RU2287803C2 - Multiple-component ir-range gas analyzer - Google Patents

Abstract

FIELD: measuring technique.

SUBSTANCE: IR gas analyzer can be used for measuring concentrations of gases in multiple-component mixtures. Multiple-component IR-range gas analyzer has electromagnet radiation source made in for light-emitting diode array, which irradiates reference and working waves. Light-emitting diode array is connected with excitation current pulse oscillator which is synchronized by microprocessor. Radiation of reference and working waves first enters additional pyroelectricity-type photoreceiver, and then it passes through gas dish and enters basic pyroelectricity-type photoreceiver. Outputs of any pyroelectric photoreceiver are connected with signal processing unit through pre-amplifiers and sync detectors. Signal processing unit has analog-to-digital converter. Output of analog-to-digital converter is connected with input of microprocessor. One output of microprocessor is connected with digital-to-analog converter, which is connected with light-emitting diode array's current pulse forming circuit to adjust intensity of radiation of light-emitting diodes of array according to preset algorithm. Three other rest outputs of microprocessor are connected with inputs of light-emitting diodes' current pulse forming circuit to control operation of light-emitting diodes according to certain sequence.

EFFECT: improved precision of measurement.

5 dwg

Description

The invention relates to the field of measuring equipment, namely, devices for determining the concentration of multicomponent gas mixtures in the atmosphere of a residential area, industrial premises, etc. both in the operator maintenance mode and in the remote access mode via switched data transmission lines.

A known gas analyzer containing optically coupled radiation source, a spectral modulator, optical filters for extracting the wavelengths of the working and reference radiation, a gas cuvette, a focusing system, a radiation receiver unit, a synchronization unit and a control unit, signal processing (SU, No. 1674621, G 01 N 21/61, publ. 02.20.95).

The disadvantage of the above gas analyzer is the use of an optical-mechanical modulator in the design of the device, which reduces the accuracy of determining the concentrations of the gas components of the test mixture due to the appearance of mechanical vibrations of the optical nodes, which affects the accuracy of the measurements.

A known optical absorption gas analyzer containing a gas cuvette, optical filters, photodetectors, a signal processing unit, two sources of electromagnetic radiation with a first working wavelength λ ₁ corresponding to the absorption region of the measured gas, and a second reference wavelength λ _{2 the} corresponding transparency region for the measured gas (reference radiation), and the first source of electromagnetic radiation is located in front of the gas cell along its radiation, and the second source of electromagnetic radiation installed outside the gas cell behind the photodetectors, interference filters are introduced that transmit electromagnetic radiation with wavelengths λ ₁ and λ ₂ , installed in pairs on two opposite sides of two photodetectors, the outputs of which are connected through an amplifier to a signal processing unit containing an analog-to-digital converter (ADC) microprocessor (MP) and an indication device (UI), while the ADC output is connected to the MP input. (RU No. 2109269, G 01 N 21/61, publ. 20.04.98).

The disadvantage of this gas analyzer is the difficulty of tuning, the decrease in measurement accuracy due to the temperature shift of the maximum radiation intensity and, accordingly, the working wavelengths of two independent radiation sources, as well as the fundamental impossibility of multicomponent gas analysis due to the use of fairly narrow-band IR LEDs.

Known pyroelectric flow detector for use in an infrared gas analyzer (US 6320192, publ. 20.11.2001). In this infrared gas analyzer, in addition to the main cell, two additional compartments are arranged coaxially and are filled with gas having the same absorption characteristics as the measured gas in the main cell. Both additional volumes are connected to each other through a gas channel in which a pyroelectric detector is integrated. The source of infrared radiation is modulated by a mechanical modulator.

The disadvantage of the above IR gas analyzer is the complicated design of the measuring gas cell, the use of an optical-mechanical modulator in the device design reduces the accuracy of determining the concentrations of the gas components of the test mixture due to the appearance of mechanical vibrations of the optical nodes, which affects the reliability and rigidity of the structure.

Known non-dispersive multichannel infrared gas analyzer (RU No. 2187093, G 01 N 21/61, publ. 10.08.02) containing a source of electromagnetic radiation with the presence of reference and working wavelengths, interference filters for separating the reference and working wavelengths, located at the radiation of the cell with focusing lenses at the input and output, the main resistive-type photodetector, installed behind the cell, for receiving radiation of the reference and working wavelengths from the source, the output of which is connected through an amplifier to the signal processing unit, containing m ADC, MP (microprocessor) and CI (display device). The IR radiation source is mounted on a Peltier thermoelectric refrigerator and is an LED matrix containing LEDs for generating pump radiation exciting photoluminescent converters, and interference filters to extract the reference and working radiation wavelengths, and as a part of the IR radiation source (RU 2208268, H 01 L 33/00, published July 10, 03) an additional photodetector is included for detecting pump radiation, LEDs are connected to a pump pulse generator of an LED matrix, synchronized microprocessor, and the Peltier refrigerator is connected to an additionally installed thermal stabilization unit. In this case, the main photoresistive photodetector is mounted on an additional Peltier thermoelectric refrigerator.

