Measurement apparatus for measuring concentration of a substance in a fluid

Abstract

A measurement apparatus including a cavity and measurement gap (113) arranged in the cavity (110), wherein the measurement gap (113) and the cavity (110) are designed for a fluid to flow through, wherein width of the measurement gap (113) is less than 1 mm; an optical source (101) configured to provide an optical signal to the measurement gap; a detector (101) configured to detect an optical signal that has passed through the measurement gap (113); and a control device (103) configured to determine concentration of at least one substance in the fluid based on the optical signal detected by the detector (102). The cavity may comprise one or more recesses for optical fibers, the recesses protruding into the cavity.

Description

[0001] MEASUREMENT APPARATUS FOR MEASURING CONCENTRATION OF A

[0002] SUBSTANCE IN A FLUID

[0003] TECHNICAL FIELD

[0004] The present disclosure generally relates to measurement of concentration of a substance in a fluid. The disclosure relates particularly, though not exclusively, to a measurement apparatus and method suited for accurate industrial scale fluid monitoring such as for example monitoring fluids of a pickling process.

[0005] BACKGROUND

[0006] This section illustrates useful background information without admission of any technique described herein representative of the state of the art.

[0007] Pickling is a metal surface treatment method used to remove metal oxides and / or impurities from metal pieces. The pickling process is performed by immersing the metals into baths of acids or alkalis. It is to be noted that pickling can also be performed by spraying the acids or alkali on the surface of the metals to be pickled. The bath of acids or alkalis is called a pickling liquor and the composition of the pickling liquor changes during the pickling process. The left over pickling liquor is called a picking sludge. The pickling sludge can be disposed of, according to local regulation, or be recycled or reused for beneficial reuse, where regulation permits this.

[0008] In steelmaking processes, the pickling liquor baths remove iron oxides and impurities from the steel surface. The concentration of acids in the pickling liquor baths decreases during the pickling process and ferrous salts are generated (e.g. Fe2+) and consequently the process may slow down.

[0009] To ensure pickling process quality, there is a need to monitor concentrations of the pickling liquor baths and to add fresh acid when needed. Adding too much acid is to be avoided, though, to avoid wasting the acid and to avoid excess amount of acid in the picking sludge, which is hazardous waste requiring appropriate treatment.

[0010] Conventionally, the pickling liquor has been monitored by manual titration and lab measurements. Manual titration may take a lot of time and therefore there is a need for alternative measurements. Further, manual titration and manual control of the pickling process may clearly cause human errors in timing of the measurements and resulting adjustments of the process may not be optimal.

[0011] Now, there are provided some new considerations for measurement of concentrations suited for industrial scale fluid monitoring such as for example monitoring fluids of a pickling process.

[0012] SUMMARY

[0013] The appended claims define the scope of protection. Any examples and technical descriptions of apparatuses, products and / or methods in the description and / or drawings not covered by the claims are presented not as embodiments of the invention but as background art or examples useful for understanding the invention.

[0014] According to a first example aspect there is provided a measurement apparatus, comprising: a cavity and a measurement gap arranged in the cavity, wherein the measurement gap and the cavity are designed for a fluid to flow through, wherein width of the measurement gap is less than 1 mm; an optical source configured to provide an optical signal to the measurement gap; and a detector configured to detect an optical signal that has passed through the measurement gap; and a control device configured to determine concentration of at least one substance in the fluid based on the optical signal detected by the detector.

[0015] In some example embodiments of the first aspect, the optical signal is configured to interact with at least one substance in the fluid flowing through the measurement gap.

[0016] In some example embodiments of the first aspect, the measurement apparatus further comprises a first optical fiber configured to convey the optical signal from the optical source to the measurement gap, and a second optical fiber configured to convey the optical signal from the measurement gap to the detector. The first optical fiber and the second optical fiber may be substantially aligned. In some example embodiments of the first aspect, at least one of the first and second optical fibers protrudes into the cavity. In an embodiment, the structure of the walls of the cavity may be such that the optical fiber(s) pass through the walls of the cavity so that the optical fiber(s) are in contact with the fluid, when there is fluid inside the cavity. In another embodiment, the structure of the walls of the cavity may be such that the walls comprise one or more recesses for the optical fibers, the recesses protruding into the cavity. In such embodiment, the optical fibers are not in direct contact with the fluid, when there is fluid inside the cavity.

[0017] In some example embodiments of the first aspect, the width of the measurement gap is equal to or larger than 10 pm.

[0018] In some example embodiments of the first aspect, the width of the measurement gap is 10 pm - 500 pm.

[0019] In some example embodiments of the first aspect, the measurement apparatus comprises two or more of said measurement gaps.

[0020] In some example embodiments of the first aspect, the cavity comprises walls made of transparent material configured to allow the optical signal to pass through. The transparent material may be for example glass.

[0021] In some example embodiments of the first aspect, the control device is configured to control one or more valves to alternate between a measurement cycle and a rinsing cycle, wherein a monitored fluid is allowed to enter the cavity through the one or more valves during the measurement cycle and a cleaning fluid is allowed to enter the cavity through the one or more valves during the rinsing cycle. The valves may be part of the measurement apparatus or the control device may control external valves.

[0022] In some example embodiments of the first aspect, the optical source provides optical signal on a wavelength range from ultraviolet wavelengths to infrared wavelengths or near infrared wavelengths.

[0023] In some example embodiments of the first aspect, the optical source comprises one or more light-emitting diodes (LEDs).

[0024] In some example embodiments of the first aspect, the determined concentration of the at least one substance is determined based on known samples.

[0025] In some example embodiments of the first aspect, the measurement apparatus further comprises one or more sensors configured to measure one or more operating context parameters of the fluid flowing through the cavity.

[0026] In some example embodiments of the first aspect, the control device is further configured to take one or more operating context parameters of the fluid into account in the determination of concentration of the at least one substance.

[0027] In some example embodiments of the first aspect, the one or more operating context parameters of the fluid are taken into account by a correction based on known samples and known operating context parameters thereof.

[0028] According to a second example aspect there is provided a measurement apparatus, comprising: a cavity and a measurement gap arranged in the cavity, wherein the measurement gap and the cavity are designed for a fluid to flow through; a first optical fiber configured to convey an optical signal from an optical source to the measurement gap in the cavity and a second optical fiber configured to convey an optical signal from the measurement gap in the cavity to a detector for an optical measurement of the fluid, wherein at least one of the first and second optical fibers protrudes into the cavity.

[0029] In some example embodiments of the first aspect, the optical signal is configured to interact with at least one substance in the fluid flowing through the measurement gap.

