DE3615259A1 - Method and system for the continuous determination of the concentrations of molecular compounds in liquids and gases - Google Patents

Abstract

An improved process is proposed for the continuous measurement of the concentrations of various molecular compounds in gaseous and liquid process streams, in which the optical beams from at least two radiation sources are measured before and after passing through the process medium by at least two detectors and their radiation intensities are determined. In one system operating according to this method, at least four optical beams are used for concentration determination and, besides the formation of ratios of four measured values, in particular the dark currents, i.e. the temperature-dependent signals arriving at the detectors when the radiation sources are blacked out, are included in the evaluation.

Description

The invention relates to a method and a system for continuous Determination of the concentrations of various optically absorbing Molecular compounds in liquid and gaseous mixtures, where preferably optical radiation in the ultraviolet, in the visible, is used in the near infrared and in the infrared range. From the Attenuation of the optical radiation when passing through the die the transparent substance containing the investigating substance becomes the concentration  of the desired substance.

Such systems consist of an optical radiation source, that emits a certain wavelength range, with wavelengths, those of the molecular compound to be determined optically be absorbed, furthermore from an optically transparent cell, which is flowed through by the respective process medium, and a radiation-sensitive detector that is sensitive to changes in the optical Radiation intensity responds and this in an electronic signal converted for further processing.

According to the prior art, so-called process photometers are used for determining the concentration of various optically absorbing substances in gases and liquids. In addition to two-beam systems using pneumatic or gas dynamic detector principles, single-beam / bifrequency or single-beam / gas filter correlation techniques using pyroelectric or semiconductor detectors are known. A very decisive disadvantage of the two-beam photometer on the market is that the inevitable contamination of the optical windows or the wall of the measuring cell by the process medium and other accompanying substances, such as. B. dust, chemical compounds, which also optically absorb in the area of interest, both lead to inevitable zero drifts and drifts of the sensitivity. Although these disadvantages are largely eliminated by the abovementioned single-beam photometers, a further serious disadvantage of the systems according to the prior art is that the single-beam systems, like the two-beam systems, are based on mechanical choppers for radiation modulation. In addition to high production costs, these mechanical chopper systems have unavoidable imbalances which cause a disturbance in the concentricity and thus phase fluctuations, ie jitter effects, of the rotating or oscillating chopper. For this reason, the measuring accuracy and zero point stability of these systems have very unsatisfactory values for many practical applications. In addition, aging and wear effects in the mechanical bearings of the chopper lead to time-dependent varying jitter effects of the chopper and thus to unavoidable time drifts in the measurement signal. Furthermore, the electrical drive and the mechanical mounting of the chopper represent very expensive and very sensitive wearing parts, which seriously question the safety of prior art photometers, particularly when used in process technology. In addition, the photometers according to the prior art have serious measurement errors due to aging or temperature fluctuations of the radiation source. The latter are to a certain extent eliminated by using the bifrequency method or the gas filter correlation technique as a result of the quotient formation of the signals of the measuring and reference channels. However, in the bifrequency method, too, measurement errors occur, for example, when the thermal radiator is slightly cooled, since the intensities of the measurement and reference wavelengths change according to Planck's law of radiation with different factors and therefore the quotient also experiences a change. In addition, another disadvantage to be mentioned in the devices according to the bifrequency method as well as according to the correlation technique is that a beam weakening, e.g. B. due to an unwanted absorption due to contamination effects of the optical window of the measuring cell or by a change in the reflection properties of the cell wall, as such not to be recognized, or from a real absorption of the molecular compound to be measured is indistinguishable and therefore also to incorrect measurements leads. Furthermore, all radiation detectors have a so-called dark effect, ie they also lead to a measurement effect when the beam is darkened by the chopper. This measurement effect is temperature-dependent and therefore generates false signals when the temperature of the detector varies, which simulates a concentration variation of the molecular compound to be measured. The process photometers according to the prior art are therefore unsuitable for solving numerous problems of modern process and process engineering, environmental technology, and for checking workplaces with regard to hazardous substances. The latter is shown quantitatively by the fact that the current systems have a long-term fluctuation in the so-called decadal extinction of at least ± 1 · 10 ^-3 to 1 · 10 ^-2 , while long-term fluctuations of at most 1 · 10 ^{-4 are} permissible for a large number of measurement problems. The decadal extinction E is calculated according to the relationship:

E = ¹⁰ log ( I ₀ / I )

with I ₀ as the unattenuated intensity and I as the intensity weakened by the molecular component to be measured.