The disadvantage of the above gas analyzer is the use of a photoresistive photodetector, which, as you know, has an uneven spectral sensitivity characteristic. Using a Peltier refrigerator increases sensitivity, but does not solve the problem of temperature stability over a wide temperature range. The larger the temperature range, the greater the dynamic range a Peltier refrigerator should have. This increases the energy consumption of the gas analyzer. The strong spectral nonlinearity of the photoresistor in the region of 2-5 μm reduces the measurement accuracy when using differential methods of signal registration, which can significantly increase the detection ability of a non-dispersive multichannel IR gas analyzer.

As is known, differential techniques involve comparison in the process of measuring an unknown concentration of a determined component with a reference point, moreover, the comparison should be performed either simultaneously on spatially separated photodetectors, or on a single photodetector with time division of the inclusion of infrared radiation sources at different wavelengths.

The technical solution is based on the task of creating a non-dispersive multichannel IR gas analyzer, in which, using a semiconductor IR emitter made in the form of an LED matrix, in which the short-wave radiation of the pump is converted by means of photoluminescent film converters into infrared radiation, which coincides with the absorption bands of the detected components of the gas mixtures, pyroelectric detectors are used that have uniform spectral sensitivity in the region whith 2-10 microns, which improves the accuracy of determining the concentration of the components of the multicomponent gas, and thus, expansion of the application field is achieved by increasing the accuracy of determining the concentration of the multicomponent gas components.

The above goal is achieved due to the fact that in a non-dispersive multichannel gas analyzer containing an electromagnetic radiation source, which is made in the form of a semiconductor source of infrared radiation and is an LED matrix, with the presence of reference and working wavelength emitters generated by interference filters (RU No. 2208268, H 01 L 33/00, published July 10, 03), a gas cell located along the source radiation, the main pyroelectric type photodetector installed behind the cell, for receiving op orna and working wavelengths from the source, an additional pyroelectric type photodetector installed in front of the cuvette, on which part of the radiation of the LED matrix is allocated, for receiving the reference and working wavelengths. The semiconductor source of infrared radiation is connected to the current pulse generation circuit of the LED array synchronized by the microprocessor. In this case, the main and additional pyroelectric type photodetectors are connected via pre-amplification and useful signal extraction circuits to the microprocessor, which synchronizes the operation of all nodes.

An increase in the accuracy of determining the concentration of the measured components is achieved through the use of pyroelectric detectors as the primary and secondary photodetectors, a certain method for controlling the LED matrix of LEDs, and a method for measuring the concentration of each gas component of the mixture.

The use of a pyroelectric photodetector is determined by its two fundamental properties:

a) an almost uniform spectral characteristic of sensitivity in the region of 2-10 microns;

b) the pyroelectric receiver responds only to changes in the incident thermal radiation. The last property of pyroelectric receivers allows us to construct a measuring circuit equivalent to a two-channel two-beam differential circuit.

Let the light intensity at the working wavelength corresponding, for example, to the CO ₂ absorption line, be equal to I _CO2 , and in the reference channel I _ref ; the transmission (or aperture) of the optical system (cuvette) at the working wavelength is a _p , and in the reference channel is a _ref , the sensitivity of the photodetector S _p and S _ref, respectively. If there is no detectable component in the cuvette in the mixture, it is necessary and sufficient to achieve the following conditions:

If there is a detectable component in the working cell for which the equivalent layer thickness is:

where C is the concentration of the determined component in the mixture; l _p is the length of the cell; P is the total pressure in the cell; P _n - normal atmospheric pressure, the signal from the photodetector at the input of the measuring device is:

where K (λ) is the spectral absorption coefficient;

Δλ is the passband of the interference filter corresponding to the absorption band of CO ₂ .

The invention is illustrated in figures 1, 2, 3, 4, 5.

Figure 1 is an optical diagram showing the relative position of the optical elements of a multicomponent IR gas analyzer.

Figure 2 shows a block diagram of a multicomponent IR gas analyzer (1 - matrix of LEDs; 2 - main pyrodetector PYR-1; 3 - additional pyrodetector PYR-2; 4 - circuit for generating current pulses of matrix LEDs; 5 - microprocessor with ADC; 6 - DAC; 7 - synchronous detector of the reference channel; 8 - synchronous detector of the measuring channel; 9 - indicating device; 10 - gas cell).