[0030] In an embodiment, the structure of the walls of the cavity may be such that the optical fiber(s) pass through the walls of the cavity so that the optical fiber(s) are in contact with the fluid, when there is fluid inside the cavity. In another embodiment, the structure of the walls of the cavity may be such that the walls comprise one or more recesses for the optical fibers, the recesses protruding into the cavity. In such embodiment, the optical fibers are not in direct contact with the fluid, when there is fluid inside the cavity. In some example embodiments of the second aspect, the cavity comprises walls made of transparent material configured to allow the optical signal to pass through, and the walls comprise one or more recesses for the optical fibers, the recesses protruding into the cavity. The transparent material may be for example glass.

[0031] In some example embodiments of the second aspect, the apparatus further comprises a control device configured to determine concentration of at least one substance in the fluid based on the optical signal detected by the detector.

[0032] In some example embodiments of the second aspect, the width of the measurement gap is less than 1 mm. Further, the width of the measurement gap may be equal to or larger than 10 pm.

[0033] In some example embodiments of the second aspect, the width of the measurement gap is 10 pm - 500 pm.

[0034] In some example embodiments of the second aspect, the measurement apparatus comprises two or more of said measurement gaps.

[0035] In some example embodiments of the second aspect, the control device is configured to control one or more valves to alternate between a measurement cycle and a rinsing cycle, wherein a monitored fluid is allowed to enter the cavity through the one or more valves during the measurement cycle and a cleaning fluid is allowed to enter the cavity through the one or more valves during the rinsing cycle. The valves may be part of the measurement apparatus or the control device may control external valves.

[0036] In some example embodiments of the second aspect, the optical source provides optical signal on a wavelength range from ultraviolet wavelengths to infrared wavelengths or near infrared wavelengths.

[0037] In some example embodiments of the second aspect, the optical source comprises one or more light-emitting diodes (LEDs).

[0038] In some example embodiments of the second aspect, the determined concentration of the at least one substance is determined based on known samples.

[0039] In some example embodiments of the second aspect, the measurement apparatus further comprises one or more sensors configured to measure one or more operating context parameters of the fluid flowing through the cavity.

[0040] In some example embodiments of the second aspect, the control device is further configured to take one or more operating context parameters of the fluid into account in the determination of concentration of the at least one substance.

[0041] In some example embodiments of the second aspect, the one or more operating context parameters of the fluid are taken into account by a correction based on known samples and known operating context parameters thereof.

[0042] According to a third example aspect there is provided a method, comprising: providing an optical signal to a measurement gap arranged in a cavity, wherein the measurement gap and the cavity are designed for a fluid to flow through, wherein width of the measurement gap is less than 1 mm; detecting an optical signal that has passed through the measurement gap; and determining concentration of at least one substance in the fluid based on the detected optical signal.

[0043] In some example embodiments of the third aspect, the method further comprises conveying the optical signal from the optical source to the measurement gap by a first optical fiber, and conveying the optical signal from the measurement gap to the detector by a second optical fiber, wherein at least one of the first and second optical fibers protrudes into the cavity.

[0044] In some example embodiments of the third aspect, the optical signal provided to the measurement gap comprises a wavelength range from ultraviolet wavelengths to infrared wavelengths or near infrared wavelengths.

[0045] In some example embodiments of the third aspect, the determined concentration of at least one substance is determined based on known samples.

[0046] In some example embodiments of the third aspect, the method further comprises obtaining one or more operating context parameters of the fluid flowing through the cavity, and taking the one or more operating context parameters of the fluid into account in the determination of concentration of at least one substance.

[0047] In some example embodiments of the third aspect, the one or more operating context parameters of the fluid are taken into account by a correction based on known samples and known operating context parameters thereof.

[0048] According to a fourth example aspect there is provided a method comprising: monitoring fluids of a pickling process by the measurement apparatus of the first or second aspect or any embodiment thereof and / or by the method of the third aspect or any embodiment thereof.

[0049] In some example embodiments of the fourth aspect, the method further comprises automatically controlling the pickling process responsive to results of the monitoring. The method may additionally or alternatively include using the monitoring results for evaluating reuse of fluids of the pickling process.

[0050] In some example embodiments of the first, second, third, or fourth aspect, the operating context parameters may include one or more of: temperature, pressure, for example.

[0051] In some example embodiments of the first, second, third, or fourth aspect, the determination of the concentration of the at least one substance may involve use of multivariate regression analysis in comparison to the known samples. Additionally or alternatively, suitable machine learning algorithms or artificial intelligence tools may be used for this purpose.

[0052] In some example embodiments of the first, second, third, or fourth aspect, the at least one substance comprises one or more of, or a mixture of two or more of: ferrous ions, ferric ions, aluminum ions, titanium ions, copper ions, chromium ions, nickel ions, magnesium ions, manganese ions, zirconium ions, chloride ions, sulphate ions, sodium ions, acetic ions, citric ions, nitric ions, hydrofluoric ions, potassium ions, a substance with a known absorption pattern.

[0053] Different non-binding example aspects and embodiments have been illustrated in the foregoing. The embodiments in the foregoing are used merely to explain selected aspects or steps that may be utilized in different implementations. Some embodiments may be presented only with reference to certain example aspects. It should be appreciated that corresponding embodiments apply to other example aspects as well. BRIEF DESCRIPTION OF THE FIGURES

[0054] Some example embodiments will be described with reference to the accompanying figures, in which:

[0055] Fig. 1A shows a block diagram of a measurement apparatus according to one or more example embodiments of present disclosure;

[0056] Figs. 1 B-1 E show example structures according to one or more example embodiments of present disclosure;

[0057] Fig. 2 shows a block diagram of another measurement apparatus according to one or more example embodiments of present disclosure;

[0058] Fig. 3A shows a cross-section of a measurement cavity according to one or more example embodiments of present disclosure;

[0059] Fig. 3B shows a cross-section of another measurement cavity according to one or more example embodiments of present disclosure;

[0060] Fig. 4A shows a cross-section of yet another measurement cavity according to one or more example embodiments of present disclosure;

[0061] Fig. 4B shows a cross section of a fluid pipe with a measurement cavity according to one or more example embodiments of present disclosure;

[0062] Fig. 5 shows a block diagram of a measurement apparatus according to one or more example embodiments of present disclosure;

[0063] Fig. 6 shows a block diagram of a control device according to one or more example embodiments of present disclosure; and

[0064] Figs. 7A and 7B show flow charts of methods according to one or more example embodiments of present disclosure; and

[0065] Fig. 8 is a graph illustrating observed accuracy of concentration determination in an example case.

[0066] DETAILED DESCRIPTION

[0067] In the following description, like reference signs denote like elements or steps.

[0068] Various embodiments of present disclosure relate to measurement apparatuses and methods that may be used in connection with monitoring fluids of a pickling process. The monitoring results may be used for controlling and optimizing the pickling process. Additionally or alternatively, the monitoring results may be used for evaluating reuse of fluids of the pickling process. The reuse may involve a fully separate process that is not part of the pickling process.