Based on this prior art, the invention lies Task to create a method and a system that while avoiding the disadvantages mentioned in particular both high measurement accuracy, d. H. high extinction accuracy, as well as a high long-term stability of the measured values and the zero point, as well as a high signal-to-noise ratio with low Enable costs.

According to the invention, a method is used to achieve the stated object of the type mentioned, which thereby is characterized in that the alternating radiation pulses different wavelength composition not mechanically, but be generated electronically and that the dark value of the Detector is detected at every period and beyond that at least one reference detector is used for monitoring and keeping the intensity of the radiation sources constant emitted radiation is used. In concrete terms This is done by both Measurement wavelength and the reference wavelength is generated and the associated optical radiation passes through the measuring cell and each individual pulse is detected by the measuring detector. Simultaneously, d. H. during each measurement period, the intensities are also the optical radiation generated by the radiation sources from a reference detector in front of the cell  detected. After analog-digital conversion and previous amplification are the four measured values according to the method according to the invention in combination with the dark values of the detectors Microprocessor support used for concentration calculation.

In a preferred embodiment of the method according to the invention the temperature of the process medium during each measurement period determined and included in the evaluation, so errors caused by temperature fluctuations in the absorption coefficient of the relevant molecular compound occur do not falsify the measurement result, in contrast to the systems according to the state of the art.

The immunity to daylight as well as other optical disturbances and electrical hum and noise, especially the so-called l / f noise, can be further increased by switching the radiation sources with high-frequency, modulated light and filtering out the same high-frequency component from the received light , wherein the resonance frequency is filtered in a selective amplifier system, which is followed by a phase shifter, which is preferably followed by a low-pass filter and a voltage-frequency converter.

The method according to the invention is also compared to the systems according to the prior art characterized in that the otherwise usual, taking into account Beer’s law:

I = I ₀ · e ^{- β · c · l}

with β as the absorption coefficient, c as the concentration of the molecular compound and l as the optical path length of the measuring cell, the logarithmic level required is eliminated by the generally nonlinear relationship between concentration and extinction directly as a table of values or as an analytical function, e.g. B. as a polynomial n -degree, where n can be chosen arbitrarily, is stored in a memory which is located in the access area of the electronic signal evaluation.

Further characteristics of the invention are that the measuring and reference detector as well as the radiation sources are always at the same temperature due to a thermal short circuit, so that temperature effects of the detectors are eliminated, and that the distance l between the optical windows of the measuring cell is due to spacers with a small coefficient of thermal expansion, e.g. B. made of quartz glass, is kept constant, so that this is also present in systems according to the prior art error source.

The method according to the invention is furthermore particularly advantageous Way through both digital adjustment options Sensitivity (K-factor) as well as the zero point are supported, so that the usual, caused by rotary potentiometers There are no drift problems.

In addition to light emitting diodes according to the prior art, radiation sources include, in particular, monolithically constructed double light emitting diodes, which can be excited to emit several spectral ranges, and discharge lamps, such as, for. B. mercury low-pressure or high-pressure discharges, deuterium lamps, atomic absorption lamps, and discharge lamps with phosphors for converting short-wave light into longer-wave light, furthermore pulsed lasers with emissions in the entire spectral range, in particular also in the infrared range, for example by CO ₂ laser or helium / neon laser is accessible, in question. In addition, pulsed Planck radiators with extremely low heat capacity, ie a short response time, can be used. Overall, therefore, according to the invention, an inexpensive, simple, trouble-free and high-precision concentration determination is created in online operation for gaseous and liquid media.

