Device and method for monitoring of formation of deposits on a surface

Abstract

The present invention relates to a device for monitoring of formation of deposits on a surface. The present invention also relates to a method for monitoring formation of deposits on a surface using said device. The present invention allows reliable assessment of the formation of deposits such as biofilms in real time, by eliminating deviations in results caused by unexpected turbidity changes and generates reliable output data in this respect.

Description

[0001] Device and method for monitoring of formation of deposits on a surface

[0002] The present invention relates to a device for monitoring of formation of deposits on a surface . The present invention also relates to a method for monitoring formation of deposits on a surface using said device .

[0003] Introduction

[0004] The formation of deposits on the surfaces of aqueous systems or other systems in which a fluid flows or resides is often considered undesirable and therefore requires monitoring so that adequate measures can be taken when the level of deposits exceeds a certain threshold level, so as to avoid costs for mitigation, damage and health danger .

[0005] A particular problem is microfouling by bacterial slime layers (biofilms) in heat exchangers, cooling tower filling, water pipes, swimming pools, greywater systems and (drip) watering systems . In order to determine when action is required to reduce biofilm formation or to remove biofilms it is desirable that their formation is monitored, preferably by means of real time and in-line monitoring .

[0006] It is an objective of the present invention to provide a reliable, compact and affordable device for monitoring deposits on surfaces by which the formation of deposits on surfaces can be monitored in real time .

[0007] Summary of the invention

[0008] This objective is achieved by the present invention, wherein the present invention relates in a first aspect to a device for monitoring formation of deposits on a surface, comprising a passage bounded by a passage wall, said passage configured for passing a fluid through said passage; a first light unit configured to emit light in a first emitted light path (el) through said passage and directed at a first mirror element configured to reflect light from the first lightunit in a first reflective light path (rl) to a first sensor configured to receive light reflected by said first mirror element; a second light unit configured to emit light in a second emitted light path (e2 ) through said passage and directed at a second mirror element configured to reflect light from the second light unit in a second reflective light path (r2) to a second sensor configured to receive light reflected by said second mirror element; wherein the total of the length of said first emitted light path (el) to said first mirror element and the length of the first reflective light path (rl) to said first sensor, (el + rl) , differs from the total of the length of said second emitted light path (e2 ) to said second mirror element and the length of the second reflective light path (r2 ) to said second sensor, (e2 + r2 ) .

[0009] In a second aspect the invention relates to a method for monitoring of formation of deposits on a surface using the device of the first aspect, wherein the method comprises passing a fluid through said passage; allowing the light units to emit light in an emitted light path through said passage and directed at the respective mirror elements which reflect light from the respective light unit in a reflective light path to the respective sensor to detect the light reflected by the respective mirror element; quantifying the reflective light detected on the sensors, and monitoring the quantified detected reflective light for each sensor during a time period; wherein formation of deposits on the surface of the wall of the passage and / or on the mirrors leads to a decrease of detected light and wherein particles of solid matter contained in the fluid in the passage lead to a decrease of detected light; wherein when during monitoring the quantified detected reflective light for each sensor during said time period, the amount of light detected by the first sensor diverges from the amount of light detected by the second sensor, this indicates the presence of particles of solid matter contained in the fluid in the passage .Short description of the drawings

[0010] Fig. l shows an exemplary device according to the invention in a schematic representation .

[0011] Fig. 2 shows an example of a visual output of monitoring deposits on a surface in time by quantifying the amount of light (lux) .

[0012] Detailed description of the invention

[0013] The principle of the device of the present invention is based on optical detection of light reflecting from multiple mirrors placed at a different total distance with respect to their respective light source and sensor so that the total of the length of a first emitted light path (el) to a first mirror element and the length of the first reflective light path (rl) from said first mirror element to said first sensor, (el + rl) , differs from the total of the length of a second emitted light path (e2 ) to a second mirror element and the length of the second reflective light path (r2) from said second mirror element to said second sensor, (e2 + r2 ) . In other words, el + rl differs from e2 + r2 .