[0069] The pickling process may relate to pickling metal sheets, parts, rolls, tubes, rods or other shapes of metal pieces. Metals that may be treated in this way include, without being limited to, steel including stainless steel, carbon steel or other type of steel or iron based alloys, aluminum, titanium, copper, chromium, brass, nickel, magnesium, manganese, zirconium, or alloys of these metals containing carbon or not, as examples. The baths of acids or alkalis, such as, but not limited to, hydrochloric, sulfuric, chromic, acetic, citric, nitric, or hydrofluoric acids, or sodium, potassium or chromate hydroxides may be used. Controlling and optimizing the pickling process may require monitoring concentrations one or more of the substances mentioned in the foregoing.

[0070] Various embodiments of present disclosure are based on determining concentration of at least one substance in a fluid based on optical signal that has passed through the fluid. The optical measurement is based on that as the optical signal passes through the fluid, the optical signal interacts with at least one substance in the fluid, and this interaction affects the optical signal. Consequently, concentration of the at least one substance may be determined based on knowing the optical signal that has been fed to the fluid and based on measurement of the optical signal that has passed through the fluid. The concentration may be determined based on Beer- Lambert law.

[0071] In general, accurate optical measurement of substances that have high absorption coefficients require that the optical signal travels in the fluid to be observed. If the distance is too long, the fluid may fully absorb the optical signal and no signal is detected by the detector intended to detect the optical signal that has passed through the fluid. On the other hand, the distance cannot be too small either. If the distance is too small it is possible that there are not enough possibilities for the optical signal to interact with the substance that is to be measured while the optical signal passes through the fluid.

[0072] The inventors of present disclosure have noted that optical monitoring of pickling liquor and / or determining concentrations of substances such as ferrous ions, ferric ions, aluminum ions, titanium ions, copper ions, chromium ions, nickel ions, magnesium ions, manganese ions, zirconium ions, chloride ions, sulphate ions, sodium ions, acetic ions, citric ions, nitric ions, hydrofluoric ions, and / or potassium ions may require that the distance the optical signal travels in the fluid to be observed is extremely small. Thereby an extremely narrow measurement gap, through which an optical signal is intended to travel, is needed in a measurement apparatus configured for determination of concentration in such context. In present disclosure, the term measurement gap is in general used for referring to the distance that the optical signal is arranged to travel in the fluid to be observed. The measurement gap may be defined as the distance between the point in which the optical signal enters the fluid and the point in which the optical signal exits the fluid.

[0073] Various embodiments of present disclosure provide a measurement apparatus and methods that are configured for being operatively connected to an industrial production environment. The measurement apparatuses and methods of various embodiments may suit for harsh industrial production environments and the measurement apparatuses and methods of various embodiments may tolerate corrosion, temperature swings, and vibrations, for example. In this way a real time and automated measurement of a production process is enabled. This may enable automated control and optimization of the production process.

[0074] In the following, some examples of present disclosure are discussed in connection with optical monitoring of pickling liquor and / or determining concentration of Fe2+. Optical monitoring herein refers to detecting optical signal that has passed through a fluid (e.g. pickling liquor) and interacted with at least one substance in the fluid while passing through the fluid and determining concentration of the at least one substance based on the detected optical signal. Various embodiments of present disclosure may be applied to monitoring other substances and used for other purposes, too. Various embodiments may be applied to monitoring one or more of: ferrous ions, ferric ions, aluminum ions, titanium ions, copper ions, chromium ions, nickel ions, magnesium ions, manganese ions, zirconium ions, chloride ions, sulphate ions, sodium ions, acetic ions, citric ions, nitric ions, hydrofluoric ions, potassium ions, or a mixture thereof. Further, various embodiments may be applied to monitoring some other substance with a known absorption pattern. It is to be noted that at least some embodiments may be used for separately and possibly simultaneously monitoring of two or more different substances. Further, at least some embodiments may be used for separately and possibly simultaneously monitoring of ions of the same substance with different oxidation number. For example, at least some embodiments may be used for separate and possibly simultaneous determination of concentration of Fe2+ and Fe3+ in a fluid. It is further noted that the fluid that is monitored in various embodiments of present disclosure may include various substances that are not necessarily monitored.

[0075] Various embodiments of present disclosure provide a measurement apparatus for optical measurement of a fluid flowing through a measurement gap arranged in a cavity.

[0076] In various embodiments, the measurement gap may be for example a gap whose width is less than 1 mm. In an embodiment, the gap is equal to or larger than 10 pm and less than 1 mm. In another embodiment, the gap is 10 pm - 500 pm. In yet another embodiment, the gap is 50 pm - 500 pm. For certain substance, the gap may be for example 50 pm. For some other substance, the gap may be for example 100 pm, 200 pm, 300 pm, 400 pm, or 500 pm. The suitable width may depend on the substance that is to be observed and absorption properties of the fluid to mention a few.

[0077] In various embodiments, the cavity comprises walls made of transparent material configured to allow the optical signal to pass through. The transparent material may be for example glass or some other suitable material that is transparent in desired wavelength range. The fluid flowing in the cavity is exposed to the optical signal through the transparent wall and the optical signal that has passed through the fluid is detected through the transparent wall.

[0078] The cavity may be a microfluidic cell, but also other alternatives are provided in some embodiments.

[0079] In various embodiments, the measurement gap is formed by a pair of optical fibers that protrude into the cavity that is designed for a fluid to flow through. In such an arrangement, the cavity may be wider than the mere width of the measurement gap and the fluid may flow in a wider space than the mere width of the measurement gap. The pair of optical fibers comprises a first optical fiber that conveys optical signal from an optical source to the measurement gap and a second optical fiber that conveys optical signal from the measurement gap to a detector. That is, the measurement gap is formed between the end points of the optical fibers. Benefits of such embodiments include that a particulate filter may not be needed even if the fluid to be observed would include particles that are larger than the measurement gap. Further, improved measurement accuracy may be achieved.

[0080] Further, there may be two or more pairs of optical fibers configured to form two or more measurement gaps. The widths of the two or more measurement gaps may be different. Different widths may be for measuring two or more different substances and / or for optimizing for different concentration ranges, for example.

[0081] An embodiment provides a support structure for the pair of optical fibers that protrude into the cavity. The support structure may be configured to keep the width of the respective measurement gap constant. Additionally or alternatively, the support structure may be configured to keep the pair of optical fibers aligned in relation to each other. With the extremely narrow measurement gap between the optical fibers, maintaining the width of the gap constant and keeping the optical fibers aligned may improve accuracy of the measurement.

