The OxySense range of online DO meters utilises the very latest and best optical DO 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.
The OxySense Optical Dissolved Oxygen Meter with self-cleaning and self-verification offers enhanced reliability and unrivalled low maintenance. Optical DO meters have largely replaced electrochemical DO sensors (Clark Cells) due to their better reliability and lower maintenance requirements.
The OxySense DO sensor is available with different controllers giving you the same great performance with different communication, display, and control options. With the OxySense range of online DO meters, you get everything that you need – and nothing that you don’t.
The sensing element (lumiphore) is activated, or excited when illuminated with a blue light. When activated, the lumiphore then emits blue light in an intensity that is inversely proportional to the amount of oxygen present in the water. There is also a time delay between the peak emission of blue light and peak response of fluoresced red light. The amount of delay is inversely proportional to the amount of oxygen present. This time delay can be expressed as a phase shift between the wave patterns of incident blue light and the fluoresced red light. This is in turn reported by the electronics into a ppm or mg/l reading of Dissolved Oxygen. The advantages of this technology are that it is more stable than traditional electrochemical devices and far more resistant to abrasion. By using the state of the art sensor and electronics together the reliability, accuracy, and flexibility of the OxySense Meter is far superior to that of its competitors.
- Rugged optical design DO meter
- Better performance than its competitors’ dissolved oxygen sensors
- Self cleaning option – eliminates the need for maintenance
- Self verification option – automatic sensor check
- Range of process control PID options – reduces labour costs
- Proven reliability
- Up to 36 months between calibration and maintenance events
- Data-logging and remote internet access with text alarms
The OxySense DO meters are used in Aeration Lanes and Oxidation Ditches to control the air blowers.
What is probably the worlds most reliable Dissolved Oxygen sensor from In-Situ® the RDO® sensor coupled with Pi’s award winning electronics provides up to 36 months between maintenance and calibration events and results in probably the best online DO meter available in the world today.
The self-verification option (pat pending) is revolutionising calibration routines in that this dissolved Oxygen monitor is able to check it’s own functionality on a regular basis and call for calibration when necessary.
- No chemicals or moving parts
- Optional self-cleaning sensor and self-verification
- Stable and reliable – excellent process control
- Suitable for all Dissolved Oxygen applications
- Up to 36 months between maintenance
- No calibration
The online measurement of Dissolved Oxygen in an aeration lane and an oxidation ditch is critical to the efficient running of the plant. With the electrical costs of aerating the waste water with boilers etc. often making up 50% of the total running costs of the plant, any drift or often inaccuracy of the DO meters can result in large sums wasted.
This reliance on online DO meters to control the boilers etc. means that they must be reliable, robust and free of drift; in an ideal world they would be free from maintenance, an ongoing, tedious and expensive part of many Dissolved Oxygen Monitors.
The general industry move away from amperometric Clark cells towards optical sensors has made a large improvement to monitoring DO in aeration lanes, but not all optical DO meters are the same. Many, even the industry leaders, drift over time and are subject to high levels of maintenance.
Pi has partnered with In-Situ, probably the manufacturer of the world’s most reliable and robust optical DO meter and have created an instrument designed to give them as close to a zero maintenance instrument as possible.
- Lumiphore life of 3 years plus
- Inconsiderably low drift
- Auto clean – water or air
- Alarms for dirty sensor, lack of response, sensor failure etc.
- No initial calibration required
In a trial in the UK, an instrument was installed on an aeration lane and required zero maintenance (including calibration) of 12 months whilst maintaining its accuracy.
The OxySense DO Meter is the first of its kind in the world to offer automatic in-situ sensor verification as an option. The OxySense DO Meter is able to reduce maintenance by automatically checking it’s sensor operation at user defined time intervals. Calibration on the In-Situ® sensor is normally required only once per annum so with the automatic sensor verification option and the self clean option the sensor may not need to be inspected at all for a full year.
It is probably the most reliable and cost effective DO sensor available as it;
- is extremely resistant making it last longer in the harsh environment of an aeration lane.
- can self clean and self check its response every time it cleans.
How does the self clean work?
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.
How does the self check work?
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 does the control work on the autoclean?
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.
How often do I need to change the lumiphore?
That depends on the process but three years is typical.
Is it easy to change the lumiphore?
Yes very! You just pull the old one off and push the new one on. No need to enter anything on the analyser.
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You probably know that online dissolved oxygen monitors are crucial for controlling the oxygen available to the microbes in an aeration lane or oxidation ditch but…
… did you know that typically up to 50% of the cost of running a waste water treatment plant with aeration is the cost of the electricity to run the blowers?
