Method of Measuring the Amount of Oxygen in Liquids and Gases

Technical Field

The present invention relates to improving the measurement quality of a dissolved oxygen probe used in water quality analysis. The dissolved oxygen probe, which is used to determine the oxygen concentration in water, uses luminescent radiation technology which is one of the optical measurement techniques. The measurement principle of the dissolved oxygen probe is based on the quenching of the luminescence as a result of the interaction of a luminescent material (luminophore) coated on the surface of the substrate (4) at the end of the sensor cap (3) of the probe, and the determination of the dissolved oxygen concentration in the water according to the quenching amount. The present invention relates to increasing the measurement quality by coating the substrate (4) placing at the end of the sensor cap (3) of a dissolved oxygen probe and on which the luminophore material is coated, with different coating techniques.

Prior Art

In activated sludge tanks in biological wastewater applications; oxygen concentration is one of the most important parameters and must be measured continuously. Conventional electrochemical measurement techniques are based on polarographic or galvanic measuring cells. The characteristic feature of these measurement techniques are; depletion of electrolytes and degradation of anodes during measurement. In both measurement techniques, there is inevitably a shift in the measurement results. In order to prevent shift, the known electrochemical techniques for dissolved oxygen measurement require regular maintenance by the user. Cleaning, calibration, membrane and electrolyte replacement, cleaning the anode and documenting these activities are necessary and inevitable today. However, with regular calibration and maintenance, it can be ensured that the measuring device works within the desired limit values.

In the state of the art, a dissolved oxygen probe (Low Dissolved Oxygen (LDO)) has been developed for the determination of oxygen concentration in waste water, which does not shift in the measurement time and does not need to be constantly calibrated. The working method of this probe is based on the principle of reduction in physical measurement time due to oxygen concentration as a result of luminescent radiation originating from a luminescent substance (luminophore).

Dissolved oxygen probes used in water quality determination work based on the luminescent radiation principle. According to this principle, the oxygen in the water interacts with a luminescent material coated on the substrate (4) located at the end of the sensor cap (3) of the dissolved oxygen probe. This interaction between oxygen and the luminescent material results in a phenomenon known as luminescent quenching. The amount of luminescent quenching during this event indicates the oxygen concentration in the water. The dissolved oxygen probes available in the state of the art consist of a cylindrical body (1 ), at least one blue and at least one red LED light source inside the body (1 ), at least one photodiode and an electronic signal driving, reading, analysis unit, a sensor cap (3), a substrate (4) at the end of the sensor cap, whose entire surface area is coated with luminescent material. The sensor caps (3) are attached to the probe body (1 ) and are replaceable for each use. The dissolved oxygen probe, which exists in the state of the art, works as follows; the sensor cap (3) is attached to the probe body (1 ) and contacted with the water sample to be analyzed or placed in water. When the dissolved oxygen probe comes into contact with water, the probe directs a blue light source tuned at a wavelength inside the probe onto the luminescent material coated on the substrate (4). Blue light causes the luminescent material to produce luminescent light tuned at a different wavelength. Luminescent quenching affects the time that the luminescent material lumines the light as a result of directing the light to the luminescent material. Therefore, if the signal of the blue light source changes depending on the sine curve, this affects the phase difference between the luminescent quenching excitation light, the blue light, and the luminescent light. The probe uses an optical sensor to measure the phase difference between the stimulating blue light and the luminescent light to evaluate the amount of luminescent quenching. As a result, the probe processes the phase difference to determine the oxygen concentration in the water.

In the optical luminescent dissolved oxygen (LDO) technique, which is in the state of the art, the shift of the oxygen concentration measurement results over time is minimized. The wear or decrease in damping of a luminescent material coated on the surface of the substrate (4) at the end of the sensor cap (3) of the probe affects the light intensity. However, the wear of the luminescent material does not affect the emission time of the red light emitted from the red LED light source in the probe. All optics in the probe are first adjusted to measure the reference red LED light. Thus, incorrect calibration by the user is prevented.

The optical measurement method in the LDO technique has other advantages besides the prevention of incorrect calibration. In the optical measurement method in the LDO technique, there is no need for membrane or electrolyte replacement. Instead, there is a layer coated with oxygen-sensitive luminescent material at the end of the sensor cap (3) of the probe. The sensor cap (3) of the probe can be easily changed by the user at certain times.

