Patent Publication Number: US-10788416-B2

Title: Multiple wavelength light source for colorimetric measurement

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is based on and claims the benefit of U.S. Provisional Patent Application Ser. No. 61/886,291, filed Oct. 3, 2013, the content of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     Online wet chemistry analyzers are used in a variety of industries to provide a continuous indication of an analyte in a process sample. This continuous indication can be provided locally by the analyzer and/or remotely to one or more suitable devices in order to provide control and/or monitoring of a chemical process. 
     One particular type of online wet chemistry analyzer is an online silica analyzer. These devices are configured to generate a reaction in the process sample that allows an indication of silica in the sample to be determined. Such analyzers are useful in determining silica content in boiler water, boiler feedwater, demineralized water, and steam condensate. While such analyzers are useful in a variety of industries, they are of particular use in power plant boilers. In such systems, silica can form silicate deposits that can damage turbines and other generation equipment that is used in the water-steam turbine cycle. Accordingly, power plants with high pressure turbines generally monitor silica carefully in order to ensure effective detection and removal/remediation. One particular example of an online silica analyzer is sold under the trade designation Model CFA3030 Silica Analyzer from Rosemount Analytical, an Emerson Process Management company. 
     An online silica analyzer will generally employ a known reaction to render the silica in the process sample readily detectable. One example of such a reaction is known as the molybdenum blue method. In the molybdenum blue method, molybdate (usually in the form of potassium molybdate) is used to react with silica in the process sample/solution in order to generate a compound suitable for colorimetric detection. In accordance with the molybdenum blue method, the silica content in water is measured based on the color of the silicomolybdic acid formed through the wet chemistry process. The colorimetric detection in accordance with the molybdenum blue method is governed by the Beer-Lambert law, which states that there is a logarithmic dependence between the transmission (or transmissivity), T, of light through a substance and the product of the absorption coefficient of the substance, α, and the distance that the light travels through the material (i.e. path length), 1. The Beer-Lambert law is expressed as follows: 
     
       
         
