Dissolved oxygen meters are designed to measure the amount gaseous oxygen dissolved in water. Dissolved oxygen (DO) is an important indicator of water quality. Oxygen enters the water by a number of ways such as a byproduct of photosynthesis from aquatic plants, through waves and tumbling water mixing air into the water, and by way of diffusion from the surrounding air. Oxygen dissolves easily into water without chemically reacting with it, meaning it will stay as oxygen, imparting a number of biochemical properties.
The amount of dissolved oxygen that water is capable of holding varies from about 8 to 14 mg/L depending upon the temperature of the water. Since aquatic life, like all life, requires oxygen for survival, high levels of DO contribute to healthy aquatic populations which aids natural stream purification processes including the decomposition of organic matter. Aquatic life is stressed if DO levels drop below 5 mg/L and large fish kills can occur is the levels drop much further. High levels of DO also have implications for municipal water supplies as oxygenated water tastes better with more clarity and less odor. There are drawbacks to high DO levels, however, as they can increase corrosion due to oxidation.
Low dissolved oxygen levels can be due to a few factors. Warm water, for instance, is not capable of holding much DO. Overpopulation of aquatic organisms can also contribute to low DO. Aquatic animals and bacteria can consume great amounts of DO causing levels to drop if populations are too high for the conditions (water temperature and rate of re-oxygenation). Over-fertilization is also a common cause for low dissolved oxygen levels. Run-off from agricultural fields contains high levels of phosphates and nitrates which can result in algal blooms and the proliferation of aquatic plants. Though plants produce oxygen through photosynthesis, they also consume large amounts of oxygen when it’s cloudy or dark and photosynthesis cannot occur. High plant density also leads to high densities of animals and bacteria further contributing to oxygen usage and low DO levels..
Dissolved Oxygen Sensor Technology
Dissolved oxygen meters generally consist of a probe with sensor and the electronics unit that deciphers and displays the signal sent from the sensor. The sensor, of course, is the true heart and soul of DO meters. There are two primary types of dissolved oxygen sensing technologies available: electrochemical sensors and optical sensors.
Electrochemical Sensors
Electrochemical sensors detect ions in solution based upon electrical current or changes in electrical current. They generally consist of an anode and cathode sealed in an electrolyte solution by an oxygen permeable membrane. The electrolyte solution completes the circuit between the cathode and anode. Dissolved oxygen molecules pass through the membrane and react with the cathode resulting in a small electrical signal which passes to the anode and on to the instrument. The signal is proportional to the amount of dissolved oxygen reacting with the cathode.
It is important to note that the oxygen molecules are consumed in the reaction with the cathode. This matters for two reasons. First, it allows us to assume that the oxygen pressure under the membrane is zero. Therefore, the amount of oxygen diffusing through the membrane is proportional to the partial pressure of oxygen outside the membrane. Secondly, since the oxygen is consumed, artificially low readings can occur unless there is flow within the sample which keeps water moving across the membrane.
Dissolved oxygen measurements from electrochemical sensors can be affected by barometric pressure as well as the temperature and salinity of the sample.
There are two types of electrochemical sensors:
Polarographic / Amperometric Sensors: In a Polarographic (also known as amperometric) sensor, the cathode is gold and the anode is silver. A constant voltage of 0.8 volts is supplied to the sensor which completes the circuit and polarizes the anode and cathode.
Most dissolved oxygen meters use polarographic sensors. They are seen as accurate and reliable obtain accurate results. Their readings can be unreliable in complex applications with high levels of hydrogen. Polarographic sensors require a 5-15 minute warm-up time before use or calibration.
Galvanic Sensors: With galvanic sensors, the cathode is silver and the anode another material. often zinc or lead. The electrode materials are dissimilar enough to self-polarize and react with oxygen molecules without an applied voltage, unlike polarographic sensors.
Galvanic sensors do not require warm-up time though their lifespan is shorter than polarographic sensors.
Optical Sensors
Optical dissolved oxygen sensors measure luminescence as it is affected by the presence of oxygen, relying on the well-documented principle that dissolved oxygen dampens both the longevity and intensity of the luminescence associated with carefully-chosen chemical dyes.