Oxygen is not consumed in the LDO technique. Oxygen molecules are needed only in contact with the oxygen sensitive layer. There is no need for a specific flow rate in the probe. This technique is not affected by agglomeration on the electrolyte. Since there will be no agglomeration problem in the LDO measurement principle, the dissolved oxygen does not cause a change in the measurement signal level.

The luminophore material used in the LDO technique is resistant to toxic gases such as hydrogen sulfide (H2S) and many chemicals. Therefore, the probe can be used even in difficult applications. Since in the optical measurement method of LDO technique, only the oxygen molecules in the sample to be measured need to contact the luminophore material at the end of the sensor cap (3) of the probe, a response can be obtained within seconds in this technique. If a slower response time is needed, the appropriate signal structure can be created by changing the electronics and software.

The LDO technique also shows high sensitivity to low oxygen concentration in the sample. The sensor cap (3) of the probe in the LDO technique is much more resistant to mechanical loads when compared to the membrane-covered measuring cell method in the state of the art. Another advantage of the LDO technique is that the sensor cap (3) does not need to be cleaned by the user.

The quenching of the luminescent material varies depending on how long the luminescent material has been in the water. A dry sensor typically has a stable response for up to two hours when first immersed in water. When the luminescent material is saturated with water, the luminescent response changes slowly for a given oxygen concentration. When the luminescent material is completely saturated with water, typically after about three days, the luminescent response stabilizes. A user who replaces the LDO probe in the field with a dry sensor may not get an accurate reading from the probe for up to three days. Once the probe is stabilized, the user still needs to recalibrate the instrument to ensure accuracy of readings. Most users want to accurately measure the oxygen concentration in the water as soon as the probe is turned on. In the American patent document numbered as US2007140921 A1 , a method and apparatus are disclosed for placing an LDO sensor in which the luminescent material is stable.

An American patent document numbered as US2007141695 A1 , describes a method and apparatus for visually detecting when a luminescent dissolved oxygen probe is operating. On some probes, the light source is visible, allowing the user to identify when the probe is operating by viewing the pulsed light. However, sunlight hitting the luminophore or optical sensor can cause errors in the measurement of oxygen concentration in water. Therefore, it is desirable to protect the luminophore and the optical sensor from daylight. This problem can be solved by placing the light source, optical sensor and luminophore in an opaque container. The lightly sealed container protects the light source on the probe from sight, preventing the user from visually detecting the probe while it is operating. Normally the probe must be connected to a computer or other device without a visual means of verifying that the probe is working. A user may not have access to a computer while checking or installing the probe in the field. Even if the user has access to a computer, connecting the probe to a computer to verify probe operation takes more time than a simple visual verification. In the patent document numbered as US2007141695 A1 , a system and method has been developed that will allow the user to visually detect when a luminescent dissolved oxygen probe is operating.

In the US patent document numbered US20080982 A1 , a system is described to help solve the problems that arise in measuring the phase difference between the excitation light and the luminescent light. The dissolved oxygen probe measures the phase difference between the excitation light and the luminescent light to evaluate the amount of luminescent quenching. In other words, the probe processes the phase difference to determine the oxygen concentration in the water. An automatic feedback loop is used to measure the phase difference between the excitation light and the luminescent light. In some cases, the automatic feedback loop provides additional phase difference until the excitation light and luminescent light come into phase. The amount of additional phase difference needs to match the phase difference between the excitation light and the luminescent light. In other cases, the automatic feedback loop provides additional phase difference until the automatic system detects that the exciter and luminescent light are 90 degrees out of phase. The additional phase difference is subtracted from 90 degrees to obtain the phase difference between the excitation light and the luminescent light. The luminescent material must be exposed to excitation light until the automatic system settles, and the automatic settling time can take a few seconds. However, exposure of the luminescent material to the excitation light may degrade the luminescent material. In addition, parts of the probe can cause undesirable phase shift. This undesirable phase difference adds error to the oxygen concentrations determined by the probe. As a solution to these problems, the method described in the patent document US20080982 A1 has been developed.

The invention described in the German patent document numbered as DE102019122096 A1 is not limited to the oxygen sensor operating according to the luminescence quenching principle, but describes a system in which other process variables, especially the concentrations, pH or temperature values of certain analytes such as ions, molecules, gases and other chemical compounds can be measured with some adjustments. Normally there is only one sensor point in a sensor. If a different parameter is to be measured, a different sensor must be used. In order to activate more than one parameter with a single sensor, a multi-parameter sensor that is easy to manufacture and use has been developed with the method described in the patent document DE102019122096 A1.