Figure 3 - description of a variant of implementation of the working cycle of a multi-component gas analyzer.

Figure 4 presents a timing diagram corresponding to the duty cycle of a multicomponent gas analyzer.

Figure 5 - the relative position of the emission spectra of the photoluminescent structure, the LED emitters of the LED matrix and the transmission of interference filters.

Consider the principle of operation of a multi-channel IR gas analyzer using the example of measuring 2 gas components, for example, Н ₂ О (2.6 μm) and СО ₂ (4.26 μm).

The IR radiation source 1 (FIG. 1) is a matrix of light emitting diodes emitting light with wavelengths of 2.6 μm (LED-H ₂ O), 3.9 μm (LED _ref ) and 4.26 μm (LED-CO ₂ ). Narrow-band radiation with the indicated wavelengths is formed using narrow-band interference filters mounted directly on the emitting LEDs. (Figure 5 shows the mutual arrangement of the spectra of the photoluminescent structure of each of the LED emitters of the matrix and the transmission of interference filters.) The light is sent to a cuvette 2 containing two focusing spherical mirrors 5, at the entrance of which a translucent is installed in a special compartment sealed using window 6 mirror 7, which directs part of the infrared radiation of the LED matrix to an additional pyrodetector 4 (PIR-2). The infrared radiation transmitted to the cell is directed to the main pyroelectric receiver 3 (PIR-1).

In accordance with the algorithm of FIG. 3, the timing diagram of FIGS. 4a, b, c shows the clock sequences of current pulses F1, F2, F3 controlling the respective light-emitting diodes (LED _ref , LED-H ₂ O, LED-CO ₂ ). On figg, f shows the dynamics of the total light flux at the input of the additional (PIR-2) and the main (PIR-2) pyrodetectors. On fig.4d, g electrical signals at the output of the additional and main pyrodetectors, respectively.

Description of the operation algorithm presented in figure 3.

Phase 1

In the first phase of the duty cycle, the input of both receivers receives the radiation of only one matrix LED - LED _ref , while the current pulse generation circuit (SITF), controlled by the digital-to-analog converter (DAC) signal, sets the current amplitude of the LED _ref so that the signal level of the main pyrodetector (PIR-1) turned out to be equal to the predetermined value recorded in the processor memory.

Phase 2

In the second phase of the duty cycle, two light fluxes from the LED-CO ₂ and LED _ref (or rather part of them), which passed through the cell with the detected components, are sequentially fed to the input of the PIR-1 pyrodetector, and the same two light flows consecutively to the PIR-2 receiver flow reflected by the divider 7 (figure 1), which did not experience absorption.

In the first period of this phase (transient), the SITF regulates the current amplitude of LED-CO ₂ and sets it so that the signal at the output of the additional PIR-2 pyrodetector is equal to zero. For a pyrodetector with a “flat” spectral sensitivity characteristic, this mode is realized only when the light powers of LED-CO ₂ and LED _ref at the PIR-2 input are equal to each other.

In the second period of this phase (steady state process), the signal amplitude is measured at the main PIR-1 pyrodetector. Since the light powers of LED-CO ₂ and LED _ref were aligned in the first period, then in the absence of the measured components in the cuvette at the input of the PIR-1 pyrodetector, the total radiation flux from LED-CO ₂ and LED _ref will be constant and, accordingly, the signal at the output of the PIR-pyrodetector 1 will be equal to zero. If a sample gas (CO ₂ ) is present in the cuvette, it absorbs the radiation of the LED-CO ₂ LED and does not absorb the radiation of LED _ref. in this case, an imbalance in the light powers of LED-CO ₂ and LED _ref appears and the luminous flux at the input of the PIR-1 pyrodetector turns out to be modulated (Fig. 4f), respectively, a nonzero signal appears at the output of the main PIR-1 pyrodetector (Fig. 4g).

Phase 3

The third phase of the duty cycle is completely analogous to the second phase, with the only difference being that the SPIT regulates the current amplitude of LED-H ₂ O and sets it so that the signal at the output of the additional PIR-2 pyrodetector is equal to zero.

Then the whole process is repeated.

Obviously, in the absence of the measured components in the cuvette, for various reasons, it is possible that the radiation flux at the input of the PIR-1 pyrodetector will not be constant and, accordingly, the signal at the output of the PIR-1 pyrodetector will not be zero. This difference may be due to the non-identical optical paths for light fluxes from LED-CO ₂ , LED-H ₂ O, LED _ref , as well as the spectral dependence of the reflection or transmission coefficients of the optical elements of the cuvette. But this factor can easily be eliminated by preliminary calibration with a cuvette filled with dry nitrogen. The value of the measured signal by the PIR-1 pyrodetector is stored and used in subsequent measurements as a calibration constant.