[0014] Herein the light path lengths can suitably be expressed in centimeters or millimeters .

[0015] Formation of deposits on the surface of the wall of the passage and / or on the mirrors leads to a gradual decrease of detected light over time because the amount of reflected light detected will gradually decrease because of fouling of the mirror and / or wall .

[0016] However, if present in the fluid, particles of solid matter in the passage also lead to a decrease of detected light . This could make it difficult to interpret whether and to which extent the decrease of detected light is caused by deposit formation or the presence of particles in the fluid, or in other words turbidity.

[0017] The present invention allows to make such a distinction by the requirement that el + rl differs from e2 + r2 . Herein the longer of el + rl and e2 + r2 represents a longer lightpath from light unit to mirror and back to sensor compared to the shorter one of el + rl and e2 + r2 .

[0018] For instance, if el + rl > e2 + r2 and particles are present in the fluid flowing through the passage this means that the light traveling the distance of el + rl is blocked to a larger extent by the presence of the particles in the fluid than the light traveling the distance of e2 + r2 , while on the other hand blocking of light because of deposit formation is not substantially affected by this difference of light path length from light unit to mirror and back to sensor .

[0019] During monitoring the quantified detected reflective light for each sensor during a time period, this leads to a divergence of the amount of light detected by the first sensor from the amount of light detected by the second sensor, or in other words the signal resulting from a longer light path will decrease at a higher rate and to a lower level than the signal resulting from the shorter light path .

[0020] Thus, when in the course of monitoring the amount of light detected by the first and second sensor start to diverge, e . g . by a steeper drop of signal derived from the longer light paths, this is an indication of the presence of particles in the fluid. The presence of particles in the fluid causes cloudiness or haziness of the fluid or in other words, turbidity. The detected data may be corrected for this, for instance using appropriate software, to obtain insight in deposit formation without interfering factors .

[0021] As such, the present invention allows reliable assessment of the formation of deposits such as biofilms in real time, by eliminating deviations in results caused by unexpected turbidity changes and generates reliable output data in this respect .

[0022] The fluid in the context of the invention may be a gas or a liquid, and in a preferred embodiment the fluid is an aqueous liquid.

[0023] In a particularly preferred embodiment, the deposits to be monitored are biofilms . As mentioned above, monitoringbiofilms is important to avoid costs for mitigation of biofilm formation, damage caused by biofilms and health danger resulting from biofilms .

[0024] Fluids are preferably passed through the passage with a flow that resembles the conditions of the system that needs to be monitored for deposit formation, such as piping systems, swimming pools or greywater systems . This is not always practical, for instance if such a system contains stagnant or semi-stagnant water . In any case however, a flow of 0.5 - 2 .0 m / s allows a reasonable assessment of the chances of deposit formation, in particular with regard to biofilm formation .

[0025] One application is formed by upcoming re-use of rainwater captured in large concrete tanks, the so-called grey water systems . Therefore, in a suitable embodiment the device of the invention is installed as a bypass passage of a greywater system.

[0026] Another application is in the area of swimming pools . Minimizing the amount of biofilm formation may help to lower the amount of chlorine and / or other disinfectant agents . Chlorine dosing (electrochlorination by salt or hypochlorite) will form so-called chlorination by-products which are of health concern . Therefore, in a suitable embodiment the device of the invention is installed as a bypass passage of a swimming pool .

[0027] In order to realize a longer light path with an optimal length the first mirror element is preferably positioned on the wall of the passage, and positioned opposite to the first light unit and the first sensor . In this respect, it is also preferred that the first light unit and the first sensor are positioned on the wall of the passage opposite to the first mirror element, suitably in a traverse line perpendicular to the longitudinal direction of the passage . In this respect the first mirror element may suitably be attached to the outer surface of the wall if the wall is of a transparent material, for instance by glueing. It could also be possible to attach the first mirror element to theinside of the wall, especially when the wall is not of a transparent material . With respect to the first light unit and first sensor, one or both may be positioned on the inside or the outside of the wall, preferably on the outside, so as to allow easy maintenance and replacement of the mirror if necessary, without removing the device from its connecting piping .