All disadvantages of the previously available process photometer systems are eliminated by the subject of the invention. Contribute to this in particular at that for measuring and calculating the concentration  of the molecular compound of interest four optical rays are used, the intensity of which is initially related to the Dark value of the respective detector is corrected. The four Rays come from two light sources of different wavelengths of light, one wavelength being chosen to be of of the molecular compound to be examined is absorbed while the other wavelength does not weaken the process medium happens. According to the invention, one beam of each wavelength is used both before and after the absorption cell detected by one detector each. From the regarding Measured values corrected for the dark value become a double quotient formed which is the quotient of the product of the intensity of the non-absorbed radiation behind the measuring cell and the intensity the radiation of the other wavelength in front of the measuring cell as a counter, and the product of the intensity of the absorbed Radiation behind the measuring cell and the intensity of the radiation the other wavelength in front of the measuring cell as the denominator. This process not only eliminates fluctuations in radiation sources and the transmittance of the optical arrangement eliminated, but especially not gray, d. H. wavelength-dependent contamination, which leads to a wavelength-dependent change the transmittance over time, and serious in the systems according to the prior art Cause drifting.

Another very essential feature of the invention The method consists in that in the switched-on state clocked light sources are additionally modulated, the Modulation frequency is significantly higher than the clock frequency. This ensures that interference frequencies through selective filtering the modulation frequency are hidden, resulting in a Increases the signal / noise ratio leads.

Further advantages and features of the invention result from the claims and from the description below, in the embodiments of the invention with reference to the drawings are explained in detail. It shows:

Fig. 1a is a schematic representation of the measuring head and the electronic signal processing of the system according to the invention

Fig. 1B is a block diagram of the electronic signal processing of the inventive system

Fig. 2a is a timing diagram of a measurement cycle

Fig. 2b The individual arithmetic operations during a measurement cycle

Fig. 2c The time intervals for the radiation pulses of the radiation source for generating the measuring radiation

Fig. 2d The time intervals for the radiation pulses of the radiation source for generating the reference radiation

Fig. 3 is a flowchart for processing the signals and to calculate the concentration values of the molecule of interest compound

FIG. 4a One embodiment of the opto-mechanical system according to the invention

Fig. 4b An embodiment of the optomechanical system according to the invention using miniature gas filter cuvettes

In Fig. 1a, the structure of the measuring head 1 and the electronic signal processing 2 is shown. The light from two radiation sources 3, 4 passes alternately through the cuvette 5 , through which the gaseous or liquid process medium to be examined flows through an inflow 6 and an outflow 7 . The radiation then strikes a radiation detector 8 . As light sources come sources that cover the entire optical spectral range, such as. B. Consider sources with emissions in the ultraviolet, visible, near infrared, infrared and far infrared range. Semiconductor pn components, such as, for. B. photodiodes, photo elements, photoresistors and pyroelectric detectors and thermocouples can be used.

The electrical signal of the respective radiation detector 8 , hereinafter also referred to as the measuring detector, is passed on from an amplifier system 9 , which is shown in FIG. 1b, to a first voltage-frequency converter 10 , the current values of which are recorded by a counter 11 and sent to a microprocessor 12 are passed on. Alternatively, the signal of a temperature sensor 13 is passed on to the microprocessor 12 after actuation of a switch 13 a via the voltage / frequency converter 10 and the counter 11 . In this way, influences of the temperature of the process medium on the absorption coefficients of the molecular compound to be investigated can be corrected by calculation and errors in the concentration measurement, as are present in systems according to the prior art, can be avoided. The associated mathematical correction functions or the coefficients of correction polynomials are stored in a non-volatile memory 14 , e.g. B. an EPROM for the respective molecular compound.