           
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                     l 
                   
                 
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                   10 
                   
                     
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     The absorption coefficient can be written as a product of the molar absorptivity (extinction coefficient) of the absorber, ε, and the molar concentration, c, of the absorbing species in the material where, I and I o  are the intensity of the incident light and the transmitted light, respectively. 
     SUMMARY 
     A colorimetric wet chemistry analyzer for determining a concentration of an analyte of interest in a sample is provided. The analyzer comprising includes a reaction chamber configured to receive the sample and facilitate a reaction that changes a color of the sample based on the concentration of the analyte of interest. A photometric cell is operably coupled to the reaction chamber to receive the sample and direct illumination therethrough. The photometric cell has a first illumination source configured to provide illumination at a first wavelength through the photometric cell and a second illumination source configured to provide illumination at a second wavelength through the photometric cell. The second wavelength is different than the first wavelength. A photo detector is configured to detect illumination passing through the photometric cell. A controller is coupled to the first illumination source, the second illumination source and the photo detector and is configured to provide an indication of concentration relative to the analyte of interest based on a signal from the photo detector. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic view of an online silica analyzer with which embodiments of the present invention are particularly useful. 
         FIG. 2  is a diagrammatic view of a chart of absorption spectrum for silicomolybdic acid. 
         FIG. 3  is a diagrammatic view of an online silica analyzer in accordance with an embodiment of the present invention. 
         FIG. 4  is a flow diagram of a colorimetric method of measuring silica content in a water sample in accordance with an embodiment of the present invention. 
         FIG. 5  is a diagrammatic view of a method  400  for automatically ranging a silica measurement in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
       FIG. 1  is a diagrammatic view of an online silica analyzer with which embodiments of the present invention are particularly useful. Analyzer  100  includes controller  102  that is coupled to air source  104 , pumps  106 ,  108 ,  110 , and  112 . Additionally, controller  102  is also coupled to illumination source  114  and illumination detector  116 . Typically, each pump  106 ,  108 ,  110 , and  112  includes a chemically-inert flexible membrane in a cavity. Cavity volumes are typically 5 mL for sample and standards (pumps  106  and  110 ) and 0.2 mL for reagents (pump  108 ). A vacuum/pressure pump (not shown) pushes and pulls on the membrane. Vacuum causes the chamber to fill. Pressure pushed the liquid out of the cavity into reaction chamber  118 . A number of check valves  120  are provided in order to prevent backflow. When mixing of the sample/reagent/standards is desired, controller  102  engages air source  104  to pump a quantity of air into reaction chamber  118  in order to mix the contents therein. After a suitable reaction time has passed, the treated sample is pumped, using pump  112 , to measurement cell  122 . Once the mixed sample is provided within measurement cell  122 , controller  102  engages illumination source  114  to direct light through the mixed sample toward detector  116 . In accordance with known techniques, the illumination detected by detector  116  provides an indication of the analyte (silica) in the sample. Controller  102  automatically calculates the absorbance and translates the results into a silica concentration reading. Once the measurement is complete, repeated flushes with fresh sample remove the treated sample from the measurement and reaction cells,  122  and  118 , respectively. 
     Based on the molybdenum blue method, the silica content in water is measured by the color of silicomolybdic acid formed through the wet chemistry process, as set forth above. At 810 nm, the absorptivity of the material is about 0.00035/parts per billion. One difficulty for colorimetric analyzers is to provide a significant measurement range with effective resolution. For example, in silica analyzers there is a desire to provide high sensitivity down to 0.5 parts per billion of silica content while still also being able to provide a silica concentration measurement as high has 5 parts per million (ppm). If the incident light is measured with a photodiode having an output of 100 milliamps, then at 5 ppm the transmitted light will only be 0.05 nanoamps, which is too small to measure. While it would be possible to change the analyzer design by providing an additional path having a different length through which the light passes within the mixed sample, the provision of multiple measurement cells in a silica analyzer is not favored. 
     In accordance with an embodiment of the present invention, a colorimetric analyzer is provided that uses a light source or sources having at least two distinct wavelengths of light. By providing such a plurality of light sources, a single length photometric cell can be used. Light at each wavelength, generally monochromatic such that the light has a single wavelength or extremely narrow band of wavelengths, is used for a different detection range. 
       FIG. 2  is a diagrammatic view of a chart of absorption spectrum for silicomolybdic acid. In the vertical axis, the absorptivity is provided while the wavelength is provided as the horizontal wavelength (in nanometers). As can be seen, silicomolybdic acid has two absorption peaks, one at 810 nm and the other at 670 nm. Also, the absorptivity at 670 nm is approximately two-thirds of that at 810 nm. According to the Beer-Lambert Law, the reduction of absorptivity is equivalent to the reduction of the length of the photometric cell for high concentration measurements. With the reduction of the absorptivity from 0.00035 at 810 nm to 0.00022 at 670 nm, with the same length (such as 3.6 cm) and I 0  of 100 milliamps, the current (I) at 5,000 ppm will be approximately 10 nanoamps, which is a value that can be readily measured. 