[0082] In an embodiment, there is a monolithic structure that forms the walls of the cavity and the support structure for the optical fibers. In an embodiment, the monolithic structure is made of glass or of some other transparent material and the monolithic structure comprises recesses for the optical fibers. The recesses support the optical fibers and keep them stable and aligned. The recesses protrude into the cavity and the optical fibers are placed in the recesses whereby the optical fibers protrude into the cavity and the optical signals are transmitted to the fluid and from the fluid through the monolithic structure. In this way, the optical fibers do not need to be in contact with the fluid. Instead, the monolithic structure protects the optical fibers and keeps them stable. A benefit of this structure is that disadvantages of CTE (Coefficients of Thermal Expansion) mismatch may be avoided or reduced. The disadvantages that may be avoided or reduced may include for example misalignment and / or changes in physical dimensions caused by changes in temperature. Further, the optical fibers may be easy to assemble, and the optical fibers may be easily replaced when necessary.

[0083] In an embodiment, the optical measurement is arranged to alternate between a measurement cycle and a rinsing cycle. There may be one or more valves for this purpose. The measurement is performed in a cavity designed for a fluid to flow through. A monitored fluid is allowed to enter the cavity through the one or more valves during the measurement cycle and a cleaning fluid is allowed to enter the cavity through the one or more valves during the rinsing cycle. The rinsing cycle may comprise rinsing the cavity with water to achieve that there is no more monitored fluid that absorbs light in the cavity and then rinsing the cavity with a solvent, e.g. hydrochloric acid, in order to remove possible fouling from the cavity. The rinsing cycle may be performed in between each measurement cycle to prevent or at least reduce fouling, if any. For the sake of clarity, it is mentioned that the fluid flow may be continuous, or the fluid flow may be stopped during the measurement cycle and / or during the rinsing cycle e.g. after the cavity has been filled with the fluid.

[0084] In an embodiment, the fluid to be observed may be filtered to remove particulate matter, but this is not mandatory or required in all embodiments.

[0085] In an embodiment, the optical source provides optical signal on a wavelength range from ultraviolet wavelengths to infrared wavelengths or near infrared wavelengths. The wavelength range may be for example 340-830 nm, but this is just an illustrative example range and other wavelength ranges can be used as well. The wavelength range may be designed to match the whole wavelength range of the detector that is used (e.g. spectrometer). Further, the wavelength range may be designed to match metallic ions absorption spectra and wavelength region for spectral calibration.

[0086] The optical source of various embodiments may be formed of one or more lightemitting diodes (LEDs). LEDs are well suited for this purpose due to their long lifetime. The optical source may comprise multiple LEDs. The intensity of the LEDs may be individually adjusted to obtain a flat spectrum over a desired wavelength range. LED types may be selected so that maximal coverage and maximal light intensity is obtained . For the sake of clarity, it is mentioned that other types of optical sources may be used, too. The optical signal from the optical source may be split between the measurement cavity and a reference arm for monitoring the optical source and enabling correction for drift of the optical source.

[0087] In an embodiment, there are one or more sensors configured to measure one or more operating context parameters of the fluid flowing through the cavity. There may be for example a temperature sensor configured to measure the temperature of the fluid to be observed. The temperature sensor and other sensors may be placed in the cavity near the measurement gap, but other measurement arrangements may be used, too. The temperature and / or other operating context parameters of the fluid may be taken into account in the determination of concentration of at least one substance in the fluid. The temperature and / or other operating context parameters of the fluid may be taken into account for example by a correction based on known samples. There may be a regression model trained based on known samples for performing the correction. In an embodiment, the signal absorption signature determined based on the optical signal detected by the detector is compared to a multivariate model of signal absorption signatures to determine the concentration, and then the regression model is referred to for correction of the concentration based on the temperature and / or other operating context parameters of the fluid.

[0088] By performing a correction based on the operating context parameters of the fluid, one achieves that there is no need to control the operating context parameters of the fluid during the measurement or the operating parameters of the measurement conditions. Instead, measurements may be performed in different operating contexts, such as in different temperatures or under different pressure conditions, and still it may be possible to obtain comparable and accurate measurement results.

[0089] In the following, some examples of present disclosure include a temperature sensor and correction based on temperature, but it is to be noted that some other operating context parameter may be equally used instead of or in addition to the temperature.

[0090] Further, the determined concentration of the at least one substance in the fluid may be in general determined based on known samples. There may be a regression model trained based on known samples for performing the determination of the concentration. In this way, the regression model may be used for calibration of the measurement.

[0091] In general, linear Beer-Lambert law may be used for the determination of the concentration of the at least one substance based on the optical signal detected by the detector, but it is noted that linearity is not always ensured. The use of the regression model allows taking into account nonlinear behavior, too. Accuracy of the measurement may be improved in this way.

[0092] It is noted that suitable artificial intelligence tools or machine learning algorithms may be used in addition to or instead of the regression model for the correction of the determined concentration and / or for determining the concentration reading based on the optical signal detected by the detector.

[0093] The known samples may comprise data obtained by spectroscopic measurements of nominal samples for which the actual concentration levels are already known. The use of known samples may improve accuracy of the results and may account for various components (variation of measurement components, distortion, noise removal etc).

[0094] In an embodiment, several measurements are sequentially performed, and the determined concentration is based on a rolling average of the several measurements. This suits well for embodiments where the fluid is flowing through the measurement gap during the measurement whereby the measurement is not a point-like measurement from one single part of the fluid, but instead provides more representative measurement of different parts of the fluid. This may improve the accuracy of the measurement.

[0095] Fig. 1A shows a block diagram of a measurement apparatus according to one or more example embodiments of present disclosure.

[0096] Fig. 1A shows an optical source 101 , a detector 102, a control device 103, a temperature sensor 104, valves 105 for measurement and rinsing cycles, a cavity 110, walls 111 and 111 ’ of the cavity, an optical fiber pair comprising a first optical fiber 114 and a second optical fiber 115, and a measurement gap 113 between the first optical fiber 114 and the second optical fiber 115. The first optical fiber 1 14 and the second optical fiber 115 protrude into the cavity 110. The first optical fiber 114 and the second optical fiber 115 pass through the walls 111 and 11 T of the cavity 110 into the interior of the cavity 110 to form the measurement gap inside the cavity 110.

[0097] The optical source 101 may comprise one or more LEDs or some other source of light with a suitable wavelength range. The detector 102 may be a spectrometer.

[0098] The cavity 110 is designed for a fluid to flow through. Arrows 112, 112’ illustrate the direction of the fluid flow.

[0099] The control device 103 is for example an embedded processing device that may be programmed to perform appropriate computing actions. The control device 103 is operable to receive measurement results from the detector 102 and the temperature sensor 104.

[0100] In operation, the control device 103 controls the valves 105 to allow the fluid that is to be observed to flow through the cavity 110. The first optical fiber 114 conveys optical signal from the optical source 101 to the measurement gap 113. The second optical fiber 115 conveys optical signal that has passed through the measurement gap 113 and through the fluid flowing in the cavity 110 to the detector 102. The reading of the detector is conveyed to the control device 103 and the control device 103 determines concentration of at least one substance of the fluid flowing through the cavity 110 and the measurement gap 113 based on the reading of the detector 102. The temperature sensor 104 measures temperature of the fluid flowing through the cavity 110. The control device 103 may take the measured temperature into account in the determination of the concentration of at least one substance.

