The OxySense DO analyser range of online Dissolved Oxygen Meters utilises the very latest and best optical sensor available in the world today. It is an optical luminescent device which is extremely resistant to abrasion, extremely stable, and has greatly reduced maintenance and whole life costs. Coupled with the automatic self cleaning and self verification, the OxySense online DO analyser is as close to ‘fit and forget’ as it gets.
Pi has developed a special end cap that screws onto the end of the sensor. It is smooth and low profile to reduce the possibility of ragging. An air blast cuts across the flat sensor end helping it clean.
When the sensor is being cleaned the design of the end cap means that the water is excluded from the end cap by the force and volume of air. This cleans the end cap (lumiphore) and exposes the sensor to air. The sensor then reads the DO in air (which is much greater than in water) and that response is measured. If the response is not what is expected an error is flagged for the operator to intervene.
How often and for how long the sensor is cleaned is set on the simple controls on the analyser itself. It is also possible to set a ‘hold’ for the output from the analyser to ensure that the control is not affected by the self clean. This ‘hold’ starts at the beginning of the cleaning cycle and continues for a user defined time after the cleaning cycle.
That depends on the process but three years is typical.
Yes very! You just pull the old one off and push the new one on. No need to enter anything on the analyser.
Yes, a clean pressurised water supply can be used instead of compressed air to clean the sensor, however the self verification function would not work if water is used to clean the sensor.
No, there are no moving parts beneath the water. All moving parts are easily accessible.
Measuring dissolved oxygen in water can be done using a variety of methods. Online these are generally of two types, “electrochemical” and the later type “optical”. There are variations within these two groups but fundamentally the electrochemical sensors measure the rate of an electrochemical reaction that is changed by the presence of oxygen and the optical devices measure the amount of light of a particular wavelength that is given out by an optically sensitive material that is also influenced by the amount of oxygen dissolved in water. The advantage of the optical devices is that they require less maintenance, but they are more expensive.
Dissolved oxygen in water is normallly measured with a “hand-held” device or with a continuous online analyser such as the OxySense from Pi.
Oxygen is introduced to water through plant photosynthesis in the aquatic plants, or though the dissolution of oxygen from the air. The total amount of dissolved water varies but is generally between 6.5 mg/L (ppm) and 8.5 mg/L (ppm). The warmer the water is the less oxygen can be dissolved in the watwr and the more polluted the water is the less dissolved oxygen will be present.
The colder the water the more oxygen can dissolve in water. The more green aquatic plants that are producing oxygen from photosythesis the more dissolved oxygen there will be and the fewer pollutants or organisms that use up the oxygen the more dissolved oxygen there will be. In waste water treatment dissolved oxygen is increased by blowing air through the water to replace the oxygen used up by the micro-organisms in waste water.
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Whatever the process being monitored is, there is often something in the sample water capable of fouling a sensor, and therefore causing erroneous results. The obvious solution to this problem is to clean the sensor, but how regular should inspection and cleaning programs be for each piece of instrumentation? Too regular and the inspection and cleaning regime is time consuming and unnecessarily costly. Not often enough and the instrumentation will give false results and probably fail prematurely.
Pi’s Autoclean and Autoflush systems can give trouble free and fouling free functioning of sensors for weeks, if not months, at a time.
If using air to clean a DO sensor the system can also automatically verify that the sensor is still responding correctly, removing any need to remove the sensor from the sample for months at a time.
The graph shows a three hour snapshot of some of the results that led to NIW deciding that Pi’s DO meter performed the best overall. From looking at this graph it is clear why Pi was preferred: Competitor A’s sensor displays a low, inaccurate reading and Competitor B’s sensor is afflicted by noise. These results are discussed in more details later in this Focus On.
Pi’s sensor was equipped with its Autoclean end cap (see image, right), which actively cleans the sensor with compressed air (or clean water) to remove sludge and debris from the sensor tip on a regular basis.
The trial found that the CRIUS® OxySense from Pi provided the most accurate and reliable DO meter. The sensor was kept clean by Pi’s Autoclean system, and reported accurate and precise readings with the lowest noise. A gradual increase in dissolved oxygen content is easily observed during aeration, followed by a steady decline as the air blowers are turned off.
Competitor A’s sensor struggled to keep itself clean between the regular maintenance events, as the passive self cleaning system was ineffective. The instrument appears to inaccurately report lower readings as a result of sludge and debris coating the sensor. The DO sensor detects the increased oxygen dissolved during aeration, but then falls to a reading of zero for long periods of time.
Readings from Competitor B’s sensor appear to lack precision and contain noise. This is most likely due to fouling of the sensor head (see image, right). The results had large variations between measurements, often making it difficult to distinguish the peaks during aeration. By the end of the six months, Competitor B’s sensor was found to be worn and required a replacement lumiphore end cap.
Early measurement of online dissolved oxygen in waste water was made by electrochemical cells that needed sacrificial electrodes, electrolyte, membranes and a lot of calibration. Whilst there are some companies still advocating the use of this technology it was largely replaced in the 1990s and 2000s with online optical measurement. The vast majority of online DO systems used today are optically based.
If you ask operators how often they maintain a DO meter (clean/calibrate etc.) you often get the answer “every week”. On a large plant that can mean an operator spends half their time doing nothing but maintaining the DO sensors!
| Document | Type | Size |
|---|---|---|
| OxySense | Brochure | 929kB |
| NIW DO Trial | Article | 1.2mB |
| Dissolved Oxygen Meter for Aeration Control | Technical Note | 602kB |
| AirFlush | Technical Note | 594kB |
| Probe Fouling | Technical Note | 459kB |
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