DE3623345A1 - Method for selectively measuring the concentrations of IR to UV radiation-absorbing gaseous and/or liquid components in gases and/or liquid substances, and device for carrying out the method - Google Patents

Abstract

The invention relates to a method for selectively measuring the concentrations of IR to UV radiation-absorbing gaseous and/or liquid components in gases and/or liquid substances, and a device for carrying out the method. The radiation of a light source 1 traverses a measuring chamber 2, through which the medium to be investigated flows, and is subdivided into a measuring beam 8 and a reference beam 8a by means of a filter combination 9, 10. A chopper 5 ensures that the measuring beam 8 is alternately focused together with the reference beam 8a onto a detector 4 by means of a lens 3. The characteristics of a pyroelectric detector 4 are utilised for directly imaging the difference signal of the alternating radiation, and this substantially simplifies subsequent signal processing in conjunction with improved sensitivity and higher accuracy in comparison with the previous methods. <IMAGE>

Description

The invention relates to a method for selective measurement the concentrations of IR to UV absorbing gaseous and / or liquid components in gases and / or liquid substances according to claim 1 and a device to carry out the procedure.

The use of intermittent rays is in the optical Measurement technology is common and happens primarily to separate the actual useful signals from the interference signals, the interference signals having a different frequency than that Have useful signals. The periodic interruption of the Blasting is usually done using a chopper, e.g. B. in a mechanical design from one in the beam path arranged rotating disk with openings consists.

With the methods of the type mentioned above, the measured value obtained from the comparison of two beam currents, namely a measuring beam and a reference beam. They are procedures and measuring devices known, in which the beam currents geometrically go through separate paths or where the two beam currents follow the same path in time run through (temporal beam splitting) and onto one Detector can be directed. The disadvantage of this method is in that at the required chopper frequencies a temporal overlap of successive signals occurs if the detectors themselves or those connected downstream Electronics a comparatively large time delay possess which measurement error causes or the measurement makes impossible. Since both for the measurement signal and the same detector is usually used for the reference signal the relevant signals must be in an evaluation device be separated from each other. Like the influence  successive detector signals on the measurement result can be eliminated is in DE-PS 27 27 976 specified. In an extensive measurement processing facility are those that arise during the light period Detector signals evaluated. Both the one described therein analog as well as digital solution is complex and expensive.

From DE-PS 31 37 658 a device for measuring the concentration of a gas in a gas matrix or a substance dissolved in a solvent with temporal beam splitting is known, in which measuring and reference filters are moved alternately through the beam path according to the known chopper method. The detector signals with the single and double angular frequency ( ω and 2 l ) of the chopper modulation are processed in a complex measured value processing device which includes a Fourier analyzer, a divider or quotient images and a display device.

The methods used in the prior art for Evaluation of the measurement signals supplied by the detector the fundamental disadvantage of handling large numbers on what affects the measurement accuracy to be achieved negatively.

At small concentrations, e.g. B. in the ppm range, to be able to detect liquid or gaseous components precisely and with long-term stability, the extinction E

can be calculated from the intensity I _REF and I _MESS of the reference signal and the measurement signal accordingly accurately and with long-term stability. With the extinction of E = 10 ^-4 to 10 ^-3 , which is common in the ppm range, this means that the signals ln I _REF , ln I _MEAS must be determined with at least 4 digits.

In practice, this has the consequence that only those semiconductor components (amplifiers, logarithmers, etc.) which correspond to the latest state of the art in semiconductor technology can be used in current analog technology. In addition, the critical components must be thermostatted. These demands lead to high costs of the overall system. Nevertheless, a measurement in the range between E = 10 ^-4 and 10 ^{-3 is} not possible for process engineering applications, even when using the latest analog signal processing, if one ignores the few applications in the near infrared range.

When transitioning to digital signal processing, e.g. B. using a microprocessor, the problem of high-precision analog / digital conversion.