[0028] In order to realize the shorter light path the second mirror element is preferably positioned within the passage opposite to said second light unit and said second sensor . In a preferred embodiment the second mirror element is positioned substantially in the center of the cross-section of the passage .

[0029] In a preferred embodiment said second mirror element is a two-sided mirror with a first side opposite to and facing in the direction of said second light unit and said second sensor and a second side opposite to and facing in the direction of a third light unit and a third sensor, wherein said second light unit is configured to emit light in said second emitted light path (e2) through said passage and directed at said first side of said second mirror element configured to reflect light from the second light unit in said second reflective light path (r2) to said second sensor configured to receive light reflected by said first side of said second mirror element; and wherein said third light unit is configured to emit light in a third emitted light path (e3) through said passage and directed at said second side of the second mirror element to reflect light from the third light unit in a third reflective light path (r3) to a third sensor configured to receive light reflected by said second side of said second mirror element; and wherein the total of the length of said second emitted light path (e2 ) to the first side of said second mirror element and the length of the second reflective light path (r2) to said second sensor, (e2 +r2 ) , corresponds to the total of the length of said thirds emitted light path (e3) to the second side of said second mirror element and the length of thethird reflective light path (r3) to said second sensor, (e3 + r3 ) .

[0030] In another preferred embodiment the device further comprises a fourth light unit configured to emit light in a fourth emitted light path (e4 ) through said passage and directed at a third mirror element configured to reflect light from the fourth light unit in a fourth reflective light path (r4) to a fourth sensor configured to receive light reflected by said fourth mirror element; wherein the total of the length of said first emitted light path (el) to said first mirror element and the length of the first reflective light path (rl) to said first sensor, (el + rl) , corresponds the total of the length of said fourth emitted light path (e4 ) to said third mirror element and the length of the fourth reflective light path (r4 ) to said fourth sensor, (e4 + r4 ) .

[0031] In accordance with the above preferred embodiments, it is preferred that the method according to the invention comprises allowing the first, second, third and fourth light units to emit light in an emitted light path through said passage and directed at the respective first, second and third mirror elements which reflect light from the respective light unit in a reflective light path to the respective first, second, third or fourth sensor to detect the light reflected by the respective first, second or third mirror element; quantifying the reflective light detected on the sensors, and monitoring the quantified detected reflective light for each sensor during a time period, wherein when during monitoring the quantified detected reflective light for each sensor during said time period, the amount of light detected by the second and third sensor diverges from the amount of light detected by the first and fourth sensor, this indicates the presence of particles of solid matter contained in the fluid in the passage .

[0032] The abovementioned preferred embodiments allows to obtain a double set of data by measuring two longer total light path lengths, el + rl and e4 + r4 , as well as twoshorter total light path lengths e2 + r2 and e3 + r3 . This duplicate assessment increases reliability of monitoring and subsequent data output .

[0033] Although this duplicate assessment suffices for purposes reliability, is it to be understood that triplicate or any further multiplicate tests are also envisaged to be included in the present invention, this would require additional respective, light units, mirror elements and sensors .

[0034] Further, instead of a double sided second mirror, the second mirror element may be one sided and a fourth mirror element, also one sided, may be added to realize the shorter total light path lengths e2 + r2 and e3 + r3 . In this embodiment the method according to the invention comprises allowing the first, second, third and fourth light units to emit light in an emitted light path through said passage and directed at the respective mirror elements which reflect light from the respective light unit in a reflective light path to the respective sensor to detect the light reflected by the respective element; quantifying the reflective light detected on the sensors, and monitoring the quantified detected reflective light for each sensor during a time period, wherein when during monitoring the quantified detected reflective light for each sensor during said time period, the amount of light detected by the second and third sensor diverges from the amount of light detected by the first and fourth sensor, this indicates the presence of particles of solid matter contained in the fluid in the passage .

[0035] The teachings above with regard to the positioning of the first light unit, first sensor and first mirror element apply mutatis mutandis to the fourth light unit, fourth sensor and third mirror element, respectively.