In order to rule out that fluctuations in the radiation intensities of the radiation sources 3, 4 lead to errors in the measurement result, the radiation intensities are detected by a reference detector 15 , which is located in front of the cuvette, and via an electronic reference channel 16 , consisting of an amplifier system 17 with a downstream voltage / Frequency converter 18 and counter 19 forwarded to the microprocessor system 12 . The microprocessor system corrects the fluctuations according to the steps indicated in FIG. 3. It is thereby achieved according to the invention that, despite the inevitable fluctuations in the intensities of the radiation sources 3, 4, a precise measurement result is achieved with high long-term stability. Accordingly, inexpensive light sources with low requirements for stabilizing the power supplies of the radiation sources can be used. The light sources are supplied with power via a power supply 20 , which alternately supplies the radiation sources 3 and 4 with energy via a port 21 and two logic gates 22, 23 and causes corresponding, temporally successive radiation pulses. All sources which have emissions in the spectral range between the short-wave ultraviolet and the far infrared, such as B. gas discharge lamps with continuous or linear spectra, laser systems, black emitters, ie glowing bodies, semiconductor light-emitting diodes and laser diodes, and gas discharges with a fluorescent coating. The clock frequency of the radiation source pulse control can be, for example, 2 kHz.

A great advantage of the arrangement consisting of measuring detector 8 and reference detector 15 can be seen in particular in the fact that the radiation sources can be in the transient state, ie that the intensity does not have to be kept constant during the respective radiation pulse. Radiation fluctuations caused by changes in the ambient temperature also have no influence on the measurement result.

The temperature values of the temperature sensor 13 are continuously displayed on a temperature display 25 and the concentration values of the molecular component on a concentration display 26 via an output unit 24 . Moreover, also via the output unit 24 both temperature and Konzentrationsmeßwerte to the standard interfaces 27, 28, for example. B. as V 24 interfaces, provided for process control. The standard interface 27 is provided for the temperature, the standard interface 28 for the concentration. The zero point or the sensitivity of the output signal for the concentration are entered via hexadecimal rotary switches, specifically the zero point via rotary switch 29 and the sensitivity via rotary switch 30 . The input takes place via the input unit 31 , e.g. B. a VIA module to the microprocessor 12 . As an external working memory for the microprocessor, the memory 32 , e.g. B. a RAM used.

Fig. 1b shows an embodiment of the circuitry structure of the amplifier systems 9 and 17 ( Fig. 1a) including the implementation of the control and power supply of the light sources according to the invention. The generated by the radiation sources 3, 4 ( Fig. 1a) on the measuring detector 8 or reference detector 15 electronic signal is first a preamplifier 33 , z. B. supplied to a photovoltaic amplifier, then arrives at an AC voltage amplifier 34 and via an active bandpass filter 35 and a phase shifter 37 that can be adjusted with the potentiometer 36 on a clocked inverter 38 . The inverter 38 converts the negative half-waves of the sine functions coming from the bandpass filter into positive half-waves and thereby generates a pulsating DC voltage of double frequency, which is transformed into a DC voltage in the subsequent low-pass filter 39 . The inversion of FIG. 1b is controlled by the clock of the oscillator 40 in synchronization with the power supplies 41, 42 of radiation sources 43, 44 and through the switch 45 of the inverter stage 38th The line 46 leads to the switching transistor 47 which controls the corresponding amplifier arrangement 48 of the reference channel 49 with regard to the signal inversion. By means of these arrangements according to the invention, all interference frequencies which lie outside the range of the bandpass 35 are eliminated and, moreover, all interference frequencies within the passband of the bandpass 35 which are not in synchronization with the clocked light sources are switched off. This produces a significant increase in the signal-to-noise ratio on the reference and on the measuring channel, so that an extremely high measuring accuracy results. The control of the radiation sources 43, 44 is also carried out by the oscillator 40 , whose signals via a pulse shaper 40 a and a logic gate 43, 43 a in the desired synchronization to the switching transistors 45, 47 of the clocked inverters and to the transistors 50, 51 Power supply units of the radiation sources are forwarded. The center frequency of the bandpass 35 is set via the potentiometer 35 a .