[0101] Thereafter the control device 103 may control the valves 105 to perform a rinsing cycle and thereafter a new measurement cycle may be initiated.

[0102] In one embodiment, the optical fibers of the pair of optical fibers protrude into the cavity from opposite directions as shown in Fig. 1 A. In another embodiment at least part of the inner surface of the cavity is reflective and the optical fibers of the optical fiber pair may protrude into the cavity from the same direction and the second optical fiber of the pair of optical fibers conveys to the detector the radiation reflected from the inner surface of the cavity. In such option, the detected radiation has passed twice through the fluid flowing in the cavity. In yet another embodiment only one of the first and second optical fibers protrudes into the cavity and the measurement gap is formed near one of the walls of the cavity.

[0103] Figs. 1 B-1 E show example structures according to one or more example embodiments of present disclosure. The example structures of Figs. 1 B-1 E may be applied to the measurement apparatus of Fig. 1A. In the examples of Figs. 1 B-1 E, the first and second optical fibers 114, 115 are not in direct contact with the fluid, when there is fluid inside the cavity.

[0104] Fig. 1 B shows a cavity 150, walls 151 and 15T of the cavity, an optical fiber pair comprising a first optical fiber 114 and a second optical fiber 115, and a measurement gap 113 between the first optical fiber 114 and the second optical fiber 115.

[0105] The first optical fiber 1 14 and the second optical fiber 115 protrude into the cavity 150. The cavity 150 may be formed by a transparent monolithic structure that forms the walls 151 , 15T of the cavity and that comprises first and second recesses 154, 155 for the first and second optical fibers 114, 115. The first and second recesses 154, 155 protrude into the cavity 150.

[0106] The cavity 150 is designed for a fluid to flow through. Arrows 112, 112’ illustrate the direction of the fluid flow.

[0107] In operation, the first optical fiber 114 conveys optical signal from an optical source to the measurement gap 113 through the transparent wall 151 . The second optical fiber 115 conveys optical signal that has passed through the measurement gap 113 and through the transparent wall 15T and through the fluid flowing in the cavity 150 to a detector.

[0108] Fig. 1 C shows a cavity 160, walls 161 and 16T of the cavity, an optical fiber pair comprising a first optical fiber 114 and a second optical fiber 115, and a measurement gap 163 between the first optical fiber 114 and the second optical fiber 115.

[0109] The first optical fiber 114 extends up to the wall 161 but does not protrude into the cavity 160. The second optical fiber 115 protrudes into the cavity 150. The cavity 160 may be formed by a transparent monolithic structure that forms the walls 161 ,

[0110] 161 ’ of the cavity and that comprises a recess 165 for the second optical fiber 115. The recess 165 protrudes into the cavity 160.

[0111] The cavity 160 is designed for a fluid to flow through. Arrows 112, 112’ illustrate the direction of the fluid flow.

[0112] In operation, the first optical fiber 114 conveys optical signal from an optical source to the measurement gap 163 through the transparent wall 151 . The second optical fiber 115 conveys optical signal that has passed through the measurement gap 163 and through the transparent wall 16T and through the fluid flowing in the cavity 160 to a detector.

[0113] Fig. 1 D shows a cavity 170, walls 171 and 171 ’ of the cavity, an optical fiber pair comprising a first optical fiber 114 and a second optical fiber 115, and another optical fiber pair comprising a third optical fiber 116 and a fourth optical fiber 117. Further, there are measurement gaps 173 and 178 formed in the cavity 170. The widths of the measurement gaps 173, 178 may be different from each other for example for monitoring different substances.

[0114] The first optical fiber 114, second optical fiber 115, third optical fiber 116, and fourth optical fiber 117 protrude into the cavity 170. The cavity 170 may be formed by a transparent monolithic structure that forms the walls 171 , 17T of the cavity and that comprises first and second recesses 174, 175 for the first and second optical fibers 114, 115, and third and fourth recesses 176, 177 for the third and fourth optical fibers 116, 117. The first, second, third and fourth recesses 174-177 protrude into the cavity 170.

[0115] The cavity 170 is designed for a fluid to flow through. Arrows 112, 112’ illustrate the direction of the fluid flow.

[0116] In operation, the first optical fiber 114 conveys optical signal from an optical source to the measurement gap 173 through the transparent wall 171 . The second optical fiber 115 conveys optical signal that has passed through the measurement gap 173 and through the transparent wall 17T and through the fluid flowing in the cavity 170 to a detector. The third optical fiber 116 conveys optical signal from the optical source to the measurement gap 178 through the transparent wall 171. The fourth optical fiber 117 conveys optical signal that has passed through the measurement gap 178 and through the transparent wall 171 ’ and through the fluid flowing in the cavity 170 to a detector.

[0117] Fig. 1 E shows a cavity 180, walls 181 and 181 ’ of the cavity, an optical fiber pair comprising a first optical fiber 114 and a second optical fiber 115, and a measurement gap 183 in the cavity 180.

[0118] The first optical fiber 1 14 and the second optical fiber 115 protrude into the cavity 180 from the same direction. The cavity 180 may be formed by a transparent monolithic structure that forms the walls 181 , 18T of the cavity and that comprises a recess 185 for the first and second optical fibers 114, 115. The recess 185 protrudes into the cavity 180.

[0119] At least part of the inner surface 189 of the wall 181 of the cavity 180 is reflective.

[0120] The cavity 180 is designed for a fluid to flow through. Arrows 112, 112’ illustrate the direction of the fluid flow.

[0121] In operation, the first optical fiber 114 conveys optical signal from an optical source to the measurement gap 183. The second optical fiber 115 conveys to a detector an optical signal that has passed through the measurement gap 183 and through the fluid flowing in the cavity 180 and reflected from the inner surface 189 of the wall 181. In this example, the detected radiation has passed twice through the measurement gap 183 and the fluid flowing in the cavity 180.

[0122] Fig. 2 shows a block diagram of another measurement apparatus according to one or more example embodiments of present disclosure.

[0123] Fig. 2 shows an optical source 101 , a detector 102, a control device 103, a temperature sensor 204, valves 105 for measurement and rinsing cycles, a cavity 210, walls 211 and 21 T of the cavity, an optical fiber pair comprising a first optical fiber 214 and a second optical fiber 215, collimator elements 216, 217, and a measurement gap 213 between the first optical fiber 214 and the second optical fiber 215.

[0124] The cavity 210 is a microfluidic cell, and the walls 211 , 21 T are transparent. The microfluidic cell forms the measurement gap 213 whereby the diameter of the microfluidic cell sets the width of the measurement gap. In present disclosure the diameter of the microfluidic cell is less than 1 mm. The collimator elements 216, 217 may spread the optical signal to a wider area in the cavity 210, but it is to be noted that the collimator elements 216, 217 are not mandatory.