[0036] The teachings above with regard to the positioning of the second light unit, second sensor and second mirror apply mutatis mutandis to the third light unit, third sensor and second (and optional fourth) mirror element, respectively.In a preferred embodiment the passage bounded by said passage wall in the context of the present invention is a tube . Tubes are available in various standard dimensions and may be connected with piping work using standard connection elements .

[0037] Preferably this is a tube of a transparent material . Transparent tubes may be made of glass, acrylic, poly carbonate material or plastic . Other materials may also be suitable . Tube diameters can be chosen and adapted to the project situation and may for instance be 1, 1.5. 2 , 2 .5 inch or larger in diameter . Tube length may suitably be chosen between 15 and 30 inch, such as about 20 inch . A transparent wall allows positioning of mirrors, sensors and light units on the outside of the wall, which allows easy access for maintenance and replacement .

[0038] In order to avoid formation of algae on the walls of the passage, it is preferred that measures are taken to keep the fluid flowing in the passage in the dark, for instance by using a cover, lamination or coating .

[0039] In a preferred embodiment said passage bounded by said passage wall is a tube with a tube wall of a substantially transparent material and the first mirror element is attached to the outer surface of the tube wall, and positioned opposite to said first light unit and said first sensor wherein said tube comprises a cleft extending in longitudinal direction of the tube in which said second mirror element is inserted to be positioned within the passage opposite to said second light unit and said second sensor . In this embodiment, the second mirror element may be housed in a transparent housing extending in the tube interior or it may be inserted in said cleft without housing, thus directly contacting the fluid. In the latter case, the cleft is sealed with an appropriate sealing agent or means after insertion of the second mirror element so as to avoid leakage .

[0040] With regard to the light units, it is preferred that light units are configured to emit light with a wavelengthof 510 - 530 nm, which corresponds to green light . The inventors have found that light of these wavelengths results in light signals that allow maximal distinguishability of detected signal derived from deposit formation and detected signal derived from particles in the fluid.

[0041] The light units may contain one or more light sources . Light sources are preferably realized as LEDs (light emitting diodes) , in particular emitting light with a wavelength including and between 510 and 530 nm. In a suitable embodiment, the light unit comprises two LEDs .

[0042] In order to generate a reliable detected light signal it is preferred that the light emitted is directed to the mirrors in a narrow beam. This can be realized by various ways that are known in the art .

[0043] In a further preferred embodiment, the light units and sensors are housed in clamp elements that also form support elements for the tube, for instance for fixating of the tube to a framework or other carrier element . Preferably also said first and optional fourth mirror elements are provided on the inner surface of said clamp elements . This way a multifunctional clamp is realized that allows easy access to the light units, sensors and optionally mirrors because it can be opened and closed when desired, while at the same time allowing fixation of the tube to a framework.

[0044] In order to allow a user to monitor the formation of deposits the sensors can coupled to a processor, configured to convert the received reflective light signals to an output assessable by a user . Such an output may suitably be a visual output on a screen .

[0045] The detected data are recorded and may be processed in a processor system. The processor system may be connected to WIFI and transmitted data can be gathered in a special designed app on the smart phone . For each application a warning level can be set in co-operation with the operator or user . This enables the input of the operator or user what size and thickness of biofilm is still permissible for the system. The design leaves enough transparency for settingacceptable levels or dangerous levels and can be adjusted to the user' s wishes .

[0046] In order to keep a reliable watch on the formation of deposits measurement detection measurements may take place at regular time intervals, for instance in time intervals of 3 minutes or shorter up to 2 hours or longer . Measures may take place over a continuous period of time or for a predetermined period.

[0047] If an amount of light detected by the sensors decreases to a certain predetermined threshold level, a first warning signal may be generated. This may be a visual or audio signal . This warns the user that action may be necessary. A specific warning may be generated in case of present of particles in the fluid. In other words when the amount of light detected by the second and optional third sensor diverges from the amount of light detected by the second and optional third sensor to a certain predetermined threshold and / or duration of divergence, a second warning signal may be generated.

