Abstract:
The invention concerns a method and device for automatic cleaning of opto-electronic measuring systems used in process-technology for the analysis of substances in liquids and gases by means of optical absorption and fluorescence. The method is characterized in that the soiling degree is detected by the opto-electronic sensor system itself and, with predetermined soiling degree, a cleaning liquid cleans the soiling-sensitive optical components and detects the residual soiling, which is then evaluated quantitatively and compensated for in the signal analysis.

Description:
This application claims Paris Convention priority of DE 199 17 632 filed Apr. 19, 1999 the complete disclosure of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     Conventional opto-electronic sensor systems for the analysis of substances, e.g. using absorption or fluorescence measurements, have the decisive advantages of selectivity, high sensitivity and the possibility of rapid detection of the respective types and concentrations of substances of interest in liquid, gaseous and solid substance mixtures. Linked with these decisive advantages, however, is the disadvantage that important optical components become soiled, such as e.g. optical measuring cells, optical windows, optical radiation sources and optical sensors. This can lead to measurement errors or total failure of the measurement. Prior art discloses various methods and devices, such as e.g. the multi-wavelength method using one or more reference wavelengths according to the patents P 40 30 959, P 40 30 960, EP 91 102 499, and DE 198 494 847. To compensate for the soiling effect, radiation of different wavelengths is transmitted through the substance mixtures and radiation emitted directly by the radiation sources, at the respective time, is detected by a reference sensor and taken into consideration for evaluating the signal (e.g. see the patents DE 36 15 259, P 41 38 419, EP 0383 072). 
     All of these methods meet practical limitations, since so-called non-gray soiling leads to spectral disturbances which cannot be completely eliminated by the conventional compensation methods and since severe soiling, which occurs in practice, can weaken the light intensities of the various beams to such an extent that the residual light is no longer sufficient for the measurement. This occurs e.g. unavoidably with measurement of isopropanol or isopropanol additive mixtures in the fountain solution of offset printing machines, since the salt content of the paper to be printed, suspended matter from the paper, foreign substances from the paint, chemical additives etc. contaminate the liquid to be measured. This falsifies the optical measurement and eventually prevents carrying it out. 
     Corresponding effects also occur if one tries to measure the substances of interest in the gas located above the liquid phase, i.e. in the so-called “headspace”, since the aerosols generated by the motion of the fountain solution also contain the above-mentioned contaminating and disturbing substances which deposit on the surfaces of the optical components, even when fine pore filters are used. 
     It is therefore the underlying purpose of the present invention to automatically eliminate the physically unavoidable deposit of soiling or disturbing substances on optical components of sensor systems for the quantitative measurement of the concentrations of the substances contained in liquids or gases and solids, to guarantee correct and interference-free measurement at all times. This methodical measurement is difficult since, in particular, the processing technology must not be disturbed. In particular, there must be no noticeable interruption in the measurement and subsequent dosing processes e.g. of isopropanol or additives. Furthermore, effects due to physically unavoidable residual soiling, which can vary with time depending on the type of soiling, must not distort the measurements. 
     SUMMARY OF THE INVENTION 
     Up to this point in time, this difficult object could be solved e.g. neither in offset printing nor in many other practical applications, due to 100% automation requirements, i.e. without any manual operation. The present invention achieves this goal with the cyclic use of cleaning substances and combined application of reference liquids. The invention also provides for timed triggering of the above-mentioned procedures, controlled by the opto-electronic sensor system for substance analysis and with the control of valves for activating the cleaning and reference liquid circuits. These circuits are hermetically sealed with respect to one another and from the process circuit, e.g. the fountain solution circuit of an offset printing machine. 
