Add to Data Pad
- The Event Manager Extension can be used to monitor an incoming dissolved O2 signal and perform a user defined action such as adding a comment, playing a sound, flashing a light, etc when the O2 signal rises above a set threshold level.
LabChart software (for Windows and Macintosh) together with a PowerLab data acquisition system offers up to 32 channels of real-time data acquisition, data integrity, easy selection of hardware settings, powerful online and offline analysis, procedure automation, seamless extraction of experimental data and flexible display options. Additional acquisition and analysis functionality is provided with the use of specialized LabChart Extensions and LabChart Modules. Modules are available as part of LabChart Pro while Extensions are free for download from the website for existing LabChart users.
The Multipoint Calibration Extension (Win and Mac) extends Units Conversion in LabChart to allow you to perform linear and non-linear corrections on any input using up to 12 standard points. The Extension may be used online, while data is recording, or offline with previously recorded data. The calibration points should span the highest and lowest signals that you expect to encounter in your experiment. Various fitting functions are provided by the Multipoint Calibration Extension. The functions (except Point to Point) are fitted by least squares to your calibration points. Note: Some calibration functions (especially polynomials) can work poorly if subsequently used with data outside the calibration region.
GLP and 21 CFR Part 11
For those researchers working within a laboratory requiring GLP and 21 CFR Part 11 compliance the GLP Client and GLP Server are available for use with LabChart (Windows only) and PowerLab data acquisition systems. For more information, visit the Good Laboratory Practice application page or contact your nearest ADInstruments representative.
PowerLab Data Acquisition Systems
The PowerLab is a high-performance data acquisition unit capable of recording at speeds of up to 400,000 samples per second continuously to disk (aggregate). PowerLab units are compatible with instruments, signal conditioners and transducers supplied by ADInstruments, as well as many other third-party companies. In addition to standard single-ended BNC inputs, 4 differential Pod ports are also available for direct connection of Pod signal conditioners and appropriate transducers. Research PowerLab units include:
- PL3504 PowerLab 4/35 - 4 Channels
- PL3508 PowerLab 8/35 – 8 Channels
- PL3516 PowerLab 16/35 – 16 Channels
Transducers and Accessories
Micro-Oxygen Electrodes (Clarke type)
- MLT1120 Micro-Oxygen Electrode: Features a 3 mm tip which is best suited to measuring oxygen concentration in small volume samples and is designed for applications that require a fast response time with minimal oxygen consumption. This electrode is supplied with a MLT1121 Membrane Housing Kit. An MLT1122 Analog Adapter is required to connect the electrode directly to the PowerLab (supplied separately).
- MLT1123 Micro-oxygen Electrode (Flow-Through): Features low oxygen consumption, fast response (less than 20s) requiring less than a drop of solution. This miniature electrode are ideal for monitoring oxygen in a continuous flow such as water, blood, urine, physiological fluids. "T" fitting size is 1/16" (1.5mm). Each electrode is supplied with an MLT1124 Membrane Housing Kit consisting of six T-membranes and a bottle of electrolyte solution. An MLT1122 Analog Adapter is required to connect the electrode directly to the PowerLab (supplied separately).
Galvanic Oxygen Electrodes
- MLT1115 Galvanic Oxygen Electrode: Best suited to measuring oxygen concentration in aqueous solutions. Connects directly to the BNC input of any PowerLab. The output, in air-saturated deionized water, is usually between 20 - 35mV at 25ºC with oxygen consumption of 3.45 x 10-13 mol O2/s per mV of Signal.
Other oxygen measurement systems (i.e. transcutaneous and fiber optic systems) are available from a number of manufacturers. Analog outputs from these devices may be connected to a PowerLab provided that the signal is ±10V. The signals may then be recorded, displayed and analyzed using LabChart software.
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