Method for monitoring the functioning of a turbidity sensor

Abstract

The invention relates to a method for monitoring the functioning of a turbidity sensor. The turbidity sensor comprises a first pair of a first radiation transmitter 5a and a first radiation receiver 6a and a second pair of a second radiation transmitter 5b and a second radiation receiver 6b, wherein between the first radiation transmitter and the first radiation receiver of the first pair there is a first optical path 7a with a first path length OPL1 for measuring the extinction of a medium located in the first optical path, and between the second radiation transmitter 5b and the second radiation receiver 6b of the second pair there is a second optical path 7b with a second path length OPL2 for measuring the extinction of a medium located in the second optical path, wherein the two path lengths differ such that the second optical path extends over a reflective surface 8. The method comprises the following steps: - in a comparison procedure, determining the extinction A after the first path length OPL1 and after the second path length OPL2 and storing the ratio a of the two values, where a = AOPL2 / AOPL1; - continuously detecting the extinction Ax after the first path length OPL1 and after the second path length OPL2 and determining the ratio ax of the two values at time tx, where ax = AxOPL2 / AxOPL1; - comparing the two ratio values a, ax and generating an error signal if this significantly deviates from a predefined threshold value.

Description

[0001] Method for monitoring the function of a turbidity sensor

[0002] The invention relates to a method for monitoring the function of a turbidity sensor.

[0003] Turbidity sensors are designed to analyze a medium, particularly a liquid, with regard to its optical properties. The primary focus is on determining the proportion of suspended matter. Typical turbidity sensors operate on the principle of light attenuation, in which a radiation transmitter sends light, particularly infrared light, along an optical path through the medium to a radiation receiver. Suspended matter in the medium attenuates the light due to extinction, i.e., particularly scattered light and absorption, so that the radiation intensity received by the radiation receiver is a measure of the turbidity of the medium. There are also turbidity sensors that operate on the principle of light scattering.

[0004] The turbidity sensor housing contains a window located behind the radiation emitter and in front of the radiation receiver. A filter is often also installed. Depending on the type and composition of the medium, deposits can build up over time on the outer sides of these windows facing the medium, which can affect the radiation intensity received by the radiation receiver, i.e., the measurement result.

[0005] This makes it possible to perform a redundant measurement, in which a second radiation transmitter sends light through the medium to a second radiation receiver on a second optical path. To determine the degree of potential influence of a coating on the window(s) on the measurement result, the lengths of the two optical paths differ.

[0006] DE 102016 123650 A1 discloses such a design. However, the disadvantage is that this design significantly alters the outer contour of the turbidity sensor housing. The stepped contour promotes additional deposit formation and is unsuitable for hygienic applications due to the associated limited cleanability. A turbidity sensor with two pairs of radiation emitters and receivers is also known from DE 10 2009 027 929 A1, although this design, with a total of four windows, is comparatively complex.

[0007] The object of the invention is to provide a turbidity sensor for reliable detection of deposits on the outside of windows, which can be implemented in a simple manner.

[0008] The object is achieved according to the invention by a method having the features of claim 1. Advantageous embodiments of the invention are specified in the subclaims.

[0009] The starting point of the invention is the idea that the second optical path OPL2 runs between the second radiation transmitter and the second radiation receiver via a reflection surface which is preferably arranged in a plane parallel to the first optical path OPL1.

[0010] According to the invention, the method is characterized by the following steps:

[0011] - in a calibration procedure, the extinction A is determined after the first path length OPL1 and after the second path length OPL2 and the ratio a of both values ​​is stored, where a = AOPL2 / AOPLI;

[0012] - continuous recording of extinction / absorption A x after the first path length OPL1 and after the second path length OPL2 and determining the ratio a x of the two values ​​at time t x , where a x = AXOPL2 / AXOPL1 ;

[0013] - Comparison of both ratio values ​​a, a xand generating an error signal in the event of a significant deviation from a predetermined threshold.