[0048] Detailed description of the drawings

[0049] The following drawings are meant to illustrate the present invention and not to limit the scope of the claims .

[0050] Fig. l shows an exemplary device according to the invention in a schematic representation in the form of a tube . The two circles below and the circle above the tube indicate cross-sectional views of the tube at the positions of the respective dotted lines .

[0051] In Fig. 1, the device 1 for monitoring of formation of deposits on a surface comprises tube with a transparent wall 11. A fluid is passed through said tube in the direction of the arrow. This fluid may suitably an aqueous liquid. The device comprises a first light unit 21 configured to emit light in a first emitted light path (in direction of the 3 small arrows exiting light unit 21) through the contents of the tube and directed at a first mirror element 41 configured to reflect light from the first light unit 21 in a firstreflective light path to a first sensor 31 configured to receive light reflected by said first mirror element (see small arrow entering the first sensor 31) . The first mirror element 41 is attached to the outer surface of the wall 11, for instance by glueing .

[0052] The device 1 also comprises a second mirror 42 which is a two-sided mirror with a first side 421 opposite to a second light unit 22 and a second sensor 32 , wherein said first side 421 faces in the direction of said second light unit 22 and said second sensor 32 . The two-sided mirror 42 also has a second side 422 opposite to and facing in the direction of a third light unit 23 and a third sensor 33, wherein said second side 422 faces in the direction of said second light unit 23 and said second sensor 33.

[0053] Second mirror 42 is positioned substantially in the center of the cross-section of the tube .

[0054] The second light unit 22 is configured to emit light in said second emitted light path (in direction of the 3 small arrows exiting second light unit 22) through the contents of the tube and directed at said first side 421 of said second mirror element 42 that is configured to reflect light from the second light unit 22 in a second reflective light path to said second sensor 32 which detects the light reflected by said first side of said second mirror element (see small arrow entering the first sensor 31) .

[0055] The third light unit 23 is configured to emit light in said third emitted light path (in direction of the 3 small arrows exiting second light unit 23) through the contents of the tube and directed at said second side 422 of said second mirror element 42 that is configured to reflect light from the third light unit 23 in a third reflective light path to said third sensor 33 which detects the light reflected by said first side of said second mirror element (see small arrow entering the first sensor 31) .

[0056] The exemplary device of Fig. 1 also comprises a fourth light unit 24 configured to emit light in a fourth emitted light path (in direction of the 3 small arrows exiting lightunit 24 ) through the contents of the tube and directed at a third mirror element 43 configured to reflect light from the fourth light unit 24 in a fourth reflective light path to a fourth sensor 34 configured to receive light reflected by said first mirror element (see small arrow entering the fourth sensor 34 ) . Just like the first mirror element 41, also the third mirror element 43 is attached to the outer surface of the wall 11, for instance by glueing . In order to monitor the formation of deposits such as biofilms using the device of Fig. 1 the light units 21, 22, 23, 24 are allowed to emit light in an emitted light path through said passage and directed at the respective mirror elements or sides thereof 41, 42 (421 / 422 ) , 43 which reflect light from the respective light unit 21, 22 , 23, 24 in a reflective light path to the respective sensor 31, 32 , 33, 34 to detect the light reflected by the respective mirror element . In accordance with this method the reflective light is detected by the sensors 31, 32 , 33, 34 , and the quantified detected reflective light is monitored for each sensor during a time period. Herein, the formation of deposits on the surface of the wall of the passage and / or on the mirrors leads to a decrease of detected light; and wherein particles of solid matter contained in the fluid in the passage lead to a decrease of detected light .