In Fig. 2a to 2d, the timing of activation of the individual components is shown of the procedure. FIG. 2a shows the five time intervals 52, 53, 54, 55, 56, 57, 58 which are run through during a total period 59 of the time control. From FIG. 2b it can be seen that in the time interval 60 the sensitivity setting 30 of the concentration measurement specified according to FIG. 1a and the zero point setting 29 are read into the memory 32 or the microprocessor 12 via the input unit 31 ( FIG. 1a). In the time interval 53 , the radiation values 61, 62 of the measurement wavelength of the radiation source 3 ( FIG. 1 a ) are recorded on the reference detector 15 and on the measurement detector 8 . In the time interval 55 , the corresponding radiation values 63, 64 of the reference wavelength, which is emitted by radiation source 4, are detected both by the reference detector and by the measurement detector. In the time interval 56 , the signal 65 of the temperature sensor 13 is detected ( FIG. 1a). The entire measurement period is concluded after the dark values 66, 67 of the radiation detectors 8, 15 ( FIG. 1a) have been recorded, with the radiation sources switched off. In the switch-on phases 68, 69 , the radiation source 43, 44 are additionally modulated at high frequency. From FIG. 2c it can be seen that the radiation 68 of the radiation source 3 during the time intervals 52, 53 , the radiation 69 of the radiation source 4 during the time intervals 54, 55 are switched on. According to FIG. 2a, after the entire system is switched on 70, the electronic system is set to the initial state 71 and the first measurement cycle is started automatically by an initialization 72 . In the event of a power failure or a fault in the signal sequence, the system is automatically put back into operation via the initialization 72 .

FIG. 3 shows the flowchart for the software control and the computing routines of the microprocessor running in the time interval 59 ( FIG. 2a). First, the associated dark value 66 of the measuring detector is subtracted from the measured value 61 ( FIG. 2) of the measuring wavelength λ _{1 of} the radiation source 3 at the measuring detector 8 ( FIG. 1a). This gives the corrected measured value 73 of the wavelength λ _{1 of} the radiation source 3 . The associated dark value 67 of the reference detector 15 is then subtracted from the measured value 62 ( FIG. 2b) of the measuring wavelength of the radiation source 3 at the reference detector 15 ( FIG. 1a). The corrected reference value 74 of the measurement wavelength of the radiation source 3 at the reference detector 15 is thereby obtained. Subsequently, the measured value 63 (FIG. 2) the reference wavelength of the radiation source 4 g ₂ at the measuring detector 8, the associated dark value 66 of the measurement detector 8 is subtracted. This gives the corrected reference value 75 of the reference wavelength λ _{2 of} the radiation source 4 on the measuring detector. Finally, the reference wave length is measured from 64 λ ₂ of the radiation source 4 at the reference detector 15, the corresponding dark value 67 of the reference detector 15 subtracted. The corrected reference value 76 of the reference wavelength λ _{2 of} the radiation source 4 is obtained at the reference detector 15 .

Subsequently, the corrected measurement value 73 of the measurement wavelength at the reference detector is divided by the corrected measurement value 74 of the measurement wavelength at the measurement detector, and the corrected measurement signal 77 of the measurement wavelength λ _{1 of} the radiation source 3 is obtained . In the same way, the corrected measured value 75 of the reference wavelength λ ₂ at the reference detector is then divided by the corrected measured value 76 of the reference wavelength λ ₂ at the measuring detector. The corrected reference signal 78 of the reference wavelength of the radiation source 4 is obtained . Finally, the corrected reference signal 78 is divided by the corrected measurement signal 77 and the result signal 79 is obtained , the course of which, depending on the concentration of the molecular compound of interest, represents the so-called calibration curve. In a further embodiment of the invention, this signal 79 can be subjected to decadal logarithmization, so that a linear calibration curve results, at least for small concentrations of the molecular compound, according to Beer law. However, the latter is not absolutely necessary, since the exponential course of the calibration curve, which is present without logarithmization, can be stored directly in the non-volatile memory 14 ( FIG. 1a), so that a concentration signal from the microprocessor can be uniquely assigned to each result signal. The procedure described in Figure is repeated every period 59 ( Figure 3). To further improve the signal-to-noise ratio, the mean value is formed from a plurality of successive result signals 79 , with a special embodiment in which a moving mean value formation is preferred, based on the fact that after the last measurement period the new result value is incorporated into the mean value formation during the oldest Value is disregarded, so that the total number of averaged values is a constant.