[0014] Both optical paths OPL1 and OPL2 pass through the same arrangement of filter and optical window. They differ in the distance traveled through the medium and in the reflection at the reflecting surface. Preferably, the first path OPL1 and the second path OPL2 form an angle α. The relationship between the two path lengths can be approximated mathematically as:

[0015] OPL1 = OPL2 * k « OPL2* 1 / cos(a). In an advantageous refinement, the reflection surface is made of polished stainless steel to achieve particularly good mirror properties.

[0016] By measuring absorption at two different path lengths, it is possible to distinguish between absorption due to turbidity and absorption due to coating. The absorption is made up of the absorption in the device Acerat, the absorption due to coating Aßeiag and the absorption due to turbidity A ibung. The last two are unknown Aunknown = An-übung + Aßeiag. Acerat is constant and known, the value of Aßeiag is the same for both path lengths, the value of Amibung scales with the quotient of the path length difference. If the quotient of Aunknown for both path lengths deviates from the quotient of the path lengths, coating must be present. The coating can be present either on the windows or on the reflecting surface. If coating only forms on the reflecting surface, assuming constant turbidity, the received signal on the second path decreases, while the signal on the first path remains the same.A change in turbidity could be eliminated by calculating a quotient.

[0017] A further advantageous development provides that the first radiation receiver and the second radiation receiver are identical, which reduces the number of required components.

[0018] The invention makes it possible to realize different path lengths without changing the outer contour of the sensor housing, in particular without corners, edges, steps or the like, so that the turbidity sensor can also be used in hygiene applications without any problems.

[0019] By means of the method according to the invention, a cleaning process is only to be carried out if the sensor gives a corresponding indication.

[0020] The invention is explained in more detail below using exemplary embodiments with reference to the drawings.

[0021] They show schematically:

[0022] Figure 1 shows a turbidity sensor according to the invention;

[0023] Figure 2 shows a sectional view of a sensor tip of the turbidity sensor according to the invention. In the following description of the preferred embodiments, the same reference numerals denote the same or comparable components.

[0024] Figure 1 shows a turbidity sensor 1 according to the invention in a side view from the outside. The sensor 1 comprises a housing 2. Part of the housing 2 is a process connection 4 in the form of an external thread, with which the sensor 1 is connected to a container containing the medium to be measured, i.e. a pipeline, a tank or the like. This connection is usually made by means of a flange formed on the container or a corresponding adapter. The housing or sensor tip 1a projects accordingly into the container and thus into the medium. The sensor tip 1a has a slit-like recess in the region of two housing sections 2a, 2b, which represents the actual measuring environment.The medium to be measured is then located in this recess and thus in the optical path 7a between a first radiation transmitter 5a located in the first housing section 2a and a second radiation receiver 6a located in the second housing section 2b, as well as in the optical path 7b between a second radiation transmitter 5b located in the first housing section 2a and a second radiation receiver 6b located in the second housing section 2b. The essential difference between the two optical paths 7a, 7b is that they have a different path length, which results from the fact that the second optical path 7b runs over a reflection surface 8.

[0025] The turbidity sensor is shown here as a so-called transmitter device, which has no display or operating unit and merely outputs an analog voltage or current signal corresponding to the measurement result via a plug connection 3, which is made available to a higher-level control unit, e.g. a PLC, for further processing and evaluation.

[0026] Figure 2 shows a sectional view of the sensor tip 1a of the turbidity sensor 1 according to the invention. A window region 9 is arranged in each of the housing sections 2a, 2b, in the radiation direction in the area behind the two radiation transmitters 5a, 5b, and in the radiation direction in the area in front of the two radiation receivers 6a, 6b, through which the two optical paths 7a, 7b pass. The radiation intensity received by the first radiation receiver 6a can then be evaluated as a measure of the turbidity of the medium.