Some technical elements such as LEDs and luminophore material in the probe age depending on time. Optical sensors, with increasing aging, suffer from water penetration. Damage to optical sensors causes sensor deviations during measurements. On the other hand, separating the membrane in optical sensor devices from the substrate surface on the sensor cap is another disadvantage. This is not a desirable situation, especially in the food and pharmaceutical industry. Gaps or cracks on the membrane accelerate aging. It is therefore very important to detect these defects early. With the invention described in document LIS20210318249 A1 , a system and method has been developed for detecting the replacement time of optochemical sensors or some of their parts.

In the present invention, unlike the techniques described above, a method is described, in which the oxygen measurement sensitivity is increased by coating 10%-90% of the surface of the substrate (4) located at the end of the sensor cap (3) in a dissolved oxygen probe, existing in the state of the art, with a non-fluorescent dye and the remaining surface area with luminophore; and the obtained signal is increased by converting the surface of the substrate (4) into different geometric forms such as concave spherical and/or ellipsoidal and coating it with luminophore material.

Technical Problem that the Invention Aims to Solve

Dissolved oxygen parameter is a very important parameter for surface waters (lakes, rivers, etc.), marine ecosystems, wastewater, drinking water and fish farms. Dissolved oxygen is critical to aquatic life and its deficiency is one of the most important indicators of surface water pollution. The fact that the dissolved oxygen parameter is lower than the required value causes problems for aquatic ecosystems, wastewater treatment plants and fish farms. Therefore, it is necessary to monitor the dissolved oxygen parameter regularly or continuously. Existing dissolved oxygen measurement devices available in the state of the art use the optical fluorescence extinction time method. The measuring devices work with the method of determining the dissolved oxygen amount by taking the differences of the measurement phase created by the blue light source from the reference phase created by the red light source in the device.

In the existing methods in the state of the art; both red light and blue light in the probe are applied as lighting source to the luminophore material layer, which is coated over the entire surface of the substrate (4) at the end of the sensor cap (3) of the dissolved oxygen probe. The amount of light obtained from the lighting sources used is insufficient, the density of the oxygen-sensitive luminophore material on the surface of the light-treated luminophore material coated substrate (4) differs from point to point, and the luminophore material cannot be coated homogeneously on the light-applied substrate (4). Oxygen measurement accuracy errors occur due to the inhomogeneity of the signal obtained from the photodetector in the oxygen sensor and the light coming from the light source on only a small surface of the luminophore material coated substrate (4) surface, and it cannot be avoided that the signal obtained from the photodetector is of small amplitude close to the noise level. For these reasons, dissolved oxygen measurement devices available in the state of the art are insufficient in terms of amplitude and accuracy. The lack of repeatability and the accuracy of dissolved oxygen measurement in dissolved oxygen measurement devices is a problem. The present invention provides a solution to the problem that the dissolved oxygen measurement accuracy cannot be achieved.

In the present invention, a method is described, in which the measurement sensitivity is increased by coating 10%-90% of the surface of the luminophore coated substrate (4) placing at the end of the sensor cap (3) of a cylindrical dissolved oxygen probe with a non-fluorescent dye; and in which the obtained signal is increased by using luminophore material surfaces with concave spherical and/or ellipsoidal geometric form.

Description of the Figures

Figure 1. Schematic view of a cylindrical dissolved oxygen probe

Figure 2. View of the substrate (4) at the end of the sensor cap (3)

Figure 3. Concave spherical (5) and ellipsoidal (6) form of the substrate (4) at the end of the sensor cap (3)

Figure 4. Method of coating two areas of the substrate (4)

Figure 5. Method of coating three areas of the substrate (4)

Description of the References in Figures

1 : Body

2: Luminophore material coated on the substrate (4)

3: Sensor cap 4: Substrate

5: Concave spherical substrate

6: Ellipsoidal substrate

7: Area of the substrate (4) coated with luminophore

8: Area of the substrate (4) coated with a non-fluorescent dye

9: Area of the substrate (4) m-fold coated with the luminophore

10: Area of the substrate (4) n-fold coated with the luminophore

11 : Area of the substrate (4) coated with a non-fluorescent dye

Description of the Invention

In the state of the art, the circular surface coated with a luminophore material located at the end of the sensor cap (3) of a cylindrical dissolved oxygen probe (Figure 1 ) is known as the substrate (4). The substrates (4) can be produced from materials such as plexiglass, borosilicate glass and quartz glass. Luminophore is a material coated on the substrate (4) of the oxygen probes, which determines the oxygen value during dissolved oxygen measurement. The coating of the substrate (4) with luminophore material is carried out by the spray coating method. The luminophore material consists of a platinum or palladium porphyrin and a polymer (polystyrene, PMMA, etc.) matrix.