[0057] Because the light detected by the first sensor 31 and fourth sensor 34 has traveled over a longer path length than the light detected by the second sensor 32 and third sensor 33, the presence of particles of solid matter contained in the fluid in the tube, such as sudden turbidity, results in a divergence in the light signal detected by the first sensor 31 and fourth sensor 34 on one hand and the second sensor 32 and third sensor 33 on the other hand. In practice this means that the lux signal of the first sensor 31 and fourth sensor 34 decreases at a higher rate than the signal of the detected by the second sensor 32 and third sensor 33, because the light detected by the first sensor 31 and fourth sensor 34 has been hindered by more particles .The exemplary embodiment of Fig. l allows to obtain a double set of data by measuring two longer total light path lengths with sensors 31 and 34 as well as two shorter total light path lengths with sensors 32 and 33. This duplicate assessment increases reliability of monitoring .

[0058] If during monitoring the quantified detected reflective light for each sensor 31, 32, 33, 34 during said time period, the amount of light detected by the second 32 and third sensor 33 diverges from the amount of light detected by the first 31 and fourth sensor 34 , this indicates the presence of particles of solid matter contained in the fluid in the tube, such as sudden turbidity. In other words, when in the course of monitoring the amount of light detected by the first and second sensor start to diverge, e . g . by a steeper drop of signal derived from the longer light paths, this is an indication of the presence of particles in the fluid. The detected data may be corrected for this to obtain insight in deposit formation without interfering factors . As such the present invention allows reliable assessment of the formation of deposits such as biofilms, by eliminating deviations in results caused by unexpected turbidity changes and will generate reliable output data in this respect .

[0059] Fig. 2 shows a possible representation of output using a simple setup wherein a device for monitoring biofilms, comprises a first light unit configured to emit light in a first emitted light path through said passage and directed at a first mirror element configured to reflect light from the first light unit in a first reflective light path (rl) to a first sensor configured to receive light reflected by said first mirror element; and a second light unit configured to emit light in a second emitted light path (e2 ) through said passage and directed at a second mirror element configured to reflect light from the second light unit in a second reflective light path (r2 ) to a second sensor configured to receive light reflected by said second mirror element; and wherein the total of the length of said first emitted light path (el) to said first mirror element and thelength of the first reflective light path (rl) to said first sensor, (el + rl) , differs from the total of the length of said second emitted light path (e2) to said second mirror and the length of the second reflective light path (r2 ) to said second sensor, (e2 + r2) . In the example of Fig. 2 el + rl > e2 + r2 . Such a device is basically configured as shown in Fig . 1, but without the third and fourth sensor, and respective light units and mirror elements or mirror element sides .

[0060] In an ideal situation, where no turbidity is present, only biofilm formation is detected which is shown by the gradual decrease represented by line A.

[0061] Initially, this is also the case for lines B and C, which represent signals detected by the first and second sensor, respectively. However, if a sudden turbidity develops at the position of the first vertical line, the signals detected by the first sensor (line C) show a steeper and more pronounced decrease than the signals detected by the second sensor (line B) . When the turbidity is passed and reduced to zero at the position of the second vertical line, the signals lines B and C align again .

Claims

CLAIMS1. Device for monitoring formation of deposits on a surface, comprisinga passage bounded by a passage wall, said passage configured for passing a fluid through said passage;a first light unit configured to emit light in a first emitted light path (el) through said passage and directed at a first mirror element configured to reflect light from the first light unit in a first reflective light path (rl) to a first sensor configured to receive light reflected by said first mirror element;a second light unit configured to emit light in a second emitted light path (e2) through said passage and directed at a second mirror element configured to reflect light from the second light unit in a second reflective light path (r2 ) to a second sensor configured to receive light reflected by said second mirror element;wherein the total of the length of said first emitted light path (el) to said first mirror element and the length of the first reflective light path (rl) to said first sensor, (el + rl) , differs from the total of the length of said second emitted light path (e2 ) to said second mirror element and the length of the second reflective light path (r2 ) to said second sensor, (e2 + r2 ) .

2. Device according to claim 1, whereinsaid first mirror element is positioned on the wall of the passage, and positioned opposite to said first light unit and said first sensor; andsaid second mirror element is positioned within the passage opposite to said second light unit and said second sensor .

3. Device according to claim 2, wherein said second mirror element is positioned substantially in the center of the cross-section of the passage .