… did you know that often the cost of purchasing many online DO meters is less than 10% of its 10 year total cost of ownership?
… did you know that an online DO meter that is reading 0.1 ppm low could cost more than its purchase and installation costs in increased electricity costs in just a few months?
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. For a description of how they work click here.
Why is controlling the blowers so important?
Simply put, the microbes need enough oxygen to do the job. More oxygen than this and we are wasting energy and promoting the growth of unwanted biologies in the process. Many plants are estimated to waste significant energy by over aerating their process.
What do you need from an online DO meter?
Reliability, Dependability and Reproducibility are the three most important things that a plant needs if it is to optimise its treatment (i.e. control the DO in the aeration lanes to a precise and accurate level).
Typically an aeration basin will have ‘lanes’ with more then one DO sensor. If these DO meters are reading very different levels and do not track each other it is very difficult for a plant to know how to implement a control strategy. DO meters fail low, an unfortunate consequence of the technology. A reliable, dependable and reproducible DO meter will be consistent and will check it’s own responsiveness so an operator doesn’t need to worry.
The Process Instruments In-Situ® sensor has what is probably the most robust lumiphore (sensing material) of any available DO meter. The lumiphore is the disc of material that is used to sense the amount of oxygen present and it needs to be tough to withstand the high energy, aggressive environment that they work in.
Below is a graph of how long the In-Situ® lumiphore works during a SANDBLASTING experiment against two other leading suppliers of online dissolved oxygen monitors. The results are staggering. The Pi/In-Situ® sensor lasts more then 20 times longer! You can’t scratch our lumiphore off with a thumbnail!
Dependability means Clean
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!
The Pi online DO analyser has an optional cleaning system that can clean the lumiphore and keep it clean potentially for the entire life of the sensor. Utilising compressed air from site or a small robust compressor available from Pi, the autoclean keeps the sensor clean and that’s not all…
Dependability means Self-Checking
If you ask the operator why they clean/calibrate the sensor weekly you will often get the answer “so we know it’s working”.
In modern plants the DO levels are kept low to reduce the energy required for the blowers. This means that the DO sensors can often read close to 0 mg/l. How do we know that the sensors are reading correctly and aren’t faulty? The traditional solution is for an operator to pull the sensor out and watch it respond to air, and when it’s out the operator may as well clean it and calibrate it.
What if there was a way to check the response to air without taking it out of the aeration lane?
Well there is! The Pi autoclean system not only automatically cleans the system but whilst doing so, also checks the response to air. Any issues are reported to the operator who no longer has to maintain the sensors every week. Click here for a video.
How long can they last without any operator interference?
On a recent trial in a UK Utility the Pi OxySense was installed for 13 months alongside their existing DO meter and during that time the sensor was removed once for manual cleaning when the power to the autoclean system was inadvertently off for a week. That was the only time the sensor was removed from its position in the aeration lane and throughout the entire trial the instrument was reliable, dependable, and consistently accurate.
Northern Ireland Water (NIW) recently trialled three dissolved oxygen analysers (DO meters) in parallel. These were installed at Culmor Waste Water Treatment Plant side by side in the same aeration lane, where measurements were recorded every minute for 6 months.
After processing more than a million points of data, the OxySense from Process Instruments (Pi) was shown by NIW to be the best solution.
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.
The trial plant had used Competitor B’s DO meters for some time in all their aeration lanes, however they found that the sensing end cap required replacing quite frequently (every 6 months). The replacement costs of £90 ($120, €105) per sensor started to accumulate and the plant decided to seek an alternative.
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.
“When the probes were removed from the lane it was obvious that Pi’s cleaning system worked well and would massively reduce maintenance time and cost.” Peter Martin, M&E Manager, NIW.
Competitor A’s sensor was attached to a flexible mounting which allowed the sensor to move with the sludge and aeration bubbles, theoretically cleaning the sensor passively.
Competitor B’s sensor did not have any automatic cleaning functionality.
All three sensors periodically received a manual clean as part of the plant’s regular maintenance schedule, ensuring the test was fair for the sensors without a cleaning system.
Measurements from the 25th of May 2015 have been used to demonstrate the variation between the different analysers. This date set was chosen to be representative of the whole trial.
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, left). 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.
“It was clear that Pi’s sensor was more robust than the (Competitor B) sensor. After 6 months, I fully expect it to complete its minimum expected lifetime of 2 years.” Peter Martin, M&E Manager, NIW.