In the present invention, the substrate (4) has at least one area (7) coated with a luminophore and at least one area (8) that is coated with a non-fluorescent dye. The area of the substrate (4) that is coated with a non-fluorescent dye (8) is at least 10% and/or at most 90% of the total surface of the substrate (4), and coated under a blue light source with a non-fluorescent dye that has approximately the same red color as the luminophore material. The rest of the surface area of the substrate (4), the area coated with luminophore (7), is coated with luminophore material. For example, when 40% of the surface of the substrate (4) is coated with a non-fluorescent dye, 60% of it is coated with luminophore material. The coating thickness of the coated luminophore material and the non-fluorescent dye should be 1 mm or less than 1 mm (<1 mm). The non-fluorescent dye can be any dye that does not show fluorescence. In this way, in the scope of the present invention, when measuring with a dissolved oxygen probe having a substrate which has areas coated with luminophore material and a non- fluorescent dye (Figure 4), the red sinusoidal light source in the probe is applied to the area (8) coated with a non-fluorescent dye. The blue colored sinusoidal light source inside the probe is also applied to the area (7) of the substrate (4) coated with luminophore material. The only difference between both areas (7 and 8) is whether the substrate (4) is coated with a luminophore material or not. This is mathematically because the only reason for the phase difference is the amount of oxygen interacting with the luminophore material.

During the measurement, the offset from the optical path and electronic circuit, systematic errors with “short time” and “long time” times that can change with time and environmental conditions create the reference phase value created by the red light source. The offset from the optical path and electronic circuit, systematic errors that can change with time and environmental conditions, and oxygen interacting with the luminophore create the measurement phase value created by the blue light source. Thus, the error terms are theoretically eliminated. In practice, with the solution method in question in the present invention, it is possible to reduce the accuracy error values on the dissolved oxygen measurement to a negligible level.

In another embodiment of the present invention; the surface of the substrate (4) can be coated in three parts. These parts are coated with m-fold luminophore, n-fold luminophore and non-fluorescent dye. When the substrate (4) is coated in three parts, the ratio of each area coated with different layers (m, n and non-fluorescent dye) to the total surface area is between 10%-80%.

The area (9) of the substrate (4) coated with luminophore in m layers is coated with 10-30 layers (m) of luminophore material. The area (10) of the substrate (4) coated with luminophore in n layers is coated with luminophore material in 2-5 layers (n). As the number of areas to be coated with the luminophore material in different layers increases, the number of blue light sources in the probe increases in direct proportion (1 :1 ratio). Thus, the two areas (9 and 10) of the substrate (4) will have a luminophore material density of a certain “m/n” ratio (Figure 5). The only difference between both surfaces (9 and 10) is the coating thickness difference of the luminophore material in the ratio of “m/n”. The area (11 ) of the substrate (4), which is coated with a non- fluorescent dye, is not coated with a luminophore material, but only with a non- fluorescent dye. Also in this application of the invention, the coating thickness of the coated luminophore material and the non-fluorescent dye should be 1 mm or less than 1 mm (<1 mm).

In this embodiment of the invention, the substrate’s (4); i. One-third area and/or less than one-third area is coated m (m = 10-30) times (9) with luminophore material ii. The other one-third area and/or less than one-third area is coated n (n = 2- 5) times (10) with luminophore material iii. The other one-third and/or less than one-third of the area is coated with a non-fluorescent dye (11 ).