The required high-precision representation in at least 4 digits after the comma leads to the use of the currently fastest and most expensive A / D converter with converter frequencies in the megahertz range.

The invention is based on the object of a method develop with which the measuring sensitivity improves and the system costs for the measuring device are reduced.

This object is achieved by means of the characteristic Part of claim 1 specified process steps and the characterizing part of those specified in claim 5 Device solved.  

The remaining claims give advantageous developments and Embodiments of the method according to the invention and the Arrangement for performing the same.

In the method according to the invention is advantageous Way the previously disturbing and undesirable property of pyroelectric detector, the inevitable response time (Inertia) for emitting a measurement signal eliminated, that the difference between the intensity of the measuring beam and the intensity of the reference beam directly at the detector is formed. The considerable amount of circuitry previously required in the simplest for the evaluation of the measurement signals Trap reduced to a single amplifier.

This is achieved in that the measuring beam and the reference beam are alternately interrupted on the way to the detector by means of a specially designed π chopper, so that the difference in intensities is generated directly in the detector element.

In particular, according to Eq. (1) the extreme to make a small difference between two large numbers, d. H. e.g. B .:

ln I _REF = 9.9999 Volt - ln I _MESS = 9.9998 volts (2)

so

E = ln I _REF - ln I _MEAS = 0.0001 volt = 1 · 10 ^-4 volt.

By amplifying the difference signal, e.g. B. by a factor of 10 ⁺³ , instead of Eq. 2:

I _REF - I _MEAS = 0.1 volt (3)

This means that instead of the precise representation of the numbers 9.9999 and 9.9998 are just a representation of the number 0.1  to a position after the decimal point, with unchanged accuracy of the overall result. The cost of that A / D conversion then becomes negligible or when increasing the precision of the A / D converter is then a corresponding one Possible to increase the measuring accuracy of the overall system.

The invention is described below with reference to exemplary embodiments using FIGS. 1-5. It shows

Fig. 1 the basic principle of π -Chopper system,

FIG. 2 shows the π chopper disk 5 and the filter wheel 5 a according to FIG. 1

Fig. 2a, the filter wheel 5 a in combination with another π chopper 5 b

Fig. 3, the detector signals using a pyroelectric detector,

Fig. 4 shows the detector signals when using a photoelectric detector and

Fig. 5 is a block diagram of the electrical signal processing.

To Fig. 1

The radiation from a light source 1 passes through the measuring space 2 , through which the gas or liquid to be examined flows. Then the radiation z. B. focused by a lens 3 on a detector 4 . In the beam path between light source 1 and detector 4 , in addition to the actual π chopper 5 with the associated offset hole sequences 6, 6 a (openings 7, 7 a), a filter wheel 5 a is arranged, which different combinations ( 9 ₁.. 9 _n , 10 ₁ ... 10 _n ) of optical filter pairs 9 _n , 10 _n or gas filter cells 9, 10 for the respective measuring and reference wavelength of a substance to be measured, whereby a measuring 8 and a reference beam 8 a are formed. By changing the position of the filter wheel 5 a , other wavelength combinations can be introduced into the beam path, so that several gases or liquids can be determined quasi-continuously in succession. The signals generated by the hole sequences 6, 6 a (openings 7, 7 a) have a significantly higher frequency than the discontinuous further movement of the filter wheel 5 a .

According to the invention, the radiation from the π chopper 5 shown in FIG. 2 is chopped periodically. This chopper has a first hole sequence 6 and a second hole sequence 6 a , the first openings 7 and second openings 7 a are offset from one another. The detector sees the signals A, C shown in FIG. 3 , which are assigned to the two hole sequences 6, 6 a and have a phase shift of 180 °. If a pyroelectric detector is used, the output signals are proportional to the temperature change dt / dt of the active layer of the detector. Since the detector 4 forms the sum of the two signals, no signal B appears at its output in the case described above.