     Individual embodiments are described in more detail below for illustration of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 shows an exemplary embodiment of an opto-electronic sensor system for substance analysis in liquid mixtures using the fully automatic cleaning and reference liquid system for the case of isopropanol concentration determination in the fountain solution of offset printing machines; 
     FIG. 2 shows an embodiment of an opto-electronic sensor system, including the reference liquid system, for substance analysis of gaseous mixtures with automatic cleaning of the opto-electronic system components; and 
     FIG. 3 shows a special exemplary processing technology embodiment of the automatic cleaning or reference system for the offset printing industry. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In accordance with FIG. 1, a measuring beam  1  and a reference beam  2  pass through the measuring cell  3 , continuously flushed by the process liquid  4  to be examined, and are incident on a common measuring sensor  5  using conventional process absorption spectrometry. In addition, a partial measuring beam  6  and a partial reference beam  7  are detected by a reference sensor  8  originating from the same production charge as the measuring sensors. The signals from the measuring sensor  5  and the reference sensor  8  are converted in a conventional manner, e.g. using an analog circuit  10  or e.g. a microprocessor (neither shown in FIG.  1 ), into the respective concentration signal. This could represent the concentration of isopropanol in the fountain solution mixture of an offset printing machine. When the inner wall  9  is excessively dirtied, the intensity of the reference beam  2  drops below a threshold predetermined by e.g. a threshold value setting means  11 . A cleaning circuit is then activated to transport a cleaning liquid  12  through the measuring cell  3  using a feed pump  13 . The cleaning liquid  12  may contain an alkaline substance, an acid, or a detergent. The duration of the cleaning process depends either on the process requirements, utilizing interruptions in the measurement and dosing process, or by switching on or off the cleaning process when defined limits for the intensity of the reference beam  2  are exceeded or fallen below. It is difficult to calculate the efficiency of the cleaning process in advance, i.e. since, in general, undefined residual soiling cannot be avoided following a respective cleaning. The status of the soiling is therefore detected in accordance with the present Invention by subsequent rinsing of the measuring cell  3  with a reference liquid  14  and evaluation of the light intensities transmitted by the measuring beam  1  and the reference beam  2 . For measurement of the isopropanol concentration or substitute material concentration in the fountain solution of offset printing machines, a reference measurement is carried out using water as the reference liquid  14 , which is directly extracted from the respective water circuit  17  via valves  16 ,  21 . If this measurement does not yield an isopropanol concentration of 0.0% a Vol., the sensor system readjusts automatically to this value by offset correction. In addition to or instead of water (which is normally available in regulation processing), other reference liquids, such as isopropanol/water mixtures of conventional concentration can also be used for calibration. The various circuits, i.e. the process circuit  18 , the water or reference circuit  17 , and the dosing circuit  19  are separated from one another via electromagnetically controlled valves  16 ,  20 ,  21 ,  22  to guarantee, at all times, that the liquid of one circuit is not soiled by the liquids of the two other circuits. The cleaning liquid and the reference liquids are contained in different containers  23 ,  24 . The time-switching of the valves  16 ,  20 ,  21 ,  22  is always effected to prevent cleaning agent from entering into the process circuit, the water circuit and the dosing circuit, with excess water being exclusively supplied to the process circuit. 
     In accordance with FIG. 2, the soiling of an opto-electronic substance analysis system for measuring the concentrations of various substance components in the gaseous phase  25 , also called “headspace”, is kept within narrow limits by means of an automatic cleaning system in accordance with FIG.  1  and operating according to the same method. The gaseous mixture  28 , which is representative for the composition of the substance mixture in the liquid  32 , is analyzed in a conventional manner using one or more light beams  29  generated by at least one light source  30  whose attenuation is detected by an optical sensor  31 . The cyclic cleaning of the optical components is carried out as in FIG. 1 using a cleaning liquid  33  which flows in a conventional manner through the “headspace”  25  via a valve  34 . For reference purposes, the measurement cell room  25  can be flooded with water or a reference liquid  33  of a conventional composition, analogously to FIG.  1 . As an alternative or additionally, the headspace  25  can be rinsed with a reference gas of a defined composition following cleaning. The process liquid is removed from and returned to the process (not shown) in a circuit via conduits  38 ,  39 . When the cleaning liquid  33  is similar or identical to the process liquid  32 , the cleaning liquid can be guided back into the fountain agent container  36 . Otherwise, the cleaning liquid is returned to the cleaning container  37  via an additional valve  35 . 
     In accordance with FIG. 3, the fountain agent  56  is transported, via a pump  40 , from the fountain agent container  55  to the printing group  41  and returned to the fountain agent container  55  via the return conduit  42 . Part of the fountain agent is bypassed to the measuring cell  46  via a pressure reducer  43  and valves  44 ,  45  and returned, via the valve  47  and the conduit  53 , to the fountain agent container  55 . When the cleaning process explained in FIG. 1 is triggered after the predetermined threshold value  11  is exceeded, cleaning liquid  48  is transported by a pump  50 , via the valve  45 , from the cleaning container  49  to the measuring cell  46 . Alternatively, fresh water  54 , e.g. for rinsing purposes, can be supplied to the measuring cell  46  via a pressure reducer  52  and valves  44 ,  45  and transported to the process circuit  42  via the return conduit  53 .