[0125] The optical source 101 may comprise one or more LEDs or some other source of light with a suitable wavelength range. The detector 102 may be a spectrometer.

[0126] The cavity 210 is designed for a fluid to flow through. Arrows 112, 112’ illustrate the direction of the fluid flow.

[0127] The control device 103 is for example an embedded processing device that may be programmed to perform appropriate computing actions. The control device 103 is operable to receive measurement results from the detector 102 and the temperature sensor 204.

[0128] In operation, the control device 103 controls the valves 105 to allow the fluid that is to be observed to flow through the cavity 210. The first optical 214 conveys optical signal from the optical source 101 to the measurement gap 213 through the transparent wall 211. The second optical fiber 215 conveys optical signal that has passed through the measurement gap 213 and through the transparent wall 21 T and through the fluid flowing in the cavity 210 to the detector 102. The reading of the detector is conveyed to the control device 103 and the control device 103 determines concentration of at least one substance of the fluid flowing through the cavity 210 and the measurement gap 213 based on the reading of the detector 102. The temperature sensor 204 measures temperature of the fluid flowing through the cavity 210. The control device 103 may take the measured temperature into account in the determination of the concentration of at least one substance.

[0129] Thereafter the control device 103 may control the valves 105 to perform a rinsing cycle and thereafter a new measurement cycle may be initiated.

[0130] In the following, Figs. 3A, 3B, and 4A show some further example structures of a measurement cavity suited for measurement apparatus of various embodiments of present disclosure.

[0131] Fig. 3A shows a cross-section of a measurement cavity according to one or more example embodiments of present disclosure.

[0132] Fig. 3A shows a cavity 310 with transparent walls, a temperature sensor 304, an optical fiber pair comprising a first optical fiber 314 and a second optical fiber 315, a support structure comprising sleeves 334, 335 surrounding the optical fibers 314, 315, and a casing 333. A measurement gap is formed in the cavity 310 between the first optical fiber 314 and the second optical fiber 315.

[0133] The cavity 310 is designed for a fluid to flow through. Arrows 312, 312’ illustrate the direction of the fluid flow when the apparatus is in operation. Arrows 331 , 332 illustrate the direction of optical signal through the optical fibers 314, 315 when the apparatus is in operation.

[0134] The optical fibers 314, 315 are fixedly attached to the sleeves 334, 335 to keep the width of the respective measurement gap constant and / or to keep the optical fibers 314, 315 aligned in relation to each other.

[0135] Fig. 3B shows a cross-section of another measurement cavity according to one or more example embodiments of present disclosure.

[0136] Fig. 3B shows a cavity 370, a temperature sensor 304, an optical fiber pair comprising a first optical fiber 374 and a second optical fiber 375, a support structure comprising sleeves 334, 335 surrounding the optical fibers 314, 315, and a casing 333.

[0137] The first optical fiber 374 and the second optical fiber 375 protrude into the cavity 370. The first optical fiber 374 and the second optical fiber 375 pass through the walls of the cavity 370 into the interior of the cavity 370 to form a measurement gap inside the cavity 370 between ends of the first optical fiber 374 and the second optical fiber 375.

[0138] The cavity 370 is designed for a fluid to flow through. Arrows 312, 312’ illustrate the direction of the fluid flow when the apparatus is in operation. Arrows 331 , 332 illustrate the direction of optical signal through the optical fibers 374, 375 when the apparatus is in operation.

[0139] The optical fibers 374, 375 are fixedly attached to the sleeves 334, 335 and to the walls of the cavity 370 to keep the width of the respective measurement gap constant and / or to keep the optical fibers 374, 375 aligned in relation to each other. In an embodiment, the optical fibers 374, 375 are attached by using a glue. The glue is preferably such that it tolerates for example corrosive fluids that may flow in the cavity 370.

[0140] Fig. 4A shows a cross-section of yet another measurement cavity according to one or more example embodiments of present disclosure.

[0141] Fig. 4A shows a monolithic transparent body 411 , a cavity 410 formed by the monolithic transparent body 411 , an optical fiber pair comprising a first optical fiber 414 and a second optical fiber 415, another optical fiber pair comprising a third optical fiber 416 and a fourth optical fiber 417. Further, there are measurement gaps 413 and 423 formed in the cavity 410.

[0142] The monolithic transparent body 411 may be made of glass, for example. The monolithic transparent body 411 comprises recesses 444-447 for the respective optical fibers 414-417. The recesses at least partially protrude into the cavity 410 thereby enabling the optical fibers 414-417 to respectively protrude into the cavity 410 and to form the measurement gaps 413, 423 respectively between the first optical fiber 414 and the second optical fiber 415, and between the third optical fiber 416 and the fourth optical fiber 417. The recesses 444-447 support the optical fibers 414-417 and may keep the width of the respective measurement gaps 413, 423 constant and / or keep the optical fibers 414-417 respectively aligned in relation to each other.

[0143] As a non-limiting example, the diameter of the cavity 410 may be for example 4 mm and the widths of the measurement gaps 413, 423 are less than 1 mm and may be for example 10-500 pm. In general, the widths of the measurement gaps 413, 423 may be different from each other for example for measuring concentrations of different substances or different concentration ranges of a given substance.

[0144] The cavity 410 is designed for a fluid to flow through. Arrows 412, 412’ illustrate the direction of the fluid flow when the apparatus is in operation.

[0145] Fig. 4B shows a cross section of a fluid pipe 480 with a measurement cavity according to one or more example embodiments of present disclosure. Fig. 4B shows a monolithic transparent body 411 (e.g. the monolithic transparent body 411 of Fig. 4A) in the fluid pipe 480, and a cavity 410 formed by the monolithic transparent body 411 . Further there are one or more measurement gaps formed in the cavity 410 and one or more optical fiber pairs. For the sake of clarity, these are not shown in Fig. 4B.

[0146] Arrow 412’ illustrates the direction of the fluid flow when the apparatus is in operation and arrows 431 , 432 illustrate the direction of optical signal through the measurement gaps in the cavity 410 when the apparatus is in operation.

[0147] The fluid flowing in the pipe 480 flows through the cavity 410 and through the measurement gap(s) in the cavity 410. Additionally, the fluid may flow in the pipe over and below the monolithic transparent body 411 in space 460, 460’. In this way, the fluid flow in the pipe 480 is not necessarily disturbed by the measurement.

[0148] Fig. 5 shows a block diagram of a measurement apparatus according to one or more example embodiments of present disclosure. It is to be noted that applicable parts of the measurement apparatus of Fig. 5 may be combined with features of any one of the previously disclosed measurement apparatuses, such as the measurement apparatuses of Figs. 1A-4B.