[0027] By measuring absorption at two different path lengths 7a, 7b, a distinction can be made between absorption due to turbidity and absorption due to deposits. The absorption is composed of the absorption in the device Acerat, the absorption due to deposits Aßeiag, and the absorption due to turbidity A ibung. The last two are unknown (Aunknown = An-übung + Aßeiag). Acerat is constant and known; the value of Aßeiag is the same for both path lengths 7a, 7b, and the value of Amibung is scaled by the quotient of the path length difference. If the quotient of Aunknown for both path lengths differs from the quotient of the path lengths, deposits must be present. The deposit can be present either on the windows 9 or on the reflection surface 8.

[0028] In a specific embodiment, the optical path length OPL2 of the second path 7b differs from the optical path length OPL1 of the first path 7a in the medium by a factor of a = 1.4. For example, if the sensor measures an AOPLI value of 1 AU via the first path 7a, the measured value via the second path 7b will be AOPL2 of 1.4 AU if no deposits are present. In this case, the quotient of the measured values ​​of the two path lengths is AOPL2 / AOPLI = 1.4 and corresponds to the factor a. This value is preferably stored at the factory when the sensor 1 is delivered, advantageously in the sensor 1 itself.

[0029] If a coating is present, the measured value for both path lengths 7a, 7b increases by a value Aßeiag. Assume that this value is Aßeiag = 0.1. In this case, AOPLI = 1.1 and AOPL2 = 1.5. In the presence of a coating, the quotient AOPL2 / AOPLI = 1.36 deviates from the factor a = 1.4. Depending on the application, a threshold value can then be defined. If exceeded, sensor 1 emits an electrical and / or optical error signal, for example, to alert the user to the presence of a coating and the associated potential influence on the measurement results. List of reference symbols

[0030] 1 turbidity sensor

[0031] 1a sensor tip

[0032] 2 housings

[0033] 2a Housing section

[0034] 2b Housing section

[0035] 3 plug connection

[0036] 4 Process connection

[0037] 5a first radiation transmitter

[0038] 5b second radiation transmitter

[0039] 6a first radiation receiver

[0040] 6b second radiation receiver

[0041] 7a first optical path

[0042] 7b second optical path

[0043] 8 Reflection surface

[0044] 9 Window area

[0045] OPLi first path length

[0046] OPL2 second path length

Claims

Patent claims 1 . A method for monitoring the function of a turbidity sensor, which comprises a first pair of a first radiation transmitter (5a) and a first radiation receiver (6a) as well as a second pair of a second radiation transmitter (5b) and a second radiation receiver (6b), wherein an optical path (7a, 7b) with a first and second path length (OPL1, OPL2) for measuring the extinction of a medium located in the optical path (7a, 7b) is formed between each of the two pairs, wherein the two path lengths (OPL1, OPL2) differ from one another in that the optical path (7b) of the second pair runs over a reflection surface (8), characterized by the following method steps: - in a calibration procedure, the extinction A is determined after the first path length (OPL1) and after the second path length (OPL2) and the ratio a of both values ​​is stored, where a = AOPL2 / AOPLI ; - continuous recording of extinction A x after the first path length (OPL1) and after the second path length (OPL2) and determining the ratio a x of the two values ​​at time t x , where a x = A X OPL2 / A X OPLI ; - Comparison of both ratio values ​​a, a x and generating an error signal in the event of a significant deviation from a predetermined threshold.

2. Method according to claim 1, characterized in that the two optical paths (7a, 7b) enclose an angle a.

3. Method according to claim 1 or 2, characterized in that the reflection surface (8) is arranged in a plane parallel to the first optical path (7a) between the second radiation transmitter (5b) and the second radiation receiver (6b).

4. Method according to one of the preceding claims, characterized in that the reflection surface (8) is designed as a polished stainless steel surface.

5. Method according to one of the preceding claims, characterized in that the first radiation receiver (6a) and the second radiation receiver (6b) are identical.

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