Bring two optical interference filters 9 a and 10 a into the two partial beam paths 8 and 8 a , the measuring filter 9 a according to FIG. 1 being optically transparent at a light wavelength in which the medium to be analyzed is optically absorbed while the transmission wavelength of the reference filter 10 a is so is chosen that no absorption occurs, the signal C ( Fig. 3) of the bolt circle 6 is weakened. Accordingly, a resulting signal D appears at the output of the detector 4 , which increases with increasing partial density n of the absorbent substance in accordance with the Beers law:

I _MESS = I _REF e ^{- α n · l or .: or and thus The partial density n sought can thus be determined from the known absorption coefficient α of the medium, the optical path length l of the cuvette 2 , the detector signal (I _MEAS - I _REF ) and the known intensity I _{REF of} the reference signal according to equation ( 5 ). In order to drift the reference signal over time, e.g. B. due to drifts in the light source or the detector or by contamination of the optical windows, I _REF is measured at predetermined time intervals by pivoting the beam aperture 11 into the beam path of the measuring beam 8 and taken into account in accordance with the current value in equation ( 5 ). In order to prevent overdriving of the detector 4 , the reference beam 8 a is simultaneously weakened accordingly by a pinhole 12 . Since the radiation intensities and the optical transmissions of the filters are different at the measurement and reference wavelengths, the more intensive radiation channel is weakened with a first beam attenuator 13 until the resulting signal at the detector 4 disappears ("zero point adjustment"). While the beam stop 11 temporarily swiveled into predeterminable time intervals for the determination of I _ref, the first beam attenuator 13 remains after completion of zero point adjustment constantly in the beam. In a specific embodiment of the invention is a second beam attenuator 13 a instead of the first Strahlabschwächers 13 as adjustable unit directly on the filter 5 a is introduced. The second beam attenuator 13 a is rotatably mounted about a pivot point 14 and can be determined by means of an elongated hole 15 and screw 16 . A second special embodiment of the f- chopper is shown in Fig. 2A. The other π chopper 5 b consists of a round disk which has a π / 2 segment cutout 17 . After rectification, the resulting detector signal is passed to an A / D converter and processed further. Alternatively, further processing can be carried out using a lock-in amplifier. It is expedient to use the first and second harmonics as reference signals instead of the fundamental frequency. Instead of thermal (z. B. pyroelectric) detectors, which deliver signals proportional to the time quotient of the temperature of the radiation-sensitive element according to FIG. 3, photoelectric detectors, z. As silicon or PbS detectors can be used. The latter have output signals that are directly proportional to the temporal intensity profile I of the radiation at the detector. The resulting alternating signal can be processed according to the above procedure. The only difference in Fig. 3 is that an alternating signal results, on which a DC level is superimposed. This disturbing DC level is separated by a high pass. The detector signal is shown before and after the high-pass filter in Fig. 4 under A, G and C. A shows the signal-time profile of the measuring beam 8 and reference beam 8 a before the zero point adjustment B shows the situation after the zero point adjustment has been carried out, ie after the intensity of the stronger partial beam has been correspondingly weakened by the attenuator 13 . C shows the signal after passing through the high pass, ie after the DC component has been separated . FIG. 5 shows the block diagram of the electrical signal processing. The differential signal I _meas - I _Ref present directly at the detector 4 reaches the narrowband amplifier 19 via a high-pass filter 18 , the output signal of which is fed via a switch 22 to a first input 24 of a divider 20 . Alternatively, the signal is passed via the changeover switch 22 to a sample hold element 23 which is connected to the second input 25 of the divider 20 . As long as the switch 22 is connected to the sample-hold member 23, is the beam stop 11 in the beam path of the measuring beam while the reference beam 8 a through aperture 12 corresponding to attenuated or the gain of the narrow-band amplifier is adjusted according to the nineteenth According to formula (5), the value +1.00 is added by the adder 21 , the output signal of which is fed to a logarithmizer 26 . By multiplying the logarithmic value by a substance-dependent factor K (calibration factor) with the aid of the amplifier 27 , an output signal is obtained which corresponds numerically to the concentration 28 of the substance to be measured and can be read off from the concentration indicator 28 . The in-phase control of the narrow-band amplifier 19, which is a lock-in amplifier takes place via a light barrier 29 which, 7a of the π -Choppers radiated through the openings. 7 The beam diaphragm 11 and the switch 22 are controlled by a sequence control 30 , which is digital here and is also synchronized by the light barrier 29 .}