In a further embodiment according to the invention, the dependence of the result signal on the temperature values of the process medium 6 ascertained with the temperature sensor 13 is stored in the process cell 5 in the memory 14 , so that measurement errors which may be caused thereby are eliminated. Depending on the prevailing temperature, the microprocessor 12 multiplies the result signal with the associated correction value, which is stored in the memory 14 .

In order for non-gray, ie wavelength-dependent contamination of the optical beam path, for. B. contamination of the optical windows of the cuvette 5 , measurement errors due to different attenuation of the radiation from the light source 3 and light source 4 to avoid, is checked at each measurement cycle 59 ( Fig. 3) within the time interval 58 by the microprocessor whether the ratios of the radiation signals each of Light sources 3, 4 on the measuring detector 8 and on the reference detector 15 have not changed compared to previous ratio values. To carry out this procedure, the microprocessor 12 is signaled by a button 80 ( FIG. 1a) that the evacuation of the cuvette 5 or the flushing of the cuvette 5 with non-absorbing gas, e.g. B. nitrogen, is put into operation so that the comparison process can start. If the test shows that one or both ratios of the intensities at the measuring detector 8 and at the reference detector 15 , ie for the radiation source 3 and / or the radiation source 4 , have changed, the respective quotient is provided with a factor by the microprocessor so that the original value results. As a result, according to the invention, non-gray contamination of the windows or other optical components of the beam path is also eliminated, and measurement accuracy and, in particular, long-term stability, which has not been achieved to date.

Fig. 4a shows an embodiment of the optomechanical structure of the measuring head 1 ( Fig. 1a) of the system according to the invention. The radiation intensities of the radiation sources 81, 82 are detected both by the reference radiation detector 83 and by the measurement radiation detector 84 . The wavelength ranges of the emissions of the two radiation sources are selected such that the radiation from the measurement source 81 is absorbed by the molecular compound to be determined, while the radiation from the reference source 82 is not optically influenced by the molecular compound to be measured. In a special embodiment, the light sources 81, 82 and 3, 4 ( FIG. 1a) can be accommodated in one and the same housing. Particularly preferred is a monolithic double light-emitting diode or an infrared radiation system realized as a double metal thread, which is designed as a black radiator and whose threads have a small heat capacity, so that the sources can be pulsed at high frequencies, ie in the range of a few kilohertz .

According to the invention, the optical detectors 83, 84 are made of thermally highly conductive materials, e.g. B. copper connected by means of the brackets 86, 87, 88 so that these detectors have identical temperatures, which significantly improves the stability and accuracy of the measurement. The two clocked radiation sources 81, 82 are also located in a common, highly heat-conducting carrier 85 at an identical temperature. The electronic signal generation of the radiation sources is housed on the board 89 , the electronic signal processing of the detector signals on the boards 90, 91 .

In a further embodiment, the radiation sources 81, 82 are provided with optical narrowband filters 92, 93 in order to filter out the spectral range required for the respective measurement from a predetermined spectral emission distribution. In a further, very important exemplary embodiment, two identical light sources 94, 95 are used. In this variant shown in FIG. 4b, the spectral range which is of interest for the investigation of the relevant molecular compound is filtered out with an interference filter 96 , which covers the radiation areas of both light sources. In addition, two optically transparent miniature cuvettes 97, 98 are connected in series with the measuring cuvette 99 . One of these cuvettes is filled with the gas of the molecular compound to be examined, while the other is evacuated or contains a non-absorbent gas. Since the one cuvette completely absorbs the radiation which is weakened by the molecular connection, the corresponding signal at the measuring detector 100 is not influenced by the molecular connection which is in the cuvette 99 , in contrast to the radiation which passes through the non-absorbing miniature cuvette . As a result of the signal evaluation and quotient formation explained above, an exact measurement result is still achieved in this case even if there is an additional molecular compound in the cuvette 99 , the absorption band of which overlaps with that of the molecular compound to be examined. So-called cross-sensitivities are thus eliminated because, although the rotational-vibration absorption bands of different molecular compounds can overlap one another, the lines from which the absorption bands are composed generally have no overlap, since they are extremely narrow-band.