In this application of the invention, since the substrate surface area is coated with luminophore material in two different layers (m and n), there are 2 blue light sources and 1 red light source in the probe. A dissolved oxygen probe with a substrate (4) coated in 3 parts, with different coating layers and with a non-fluorescent dye, as in Figure 5, when contacted with the sample, one of the 2 blue sinusoidal light sources inside the probe is applied to the m-fold luminophore coated area (9), and the other blue sinusoidal light source is applied to the n-fold luminophore coated area (10). A red reference light is applied to the area (11 ) of the substrate (4) that is coated with non-fluorescent dye. Reference phase information is obtained from the area illuminated by red light. As a result, the diffusion rate of oxygen molecules in the luminophore coating changes when blue lights are applied to areas with luminophores at the “m/n” ratio. The measurement method at different diffusion rates with the “m/n” ratio provides an advantage for the measurement of dissolved oxygen concentration in dynamically rapidly changing environments. In gas environments that change the oxygen value faster than 1 second, the area coated with a non-luminophore dye (11 ), which is thinner than the area coated with m-fold luminophore (9) or the area coated with n-fold luminophore (10), allows the detection of dynamic changes by reacting rapidly in 1 -10 seconds. In the thicker layer, although the phase change takes place in 40-90 seconds, it provides more accurate and more stable results. The offset from the optical path and electronic circuit, systematic errors that can change with time and environmental conditions create the reference phase value created by the red light source. The offset from the optical path and electronic circuit, systematic errors that can change with time and environmental conditions, and oxygen interacting with the luminophore create measurement phase values created by blue light sources. In this embodiment of the present invention, the luminophore measurement method at different ratios and the reference phase measurement are used together. While the dissolved oxygen measurement gives stable results in static oxygen changes with m- fold luminophore coated area (thicker than n), it gives fast and accurate results in dynamic oxygen exchanges with n-fold coated area, which is thinner than the m-layer coated area. With this application of the invention, by using a thinner coated (n-layer) luminophore material, it is possible to dynamically and quickly measure inhomogeneous oxygen environments in the flow, as well as accurately measuring high oxygen levels with a thicker coated (m-layer) luminophore material. The three- part coating method in this embodiment of the invention is shown in Figure 5.

In another embodiment of the present invention, the geometric shape of the substrate (4) is changed to form substrates with concave spherical (5) and/or ellipsoidal (6) form. In this application of the present invention, the lights coming from the red and blue light sources in the dissolved oxygen probe are applied to the substrates with concave spherical (5) and/or ellipsoidal (6) geometric form, which are specific to the present invention, instead of the substrate (4) located at the end of the sensor cap (3) in the state of the art. Thus, since the light beams are applied to a wider area, the amplitude of the obtained light increases. Besides, the photodiode in the dissolved oxygen probe is placed at the focal point of the spherical or ellipsoid optical system. Thus, all of the light obtained is collected in the photodiode detector and the amount of light obtained in the photodiode increases. As the added benefit of this solution; since LED light sources can be driven at lower currents, the lifetime of these sources increases. Light sources do not heat up and do not cause temperature problems. Images of different coating substrates are shown in Figure 3. The substrate (4) in Figure 2 refers to the substrate available in the state of the art, the concave spherical substrate (5) and/or the ellipsoidal substrate (6) refer to the substrates in this application of the present invention. A concave spherical substrate (5) and/or an ellipsoidal substrate (6) is created by physically changing the shape of the substrate (4) at the end of the sensor cap (3) in this application of the present invention. The coating methods applied on the substrate (4) shown in Figure 4 and Figure 5 can be applied in the same shape and proportions on the concave spherical (5) and/or ellipsoidal (6) geometric forms of the substrates which are specific to the present invention.

Industrial Applicability of the Invention

In activated sludge tanks in biological wastewater treatment applications; the dissolved oxygen concentration is one of the most important parameters and must be measured continuously.

In wastewater treatment plants, approximately 50% of the energy costs are used in the aeration process of activated sludge. For biological wastewater treatment processes, oxygen transfer is often optimized in the aeration tank in order to reduce energy demand along with control and regulation strategies. Oxygen measurement must be exact and precise in general automation design. Therefore, continuous measurement of the oxygen concentration in the activated sludge is also economically essential.

The optimum amount of dissolved oxygen required for the survival of fishes in fish farms is 5-6 mg/L, and oxygen must be provided regularly to avoid fish losses. For this reason, it is necessary to measure the dissolved oxygen concentration continuously and intervene when necessary. Luminescent dissolved oxygen probes are also used in these farms, allowing fish production to continue in a healthy way. The method of the present invention improves the measurement quality of dissolved oxygen measurements in these areas.