       FIG. 3  is a diagrammatic view of an online silica analyzer in accordance with an embodiment of the present invention. Analyzer  200  bears many similarities to analyzer  100 , and like components are numbered similarly. The main difference between analyzer  200  and analyzer  100  is that analyzer  200  includes a second illumination source  202  that is also configured to introduce illumination within measurement cell  122 . Source  202  provides illumination at a different wavelength than that of source  114 . In the illustrated embodiment, source  114  provides illumination at substantially 810 nm while source  202  provides illumination at approximately 670 nm. Accordingly, if the response of detector  116  to illumination from one source is beyond the measurement limits (either too low or too high) the first source  114  can be disengaged and the second source  202  can be engaged in order to detect at a different detection level. For example, source  114  provides illumination at 810 nm. If controller  102  measures the response of detector  116  as being essentially zero current, controller  102  can disengage source  114  and engage source  202  which can provide illumination through measurement cell  122  at 670 nm. Accordingly, the detection limits of analyzer  200  are extended relative to analyzer  100  without requiring multiple measurement cells or cell lengths. While the embodiment illustrated in  FIG. 3  illustrates a pair of sources  114 ,  202 , it is expressly contemplated that additional sources can be provided. Additionally, while a single detector  116  is provided that receives illumination passing through the mixed sample within measurement cell  122 , embodiments of the present invention can also include a second detector disposed proximate sources  114  and  202  in order to directly measure the intensity of illumination prior to such illumination passing substantially through any of the mixture. In this way, embodiments of the present invention can also increase the energy provided to one or both of sources  114 ,  202  and directly compare the incident illumination with the amount of illumination that passes through the mixture. 
       FIG. 4  is a flow diagram of a colorimetric method of measuring silica content in a water sample in accordance with an embodiment of the present invention. Method  300  begins at block  302  where a user or technician selects, using a user interface of the silica analyzer, a detection range of the silica in the water sample. Examples of suitable ranges include 0-50 parts per billion, 0-100 parts per billion, 0-200 parts per billion, 0-250 parts per billion, 0-300 parts per billion, 0-500 parts per billion, 0-1.0 parts per million, 0-2.0 parts per million, 0-2.5 parts per million, 0-5 parts per million, 0-10 parts per million, 0-20 parts per million, 0-30 parts per million, 0-50 parts per million, and 0-100 parts per million. Next, at block  304 , the analyzer engages a suitable source based on the range selected at block  302 . Once the source is engaged, the analyzer utilizes a detector, such as detector  116 , to detect the silica in the water sample, as indicated at block  306  based on the known molybdenum blue method. Next, at block  308 , the analyzer provides an indication of the silica measurement determined colorimetrically based on the concentration of the silicomolybdus acid. The output can be provided locally at the silica analyzer and/or communicated over a suitable process communication loop or segment, or both. 
       FIG. 5  is a diagrammatic view of a method  400  for automatically ranging a silica measurement in accordance with an embodiment of the present invention. Method  400  begins at block  402  where a silica analyzer measures a silica content of silica in a water sample in accordance with a colorimetric technique, such as the molybdenum blue method. Next, at block  404 , the analyzer determines whether the measured silica content is beyond a threshold. If the measurement obtained at block  404  is not beyond a detectable threshold, control passes to block  412  along line  416 , where the analyzer provides the silica output. However, if the detected measurement obtained at block  402  is below a detectable level for the source, control is passed to block  406  along line  408  where the measurement is reattempted using a different source. As set forth above, the different source will have a different wavelength. For example, the first source used relative to block  402  may have light with a wavelength of approximately 810 nm, while the source used at block  406  may have light at approximately 670 nm. Once the second measurement attempt is generated using source  2 , at block  406 , control passes to block  410  where the analyzer determines whether the measurement is still beyond a detectable threshold. If the measurement is no longer beyond a measurement threshold, control passes to block  412  along block  414  where the measurement is provided either as a local output or remotely over a process communication loop or segment. As indicated in  FIG. 5 , if the second measurement attempt obtained at block  406  is still beyond a detectable threshold, control passes to block  418  along line  420 . In this instance, the analyzer can generate an error indicating that the measurement is beyond any detectable thresholds. Further, in embodiments that provide yet another source, such as a third source (having a wavelength of approximately 460 nm) the method can continue with a third measurement attempt, et cetera. 
     While embodiments of the present invention have generally been described with respect to a photometric cell for a silica analyzer using the molybdenum blue method, embodiments of the present invention can be applied to other colorimetric analyzers with wavelengths chosen based on the type of material to be detected. Essentially, any time the dynamic range of the colorimetric analyzer is desired to be extended, the absorption spectrum of the particular analyte of interest can be consulted to determine if one or more additional sources can be used to provide enhanced colorimetric detection. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.