Water and Wastewater Quality Instrumentation
1. How can dissolved oxygen be measured using fluorescence?
A very specific energy wavelength is transmitted to a ruthenium compound immobilized in a sol-gel matrix. The ruthenium will absorb this energy, changing the outer electron's energy level. The electron will then collapse back to its original energy state, emitting the energy as a photon with a different specific wavelength. This is called "fluorescing". If the intensity of the transmitted wavelength is tightly controlled, the amount of fluorescing is both predictable and repeatable.
If oxygen molecules are present the amount of fluorescing is reduced, referred to as "fluorescence quenching". By measuring the amount of quenching it is possible to determine the amount of oxygen present. BACK TO TOP
2. How long has this technology been in use?
Fluorescence technology has been used to measure oxygen uptake in the medical industry for decades. In the early 2000s, the technology was adapted for use in measuring dissolved oxygen in biological reactors. The key issues in making the technology viable in the water treatment industry were the durability of the sensing element and cost. Another important concern was packaging the system in a way that was easy to use and required very little maintenance. The Insite units have accomplished all of these objectives. BACK TO TOP
3. What is the expected life of the sensor?
Seven to ten years. During this time there are no consumables at all. No spare parts, no recharging kits, no replacement films, and no membranes or membrane cartridges. BACK TO TOP
4. How often do the sensors need to be calibrated?
We recommend that the calibration be checked at least once a year. The sensor will drift less than one percent per year. BACK TO TOP
5. Can the sensor be calibrated in the field?
Yes, the sensor can be calibrated in the field. A simple calibration to a reference takes about a minute. BACK TO TOP
6. Are the sensors interchangeable, or do the sensors have to remain with a specific analyzer?
All Insite sensors can be moved at will. The microprocessor in the sensor will "talk" to the microprocessor in the analyzer, identifying itself. The analyzer will then configure itself to operate with that sensor. BACK TO TOP
7. Will exposure to sunlight harm the sensor or shorten the life of the sensor?
No, the Insite sensor is completely unaffected by direct sunlight, unlike some luminescent type sensors that can experience significant degradation with as little as one hour of exposure to direct, or even indirect sunlight. This damage is cumulative so pulling a sensor a couple of times a month for inspection or cleaning, a five minute job, could cause the sensor to fail in as little as six months. BACK TO TOP
8. What is the difference between fluorescence and luminescence?
While the two methods are similar, there is one key difference. Fluorescence, which is the method used by Insite, is the measurement of the immediate reaction of a material in response to an excitation energy source. Luminescence is the measurement of the time it takes the material to recover after the excitation energy source is removed. This method is the one in use by all other manufacturers currently marketing "optical" type DO systems. BACK TO TOP
9. What is the accuracy of the Insite fluorescence DO system?
+/- 0.05 ppm BACK TO TOP
10. What is the minimum flow required for the sensor to properly measure DO?
Unlike Clark type cells with a membrane, the Insite DO sensors do not require any flow. Clark type DO sensors actually consume oxygen to make the measurement so a new supply of oxygen must be continuously provided. The Insite sensor does not consume anything so no flow is needed to obtain a correct reading. BACK TO TOP
11. Does the measuring area of the sensor need to remain moist, even when not in actual use?
No, the sensor can periodically become completely dry with no loss of accuracy, response time, or calibration. No soaking caps are required for storing the sensor. BACK TO TOP
12. If the actual measuring area of the sensor is physically damaged, can this be repaired?
In the very unlikely event that the measuring surface is damaged it can be repaired. The cost of this repair is around $200.00. We recently had a customer put a system (analyzer and sensor) in the back of a golf cart to take to the shop. The sensor did not quite make it into the cart and was left hanging out the back on the ground. The sensor was then dragged the entire trip back to the shop, a couple of thousand feet. The sensor looked like it was put under a grinder but it still worked. This says something about how hard it is to physically damage the sensor. BACK TO TOP
13. Can the Insite DO systems be used in very low oxygen environments, such as anoxic and anaerobic zones?
Yes, the Insite method of measuring DO is very accurate at extremely low ranges. There have been tests in which the unit performed very well in the 0.03ppm to 0.08ppm range. BACK TO TOP
14. What are the main reasons to monitor and control DO in biological reactors?
While there are almost as many specific reasons as there are treatment facilities, they generally fall into one of three general areas.
1. Reduce the amount of power required to run the blowers. This in turn will significantly reduce plant costs. In general, up to 70% of a facility's power consumption is for aeration. An industry association estimates that the average treatment facility could save up to 30% by automatically controlling aeration.
2. Optimize the conditions in the basins to achieve the correct microbiological mix. This is especially important in BNR facilities.
3. Reduce the overall maintenance required to operate the biological reactors. BACK TO TOP