The housing 106 of the measuring head is a circulating liquid 108 constant temperature, for. B. thermostated via the process medium. Alternatively, a Peltier thermostat which is not described in more detail in FIG. 4 can also be used. The cell for the process medium, the cuvette 99 , is through two optically transparent windows 101, 102 , z. B. from CaF ₂ or ZnSe or other materials. A high vacuum-tight cell is achieved via the elastic seals 103, 104 and the process medium flows through it via supply lines 107 . The constancy of the distance between the optical windows, which strongly determines the measuring accuracy, is inventively achieved by at least three spacers 105 , e.g. B. made of quartz glass or other, less elastic materials with a very low thermal expansion coefficient, guaranteed.

Claims (33)

1. A method for the continuous measurement of the concentrations of optically absorbing molecular compounds in liquid and gaseous process media, the substance mixture to be examined flowing through an optically transparent cell and the molecular compound to be measured preferably emitting light in the ultraviolet, visible, near infrared, infrared, absorbed in the far infrared range, characterized in that at least two electronically clocked light sources are used which generate at least four light beams, two of which penetrate the transparent cell and are subsequently detected and the other two beams are detected in front of the cell. 2. The method according to claim 1, characterized in that after Detection of the respective radiation intensity of the associated Radiation pulse the dark value at the detector in question measured after switching off the pulse and from the intensity values is deducted when the radiation pulse is switched on. 3. The method according to claim 1 to 2, characterized in that the intensity values corrected with respect to the dark value to one another be related, in particular the Intensity ratios of each radiation source before and after of the cell and the quotient of both relationships is calculated. 4. The method according to claim 1 to 3, characterized in that the decimal logarithm of the quotient is formed and this is multiplied by a constant factor that the sought Concentration of the molecular compound results. 5. The method according to claim 1 to 4, characterized in that with evacuated cuvette or flushed with non-absorbing gas Cell or with process liquid in the absence of to be determined molecular connection of the cuvette Ratios of intensities before and after the cell for each Light source can be determined separately and that the values with compared previous conditions and in the event of deviations Multiplying by appropriate factors to the original, d. H. earlier values are brought. 6. The method according to claim 1 to 5, characterized in that the measurement signals applied to the radiation detectors are amplified  will. 7. The method according to claim 1 to 6, characterized in that alternating with the measurement signal at the detector behind the cell the temperature value of the process medium with a temperature sensor detected and passed on to the voltage / frequency converter becomes. 8. The method according to claim 1 to 7, characterized in that the measured temperature value to correct the quotient or the Logarithm of the quotient regarding the dependency of the Absorption coefficient of the molecular compound from the temperature of the process medium is used. 9. The method according to claim 1 to 8, characterized in that the non-linear characteristic of the quotient according to claim 3, 4, 8 is stored in a non-volatile memory and the Quotient formation and the assignment of the values of the quotient to the concentrations done by the microprocessor. 10. The method according to claim 1 to 9, characterized in that the timing of the radiation sources in synchronization with temperature measurement and signal evaluation by the Microprocessor and in synchronization with the clocking of the Assemblies of the amplifier system takes place. 11. The method according to claim 1 to 10, characterized in that the radiation sources are clocked electronically. 12. The method according to any one of the preceding claims, characterized characterized in that the detector pulses corresponding to the Impulse repetition frequency can be filtered out with a bandpass, and that high frequency and low frequency interference is suppressed will. 13. The method according to any one of the preceding claims, characterized in that that the clocked radiation sources during the switch-on phases are modulated at high frequency, the Modulation frequency is significantly higher than the clock frequency.   