4. Device according to any of the previous claims, whereinsaid passage bounded by said passage wall is a tube .

5. Device according to any of the previous claims, whereinsaid passage bounded by said passage wall is a tube with a tube wall of a substantially transparent material;said first mirror element is attached to the outer surface of the tube wall, and positioned opposite to said first light unit and said first sensor; andwherein said tube comprises a cleft extending in longitudinal direction of the tube in which said second mirror element is inserted to be positioned within the passage opposite to said second light unit and said second sensor .

6. Device according to any of the previous claims, wherein said second mirror element is a two-sided mirror with a first side opposite to and facing in the direction of said second light unit and said second sensor and a second side opposite to and facing in the direction of a third light unit and a third sensor,wherein said second light unit is configured to emit light in said second emitted light path (e2) through said passage and directed at said first side of said second mirror element configured to reflect light from the second light unit in said second reflective light path (r2 ) to said second sensor configured to receive light reflected by said first side of said second mirror element; andwherein said third light unit is configured to emit light in a third emitted light path (e3) through said passage and directed at said second side of the second mirror elementto reflect light from the third light unit in a third reflective light path (r3) to a third sensor configured to receive light reflected by said second side of said second mirror element;and wherein the total of the length of said second emitted light path (e2 ) to the first side of said second mirror element and the length of the second reflective light path (r2) to said second sensor, (e2 +r2) , corresponds to the total of the length of said thirds emitted light path (e3) to the second side of said second mirror element and the length of the third reflective light path (r3) to said second sensor, (e3 + r3) .

7. Device according to any of the previous claims, further comprising a fourth light unit configured to emit light in a fourth emitted light path (e4 ) through said passage and directed at a third mirror element configured to reflect light from the fourth light unit in a fourth reflective light path (r4) to a fourth sensor configured to receive light reflected by said fourth mirror element;wherein the total of the length of said first emitted light path (el) to said first mirror element and the length of the first reflective light path (rl) to said first sensor, (el + rl) , corresponds the total of the length of said fourth emitted light path (e4 ) to said third mirror element and the length of the fourth reflective light path (r4) to said fourth sensor, (e4 + r4 ) .

8. Device according to any of the previous claims, wherein said light units are configured to emit light with a wavelength of 510 - 530 nm.

9. Device according to any of the claims 4 to 8, wherein said respective light units and sensors are housed in clamp elements that also form support elements for the tube, preferably wherein also said first and optional fourthmirror elements are provided on the inner surface of said clamp elements .

10. Device according to any of the previous claims, wherein said sensors are coupled to a processor, configured to convert the received reflective light signals to an output assessable by a user .

11. Method for monitoring formation of deposits on a surface using the device according to any of the previous claims, comprisingpassing a fluid through said passage;allowing the light units to emit light in an emitted light path through said passage and directed at the respective mirror elements which reflect light from the respective light unit in a reflective light path to the respective sensor to detect the light reflected by the respective mirror element;quantifying the reflective light detected on the sensors, and monitoring the quantified detected reflective light for each sensor during a time period;wherein formation of deposits on the surface of the wall of the passage and / or on the mirrors leads to a decrease of detected light; and wherein particles of solid matter contained in the fluid in the passage lead to a decrease of detected light;wherein whenduring monitoring the quantified detected reflective light for each sensor during said time period, the amount of light detected by the second and optional third sensor diverges from the amount of light detected by the first and optional fourth sensor, this indicates the presence of particles of solid matter contained in the fluid in the passage .

12. Method according to claim 11, wherein when the amount of light detected by the sensors decreases to acertain predetermined threshold level, a first warning signal is generated.

13. Method according to claim 11 or 12, wherein when the amount of light detected by the second and optional third sensor diverges from the amount of light detected by the second and optional third sensor to a certain predetermined threshold and / or duration of divergence, a second warning signal is generated.

14. Method according to any of the claims 11 to 13, wherein said fluid is an aqueous liquid and wherein said deposits are biofilms .

15. Method according to any of the claims 11 to 14 , wherein said device is installed as a bypass passage of a swimming pool or a greywater system.-o-o-o-

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