[0149] Fig. 5 shows an optical source 501 , a combiner 521 , a first optical switch 522, a second optical switch 523, a detector 102, a cavity 510, multiple optical fiber pairs each comprising a first optical fiber 514 and a second optical fiber 515, and multiple measurement gaps 513 between the first optical fibers 514 and the second optical fibers 515. Further Fig. 5 shows a reference optical fiber 524measurement gap.

[0150] The first optical fibers 514 and the second optical fibers 515 protrude into the cavity 510 to form the measurement gaps 513. The cavity 510 may comprise the wall and / or the support structure of Fig 1 B-1 E, 3B or 4A, for example.

[0151] The optical source 501 comprises multiple LEDs. The detector 102 may be a spectrometer.

[0152] The cavity 510 is designed for a fluid to flow through.

[0153] In operation, the optical signals of the LEDs are combined in the combiner 521 . The first optical switch is controlled to sequentially switch the optical signal from the combiner 521 to one of the multiple first optical fibers 514 or to the first reference optical fiber 524. From thereon the optical signal travels through one of the multiple measurement gaps 513 or through the reference measurement gap 526 to one of the multiple second optical fibers 515, the second optical switch 523 and the detector 102 or to the second reference optical fiber 524 and to the detector.

[0154] The multiple optical fiber pairs may be configured for determination of multiple different substances and the measurement gaps of different optical fiber pairs may be different (e.g. specific for certain substance). The first optical switch 522 splits the optical signal to these multiple optical fiber pairs by sequentially switching the optical signal to different optical fiber pair. After signal from one of the multiple optical fiber pairs has been read by the detector 102, the optical signal may be switched to the next optical fiber pair.

[0155] The reference optical fiber 524 provides a reference reading of the optical source outside the cavity and without any effects of the fluid that is to be observed. The readings from this reference arm may be used for calibration of the measurement. In an embodiment, the reference arm is continuously monitored whereby it is possible to continuously monitor that the optical source operates as intended. Further, the reference reading from the reference optical fiber 524 allows correction of the measurement results so that variation in the optical source is taken into account.

[0156] It is to be noted that for example the reference monitoring, the optical source and / or the optical switching arrangement of Fig. 5 may be combined with features of any one of the previously disclosed measurement apparatuses, such as the measurement apparatuses of Figs. 1A-4B.

[0157] Fig. 6 shows a block diagram of a control device according to one or more example embodiments of present disclosure. The apparatus 600 is for example an embedded processing device, a general purpose computing device, or some other electronic data processing apparatus. The apparatus 600 can be used for implementing at least some embodiments of present disclosure. That is, with suitable configuration the apparatus 600 is suited for operating for example as the control device 103 of Figs. 1A and 2 or for providing at least some functionality of the control device 103 of Figs. 1A and 2 or other functionality according to one or more example embodiments of present disclosure.

[0158] The apparatus 600 comprises an input / output interface 605, a processor 601 , and a memory 602. The apparatus 600 further comprises software or computer program code 603 stored in the memory 602 and operable to be executed by the processor 601. The software 603 comprises instructions for providing at least some functionality of various embodiments of present disclosure.

[0159] The processor 601 may comprise a central processing unit (CPU), a microprocessor, an embedded processor (DSP), or the like. Fig. 6 shows one processor 601 , but the apparatus 600 may comprise a plurality of processors.

[0160] The memory 602 may further comprise a model 604 for data processing stored thereon. The model 604 may be for example a regression model or a machine learning model or an artificial intelligence model.

[0161] The input / output interface 605 may be operable to receive measurement results 606 from sensors and external devices such as from the detector 102 and temperature sensor 104 of Figs. 1 -5. Further, the input / output interface 605 may be operable to output measurement results e.g. to a display panel 607.

[0162] The skilled person appreciates that in addition to the elements shown in Fig. 6, the apparatus 600 may comprise other elements, as well as additional circuitry such as memory chips, application-specific integrated circuits (ASIC), other processing circuitry for specific purposes and the like.

[0163] Figs. 7A and 7B show flow charts of methods according to one or more example embodiments of present disclosure.

[0164] Fig. 7A illustrates a method for determination of concentration of at least one substance in a fluid comprising various possible steps including some optional steps while also further steps can be included and / or some of the steps can be performed more than once. The method may be implemented by any one of the previously disclosed measurement apparatuses, such as the measurement apparatuses of Figs. 1 A-5, and any details discussed in the foregoing may be applied in the method of Fig. 7A.

[0165] The method of Fig. 7A comprises the following steps: 701 . A fluid to be observed is present in a measurement gap and an optical signal is provided to the measurement gap. The fluid may flow through the measurement gap when the method is being performed or the fluid flow may be stopped for performing the method.

[0166] 702. An optical signal that has passed through the measurement gap (and the fluid that is present in the measurement gap) is detected.

[0167] 703. Concentration of at least one substance in the fluid is determined based on the detected optical signal. The determination may involve using regression analysis. Additionally or alternatively, suitable machine learning algorithms or artificial intelligence tools may be used for this purpose.

[0168] The method may further include steps 704, 705 and / or 706.

[0169] 704. One or more operating context parameters of the fluid are obtained.

[0170] 705. The one or more operating context parameters of the fluid are taken into account in the determination of concentration of the at least one substance.

[0171] Examples of operating context parameters include temperature and pressure, but also other operating context parameters may be taken into account.

[0172] 706. The determination of concentration of the at least one substance may involve use of multivariate regression analysis in comparison to known samples. Additionally or alternatively, suitable machine learning algorithms or artificial intelligence tools may be used for this purpose, too.

[0173] Fig. 7B illustrates a method for a pickling process comprising various possible steps including some optional steps while also further steps can be included and / or some of the steps can be performed more than once. The method may involve use of any one of the previously disclosed measurement apparatuses, such as the measurement apparatuses of Figs. 1 A-5, and any details discussed in the foregoing may be applied in the method of Fig. 7B.

[0174] The method of Fig. 7B comprises the following steps:

[0175] 711. Fluids of a pickling process are (continuously) monitored. The results of the monitoring may be provided for further processing by human operators. 712. The pickling process is (automatically) controlled responsive to the results of the monitoring in step 711. The controlling may involve optimizing the pickling process.

[0176] The speed of the pickling reaction of a pickling process may depend on one or more of the following non-exclusive list of parameters: concentration of acid in a single pickling bath or in multiple pickling baths, temperature in each bath, time that metal is in contact with the acid bath, a rinsing step including temperature and type of liquid used to rinse, type of metal or metal alloy being pickled, type and concentration of acid or hydroxide being used to pickle along with pH evolution dynamics in each bath, and use of passivation agents with acids or hydroxides. The controlling step 712 may relate to controlling one or more of these.

[0177] The method of Fig. 7B may additionally or alternatively include using the monitoring results for evaluating reuse of fluids of the pickling process. The reuse may involve a fully separate process that is not part of the pickling process.