14. System for the continuous measurement of the concentrations of optically absorbing molecular compounds in liquid and gaseous process media, the substance mixture to be examined flowing through an optically transparent cell and the molecular compound to be measured preferably the light in the ultraviolet, visible, near infrared, infrared, im far infrared range, characterized in that after a preamplification ( 33 ) an AC voltage amplifier ( 34 ) and a bandpass filter ( 35 ), a phase shifter ( 37 ) adjustable on the potentiometer ( 36 ), a clocked inverter ( 38 ), a lowpass filter ( 39 ) and a voltage / frequency converter ( 10, 18 ) follow one another and that such an overall arrangement is provided both for the radiation detector ( 15 ) in front of the cell and for the radiation detector ( 8 ) after the cell. 15. System according to claim 14, characterized in that the bandpass ( 35 ) of the measuring channel and the reference channel ( 48 ) has a center frequency which is substantially higher than the clock frequency of the radiation sources ( 3, 4, 44, 44 a ) and that Modulation frequency of the radiation sources in the switched-on state coincides with the center frequency of the bandpass ( 35 ). 16. System according to claim 14 and 15, characterized in that the clocked light sources ( 3, 4, 44, 44 a ) are modulated at high frequency in the switched-on state and the modulation frequency is substantially higher than the clock frequency. 17. System according to claim 14 to 16, characterized in that the clock generator ( 40 ) is followed by a divider ( 40 a ) which actuates the switches ( 45, 47 ) of the clocked rectifier ( 38 ) and via logic gates ( 43, 43 a ) controls the switching transistors ( 50, 51 ) for the energy supply to the radiation sources ( 44, 44 a ). 18. System according to claim 14 to 17, characterized in that the logic gates ( 43, 43 a ) are controlled via in / out ports. 19. System according to claim 14 to 18, characterized in that the high modulation frequency of the power supply ( 20 ) together with the signals from port ( 21 ) to the logic gates ( 22, 23 ) is given. 20. System according to claim 14 to 19, characterized in that the signal of the temperature sensor ( 13 ) via a switch ( 13 a ) alternating with the signals of an amplifier ( 9 ) of the measurement signals of a measurement detector ( 8 ) to a voltage / frequency converter ( 10 ) can be given. 21. System according to claim 14 to 20, characterized in that the signals from a reference radiation detector ( 15 ) and a measuring radiation detector ( 8 ) via symmetrical gain channels and voltage / frequency converter ( 18, 10 ) are given to counters ( 11, 19 ) , which is followed by a microprocessor ( 12 ) as well as a non-volatile, programmable memory ( 14 ) and a further memory ( 32 ). 22. System according to claim 14 to 21, characterized in that the measured values for temperature and concentration on display instruments ( 25, 26 ) are displayed via an output unit ( 24 ), and are output via current or voltage outputs ( 27, 28 ) and that the same unit ( 24 ) the zero point and sensitivity are calibrated by means of digital setting switches ( 30, 29 ). 23. System according to claim 14 to 22, characterized in that the microprocessor ( 12 ) is addressed via a button ( 80 ) and a port ( 21 ). 24. System according to claim 14 to 23, characterized in that the counters ( 11, 19 ) and the ports ( 21, 24 ) each form a unit 25. System according to claim 14 to 24, characterized in that the radiation detectors ( 83, 84, 100, 100 a ) are brought into thermal contact via copper connections ( 86, 87, 88 ) 26. System according to claim 14 to 25, characterized in that optical windows ( 101, 102 ) of the process cell are kept at a constant distance by spacers 105 . 27. System according to claim 26, characterized in that the spacers ( 105 ) consist of quartz glass. 28. System according to claim 14 to 27, characterized in that the clocked radiation sources ( 81, 82 ) with optical filters ( 92, 93 ) are provided 29. System according to claim 14 to 28, characterized in that electronic circuit boards ( 89 ) or circuit boards ( 90, 91 ) are connected to one another by means of electrical cables via a bore made in the cuvette holder ( 86 ). 30. System according to claim 14 to 29, characterized in that two pulsed radiation sources ( 94, 95 ), an optical filter ( 96 ) and two gas cells ( 97, 98 ) are connected downstream. 31. System according to claim 30, characterized in that one gas cell ( 97 ) is filled with the gas of the substance to be examined, while the other gas cell ( 98 ) is evacuated or contains a non-absorbent gas. 32. System according to claim 14 to 31, characterized in that the measuring head ( 106 ) with the process medium ( 107 ) is flowed through via pipes ( 109 ). 33. System according to claim 14 to 32, characterized in that the ports ( 21, 24 ) and the counter ( 11, 19 ) in direct connection with both a microprocessor ( 12 ) and a programmable memory ( 14 ) and one Memory ( 32 ) with free access