Claims (13)

1. Method for the selective measurement of the concentrations of gaseous and / or liquid components in gases and / or liquid substances absorbing IR to UV radiation by means of transmission technology, the intensity of a periodically interrupted measuring beam and a reference beam interrupted with the same period being measured and the difference between them is characterized in that the measuring beam ( 8 ) and the reference beam ( 8 a) are switched alternately with the same period to the detector ( 4 ) and that a pyroelectric detector is used, the properties of which are used for direct difference signal imaging . 2. The method according to claim 1, characterized in that a phase shift of 180 ° is generated between the measuring beam ( 8 ) and the reference beam ( 8 a) . 3. The method according to claim 1 and 2, characterized in that the difference signal continuously with a stored and correction value updated at intervals is provided. 4. The method according to claim 1 to 3, characterized in that the correction value of a with darkened measuring beam and attenuated reference beam measured value is derived.   5. Device for performing the method according to claims 1 to 4 with
a radiation source ( 1 ),
a detector ( 4 ),
a measuring space ( 2 ) arranged in the beam path between the radiation source and the detector and containing the gases and / or the liquid substances,
a chopper disc ( 5 ),
a filter wheel ( 5 a) and
an evaluation device ( 14 ) for the measurement signals supplied by the detector ( 4 ),
characterized in that
a π- chopper disc ( 5 ) is present, which has at least one first opening ( 7 a) defining a measuring beam and at least one second opening ( 7 b) defining a reference beam and the openings are arranged so that the measuring beam ( 8 ) and the Reference beam ( 8 a) alternate to the detector ( 4 ), the output signal of which corresponds to the difference between the intensity of the measuring beam and the intensity of the reference beam and the filter wheel ( 5 a) has a number of cuvettes ( 9, 10 ) that can be inserted into the measuring beam path, each one contain the radiation-absorbing gaseous component and / or have optical interference filters ( 9 a , 10 a) . 6. The device according to claim 5, characterized in that the detector ( 4 ) is a pyroelectric detector. 7. The device according to claim 5, characterized in that the detector ( 4 ) is a photoelectric detector. 8. The device according to claim 5 to 7, characterized in that a beam diaphragm ( 11 ) for determining the correction value in the reference beam ( 8 a) is arranged insertable. 9. Device according to claims 5 to 8, characterized in that a pinhole ( 12 ) for preventing oversteer of the detector ( 4 ) in the reference beam ( 8 a) is arranged insertable. 10. Device according to claims 5 to 9, characterized in that a beam attenuator ( 13, 13 a) is present for zeroing the detector signal. 11. The device according to claims 5 to 10, characterized in that the filter wheel ( 5 a) on a circular path a measuring filter sequence consisting of cuvettes ( 9 ) and / or interference filters ( 9 a) and concentric to this a reference filter sequence consisting of cuvettes ( 10 ) and / or inference filters ( 10 a) . 12. Device according to claims 5 to 11, characterized in that the f chopper ( 5 ) from a disc with a first hole sequence arranged on a circular path ( 6 ) with the first openings ( 7 ) and a radially offset second hole sequence ( 6 a ) with the second openings ( 7 a) arranged on a gap between the first openings ( 7 ). 13. Device according to claims 5 to 11, characterized in that another π chopper disc ( 5 b) is present which has a π / 2 segment cutout ( 17 ) as an opening.