[0178] Fig. 8 is a graph illustrating observed accuracy of concentration determination in an example case. The graph shows an example of Fe2+ concentration determined for multiple samples that have different concentrations of Fe2+. X-axis shows Fe2+ concentration determined for a sample by laboratory titration and y-axis shows Fe2+ measurement result obtained for the same sample by a measurement apparatus according to an embodiment of present disclosure. The laboratory titrated concentration is considered to provide the ground truth value to which the measured concentration may be compared. In this example case only Fe2+ concentration was determined whereas in different setup other substances may be monitored, too.

[0179] By way of example one can see from the graph of Fig. 8 that for the sample 801 , the ground truth concentration determined by the laboratory titration 10.02 % and the measurement result obtained by a measurement apparatus according to an embodiment of present disclosure is 9.9 %. For the sample 802, the ground truth concentration determined by the laboratory titration 11 .05 % and the measurement result obtained by a measurement apparatus according to an embodiment of present disclosure is 11.3 %. For the sample 803, the ground truth concentration determined by the laboratory titration 13.01 % and the measurement result obtained by a measurement apparatus according to an embodiment of present disclosure is 13.29 %. In the example of Fig. 8 the difference between the laboratory titrated concentration and the measured concentration is in the range + / - 0.3 percent units and in percentage the difference is in the range + / - 2.7 %.

[0180] Without limiting the scope and interpretation of the patent claims, certain technical effects of one or more of the example embodiments disclosed herein are listed in the following. A technical effect is that accurate measurement of concentrations may be provided as the measurement results may be relatively close to corresponding laboratory titrated concentrations. A technical effect is a measurement suited for industrial scale fluid monitoring. A technical effect is ease of maintenance as parts of the measurement apparatus may be replaced relatively easily. A technical effect is that the measurement may be independent of operating context parameters such as temperature and pressure as the effects of operating context parameters may be corrected in the measurement results. That is, there is no need to maintain stable operating context of the fluid for the measurement. A further technical effect is a support structure that may provide self-aligning of optical fibers and that may maintain the measurement gap constant.

[0181] At least the embodiments in which one or more optical fibers protrude into the cavity provide that problems of clogging the measurement apparatus may be avoided or reduced even if there were some larger particles in the fluid that is monitored. A further effect provided in such embodiments is that a filter in the pipeline may not be needed even if there were some larger particles in the fluid that is monitored. These effects are achieved by the structure in which some part of the fluid may bypass the narrow measurement gap. That is, all of the fluid does not need to pass through the measurement gap.

[0182] Various embodiments have been presented. It should be appreciated that in this document, words comprise; include; and contain are each used as open-ended expressions with no intended exclusivity.

[0183] The foregoing description has provided by way of non-limiting examples of particular implementations and embodiments a full and informative description of the best mode presently contemplated by the inventors for carrying out the invention. It is however clear to a person skilled in the art that the invention is not restricted to details of the embodiments presented in the foregoing, but that it can be implemented in other embodiments using equivalent means or in different combinations of embodiments without deviating from the characteristics of the invention.

[0184] Furthermore, some of the features of the afore-disclosed example embodiments may be used to advantage without the corresponding use of other features. As such, the foregoing description shall be considered as merely illustrative of the principles of the present invention, and not in limitation thereof. Hence, the scope of the invention is only restricted by the appended patent claims.

Claims

CLAIMS1 . A measurement apparatus, comprising: a cavity (110, 150, 160, 170, 180, 310, 370, 410, 510) and a measurement gap (113, 153, 163, 173, 178, 183, 413, 423, 513) arranged in the cavity, wherein the measurement gap and the cavity are designed for a fluid to flow through, wherein width of the measurement gap (113, 153, 163, 173, 178, 183, 413, 423, 513) is less than 1 mm; an optical source (101 , 501 ) configured to provide an optical signal to the measurement gap, wherein the optical signal is configured to interact with at least one substance in the fluid flowing through the measurement gap (113, 153, 163, 173, 178, 183, 413, 423, 513); a first optical fiber (114, 116, 314, 414, 416, 514) configured to convey the optical signal from the optical source to the measurement gap in the cavity; a detector (102) configured to detect an optical signal that has passed through the measurement gap (113, 153, 163, 173, 178, 183, 413, 423, 513); a second optical fiber (115, 117, 315, 415, 417, 515) configured to convey the optical signal from the measurement gap in the cavity to the detector (102); and a control device (103, 600) configured to determine concentration of at least one substance in the fluid based on the optical signal detected by the detector (102), wherein the cavity comprises walls (151 , 15T, 161 , 16T, 171 , 17T, 181 , 18T, 411 ) made of transparent material configured to allow the optical signal to pass through, and the walls comprise one or more recesses (154, 155, 165, 174-177, 185, 444- 447) for the optical fibers, the recesses protruding into the cavity.

2. The measurement apparatus of any preceding claim, wherein the width of the measurement gap (113, 153, 163, 173, 178, 183, 413, 423, 513) is 10 pm - 500 pm.

3. The measurement apparatus of any preceding claim, further comprising two or more of said measurement gaps (173, 178, 413, 423).

4. The measurement apparatus of any one of claims 1-2, further comprising twoor more of said measurement gaps (173, 178, 413, 423) and two or more respective recesses (444-447) protruding into the cavity.

5. The measurement apparatus of any preceding claim, wherein the optical source (101 , 501 ) provides optical signal on a wavelength range from ultraviolet wavelengths to infrared wavelengths or near infrared wavelengths.

6. The measurement apparatus of any preceding claim, wherein the optical source (501 ) comprises one or more light-emitting diodes (LEDs).

7. The measurement apparatus of any preceding claim, wherein the control device (103) is further configured to take one or more operating context parameters of the fluid into account in the determination of concentration of the at least one substance by a correction based on known samples and known operating context parameters thereof.

8. The measurement apparatus of claim 7, wherein the operating context parameters comprise one or more of: temperature, pressure.

9. The measurement apparatus of any preceding claim, wherein the at least one substance comprises one or more of, or a mixture of two or more of: ferrous ions, ferric ions, aluminum ions, titanium ions, copper ions, chromium ions, nickel ions, magnesium ions, manganese ions, zirconium ions, chloride ions, sulphate ions, sodium ions, acetic ions, citric ions, nitric ions, hydrofluoric ions, potassium ions, a substance with a known absorption pattern.

10. The measurement apparatus of any preceding claim, wherein the walls of the cavity are formed of a monolithic structure (411 ) comprising the one or more recesses (444-447) for the optical fibers.

11. A method comprising: monitoring (711 ) fluids of a pickling process by the measurement apparatus of any one of claims 1 -1012. The method of claim 11 , further comprising automatically controlling (712) the pickling process responsive to results of the monitoring.

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