Both portable and benchtop dissolved oxygen meters can compensate for the main parameters that affect DO readings temperature, pressure, salinity, humidity. If you need to report your data, choose a meter that has the capability to log and to offload files onto a flash drive or computer. While samples may be complex, measuring your dissolved oxygen doesn't have to be!
Use this guide to help you with the what, why, and how of dissolved oxygen testing to help get you started. For help in choosing the best option for your dissolved oxygen testing needs, talk to a Hanna specialist today. She is passionate about nature, and how science is connected to the world around us. In this blog we will cover: What is dissolved oxygen? What affects dissolved oxygen?
Temperature Temperature is one of the biggest factors that directly affects dissolved oxygen. Pressure Pressure when talking about dissolved oxygen, refers to atmospheric pressure. Salinity Salinity can also affect the amount of DO in a solution. Humidity Humidity or water vapor can affect DO concentration, and the calibration of certain DO measurement technologies. Iodometry Iodometry, is a form of titration.
Colorimetry Colorimetry is a measurement of color. Electroanalytical Methods Electroanalytical methods are quite simply, dissolved oxygen probes. Galvanic probes are membrane probes, and behave like a battery.
The probes have two parts that produce the battery behavior producing a voltage. In the cap of the electrode there is a very thin membrane. This membrane is important as it is permeable to gasses and allows them through, but it keeps out everything else.
As oxygen passes through the membrane, it dissolves into the buffered electrolyte inside the probe cap. The oxygen is then reacted at a part of the electrode called the cathode, and it is given an electron. The electron is supplied by another part of the electrode called the anode.
This reaction and the exchange of electrons causes a voltage to be generated between the cathode and the anode of the probe. Once the current is formed, the meter the probe is attached to converts the reading into a DO concentration unit. Polarographic Probes work a bit differently than galvanic probes, while still being membrane probes.
Instead of behaving spontaneously like a battery, there is a voltage applied between the cathode and the anode. This supplied voltage works as a catalyst to drive the oxygen reaction. Just like the galvanic probes, the electrode has a membrane that allows oxygen through into the unbuffered electrolyte.
Once the oxygen hits the cathode, it gains an electron. This causes a current and the dissolved oxygen concentration is determined. The time-intensive nature of BOD testing makes it unable to account for rapid changes in conditions. 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 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.
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. 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. 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.
Dissolved Oxygen Meters. Learn more about Dissolved Oxygen Meters. More About Dissolved Oxygen Meters. 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. Electrochemical Sensors Electrochemical sensors detect ions in solution based upon electrical current or changes in electrical current.
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. 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.
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. Calibration Periodic calibration is the single best way to ensure accurate dissolved oxygen measurements.
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? What is the accuracy level? Components: Cathode Anode Electrolyte Membrane The cathode and anode are dissimilar metals different electropotentials. Galvanic DO Sensor. Optical DO Sensor.
Warm-up time. Response time. Power consumption. Replacement Frequency. DO Tip. DO Range. Temperature Range. Accessories Included. DO sensor tip, stainless steel DO sensor protective guard.
For laboratory use. For laboratory and field use. Compatible Meters. This nonlinearity comes from the way oxygen interacts in the polymer matrix of the dye This means that higher concentrations, oxygen solubility in the dye matrix will follow the modified Stern-Volmer equation 24 :. Optical dissolved oxygen sensors tend to be more accurate than their electrochemical counterparts, and are not affected by hydrogen sulfide or other gasses that can permeate an electrochemical DO membrane 7.
They are also capable of accurately measuring dissolved oxygen at very low concentrations 3. Optical DO sensors are ideal for long-term monitoring programs due to their minimal maintenance requirements. They can hold a calibration for several months and exhibit little if any calibration drift 5. These dissolved oxygen sensors also do not require any warm-up time or stirring when taking a measurement 7. Over a long period of time, the dye degrades and the sensing element and membrane will need to be replaced, but this replacement is very infrequent compared to electrochemical sensor membrane replacement.
Luminescence lifetime-measuring sensors are less affected by dye degradation than intensity-measuring sensors, which means that they will maintain their accuracy even with some photodegradation However, optical dissolved oxygen sensors usually require more power and take times longer to acquire a reading than an electrochemical DO sensor 7, These sensors are also heavily dependent on temperature 7.
Luminescence intensity and lifetime are both influenced by ambient temperature 23 , though most sensors will include a thermistor to automatically correct the data Electrochemical dissolved oxygen sensors can also be called amperometric or Clark-type sensors.
There are two types of electrochemical DO sensors: galvanic and polarographic. Polarographic dissolved oxygen sensors can be further broken down into steady-state and rapid-pulsing sensors. Both galvanic and polarographic DO sensors use two polarized electrodes, an anode and a cathode, in an electrolyte solution 7. The electrodes and electrolyte solution are isolated from the sample by a thin, semi-permeable membrane. When taking measurements, dissolved oxygen diffuses across the membrane at a rate proportional to the pressure of oxygen in the water 7.
The dissolved oxygen is then reduced and consumed at the cathode. This reaction produces an electrical current that is directly related to the oxygen concentration 7. This current is carried by the ions in the electrolyte and runs from the cathode to the anode As this current is proportional to the partial pressure of oxygen in the sample 15 , it can be calculated by the following equation:. If measurements are being done in a lab or still water, it is necessary to stir galvanic and polarographic DO sensors in solution.
This measurement method is dependent on flow due to the consumption of the oxygen molecules 7. When the oxygen is consumed, the sensors can produce an artificially low DO reading in no-flow situations 7. Electrochemical dissolved oxygen sensors should be stirred in the sample until the dissolved oxygen readings no longer increase.
A polarographic DO sensor is an electrochemical sensor that consists of a silver anode and a noble metal such as gold, platinum or infrequently, silver cathode in a potassium chloride KCl solution 8.
When the instrument is turned on, it requires a minute warm-up period to polarize the electrodes before calibrating or measuring. The electrodes are polarized by a constant voltage between 0. As electrons travel in the opposite direction of a current, the anode becomes positively polarized and the cathode becomes negatively polarized This polarization occurs as electrons travel from the anode to the cathode via an internal wire circuit When oxygen diffuses across the membrane, the molecules are reduced at the cathode, increasing the electrical signal 7.
The polarizing potential is held constant while the sensor detects changes in the current caused by the dissolved oxygen reduction 7.
The more oxygen that passes the the membrane and is reduced, the greater the electrical current read by the polarographic DO sensor. This is a two-part reaction — the oxidation of the silver anode and the reduction of the dissolved oxygen. In a polarographic dissolved oxygen sensor, the role of the cathode is to accept and pass on electrons from the anode to the oxygen molecules.
This current is proportional to oxygen consumed, and thus to the partial pressure of oxygen in the sample The silver anode is oxidized during this process as it gives up its electrons to the reduction reaction, but the oxidation only occurs when measurements are being taken 7.
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