Optical sensing elements have two layers. The outer layer is an oxygen permeable membrane, the second layer contains a specially selected dye that luminesces when excited with light of a proper wavelength. The probe emits a blue light of the proper wavelength which causes the dye layer to luminesce or glow red. With no oxygen present, the intensity and lifetime of the luminescence is maximal. As oxygen passes through the permeable membrane, the intensity and lifetime of the luminescence is reduced. The greater the concentration of oxygen the more the luminescence is reduced. The intensity and lifetime of the luminescence is measured by a photodiode in the probe and compared to a reference from which oxygen levels can be calculated.
Unlike electrochemical dissolved oxygen sensors, optical sensors do not consume the oxygen being measured thus eliminating the flow dependence or the need to stir the sample for accurate results.
Biochemical Oxygen Demand
Biochemical oxygen demand or BOD is the amount of dissolved oxygen needed by biological organisms to break down the organic material present in a given water sample at certain temperature over a specific time period. It is also the name of the test used to determine these oxygen requirements. Many dissolved oxygen meters include a BOD test (or probe) which is able to calculate the BOD following an established test protocol. Though not a precise quantitative test, BOD is widely used as an indication of the organic quality of water. The test can gauge of the effectiveness of wastewater treatment plants or determine the oxygen requirements of polluted waterways.
The most common way to determine BOD is a 5-day test in which the test sample is tightly sealed in an airtight bottle with the temperature maintained at 20°C. The sample is also kept out of the light to prevent photosynthesis. Dissolved oxygen measurements are made before and after the 5-day period. The BOD is calculated from the difference between those two measurements and is most commonly expressed as milligrams of oxygen consumed per liter of sample.
Like other DO testing methods, BOD testing requires stirring of the sample to prevent an oxygen depleted layer from forming near the sensor head during testing which would result in artificially low readings. Many BOD probes include an integral stirrer to overcome this.
BOD measurements are similar to Chemical Oxygen Demand (COD) measurements though there are some differences. BOD focuses more on organic matter as it relates to water quality. The time-intensive nature of BOD testing makes it unable to account for rapid changes in conditions.
Chemical Oxygen Demand
Chemical oxygen demand (COD) is a measure of the capacity of water to consume oxygen during the decomposition of organic matter and the oxidation of inorganic chemicals such as ammonia and nitrite. COD is the standard method of indirectly measuring the amount of pollution—things that cannot be oxidized—in a sample of water. COD is routinely used to test wastewater before it’s returned to the environment as well as to sample waters contaminated by domestic or industrial waste.
COD relies on the fact that nearly all organic compounds can be fully oxidized by a strong oxidizing agent in an acid solution. COD measurements generally use samples of wastewater which are incubated for a specific time at a specific temperature (e.g. 150°C for 2 hours) with a strong chemical oxidant such as potassium dichromate in a 50% sulfuric acid solution. The result of a COD test indicates the amount of dissolved oxygen consumes by the contaminants. The higher the COD, expressed as milligrams per liter, the more pollution in the test sample.
COD is closely related to BOD though BOD measures only the amount of oxygen consumed by microbial oxidation thus making it most relevant to waters rich in organic matter. Since COD measurement is much quicker than BOD measurement, COD is more often used for quick and frequent monitoring of treatment plant efficiency and water quality.
Calibration
Periodic calibration is the single best way to ensure accurate dissolved oxygen measurements. As a general rule, the data collected is only as accurate as the calibration performed prior to data collection. Calibration of DO probes consists of exposing the sensor to a sample with a known DO content. The instrument is then adjusted to read that value.
The frequency of calibrations depends a great deal on the types of sensor used. Galvanic and polarographic sensors, for example, should be calibrated every day that they are used in a spot sampling application. Optical sensors, on the other hand, have greater stability making them less susceptible to drift and allowing them to hold their calibration for many months.
Things to Consider When Selecting a Dissolved Oxygen Meter:
- Is the instrument for field or laboratory use?
- Is an environmental rating (IP) needed?
- Are any advanced calculations (BOD for example) desired?
- Is memory or a computer interface needed?
- Are probes built-in or interchangeable? Which probes are needed?
- Is barometric and/or temperature compensation included?
- What is the accuracy level?
If you have any questions regarding dissolved oxygen meters please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com or calling 1-800-884-4967.