With this algorithm of the gas analyzer, the following relationships can be written:

In phase 1:

where: Sign is the signal value stored by the microprocessor;

U _{pyr1 ref} - signal from the reference LED on the main pyrodetector PIR-1;

P _ref - power of the LED _ref LED at the input of the main pyrodetector PIR-1;

S _pyr1 is the volt sensitivity of the main PIR-1 pyrodetector for a given wavelength.

Phase 2 Period 1

Hence,

Phase 3 Period 1

hence

Phase 2 Period 2

where C is the concentration of the measured component,

k (λref) is the absorption coefficient for a given measured component at a given wavelength.

Since λ _ref is chosen, so that the absorption coefficient for the measured components is zero, and using (9), expression (10) can be reduced to

Similarly for phase 3, period 2:

The optical scheme and this operation algorithm corresponds to the differential method for determining one or more gas components, and, up to the coefficients, coincides with expression (3), while the condition of zero stability and gas analyzer sensitivity is automatically satisfied.

Conditions for the implementation of the device and algorithm:

- IR radiation source - the LED matrix must have high speed of each LED, so that when the power supply is pulsed currents in accordance with Fig.4.a, b, c, the total power is constant.

- The frequency band of the pyroelectric receivers should be wide enough (at least 100 Hz. Otherwise, the total time of each measurement cycle of all measured gas components becomes too large. At the same time, the sensitivity of the receivers should also be high to ensure the required accuracy of the adjustment of the LED currents in the matrix.

- Pyroelectric receivers should have a uniform - "flat" spectral characteristic, independent of external conditions (in particular, ambient temperature).

- Pyroelectric receivers, like IR matrix LEDs, must have a sufficiently wide dynamic range to ensure the required adjustment during long-term operation.

The multi-component gas analyzer of the infrared range shown in Fig. 2 contains a source of infrared radiation - an LED array 1, an optical cuvette with a test gas 2, a primary pyroelectric type 3 detector, an additional pyroelectric type 4 detector, a current pulse generation circuit of the LED array 11, a microprocessor with a built-in (ADC) 12, digital-to-analog converter (DAC) 13, synchronous detectors of the main channel 9 and additional channel 8, respectively including pre-amplifiers, an indie device and formation of the output signal 10.

Multicomponent gas analyzer IR range operates as follows.

From the microprocessor 12, control pulse sequences F1, F2, F3 are supplied to the current pulse generation circuit of the LED matrix 11, which determine the time parameters of the current pulses of the LEDs of the matrix 1 and their sequence. The amplitudes of the pulses of the currents of the LEDs of the matrix are independently regulated for each of the LEDs in magnitude using the DAC 13, which sets the corresponding voltage levels at the input of the current pulse generation circuit of the LED matrix 11, the codes of which are generated by the microprocessor 12 according to the control algorithm of FIG. 3. The pulses of the pump current I supplied to the LED matrix 1, and the corresponding radiation pulses are shown in figure 4. The pulses of infrared radiation enter the cell 2 with the test gas and are recorded by the pyroelectric type 3 main photodetector. Part of the radiation at the inlet of the cell is branched to an additional pyrodetector 4. The signals from the pyrodetectors 3 and 4 are fed to the input of synchronous detectors 9 and 8, which are controlled by a microprocessor - 12 clock sequence F1. The use of synchronous detectors increases the sensitivity and increases the signal-to-noise ratio, which, in turn, allows one to lower the detection threshold of the measured components of the gas mixture. In the mode of measuring unknown concentrations of gas components, the microprocessor takes into account the cell transmission constants previously measured during the preliminary calibration and calculates the concentrations. The concentration value determined in this case is displayed on the display device 10 or enters into external circuits in the form of an analog or digital output signal.

Thus, the proposed multicomponent gas analyzer of the infrared range provides high accuracy in measuring all components of the multicomponent gas due to the use of a pyroelectric type as photodetector in combination with sources of infrared radiation in the form of an LED matrix, the main pyroelectric detector registering only a differential signal proportional to the concentration of the measured gas component . At the same time, a significant simplification of the design and increased reliability in operation during long-term operation without operator intervention is achieved.

The specified device can be used to measure both one or several gas components, for example (except for the above described CO ₂ and H ₂ O) simultaneous measurement of CO ₂ , CH, CO.

Claims (1)

A multi-component infrared gas analyzer containing an electromagnetic radiation source in the form of an LED matrix emitting a reference and working wavelengths, a gas cell located along the radiation path, a main photodetector installed at the output of the cell for receiving reference and working wavelength radiation, and a signal processing unit comprising an analog-to-digital converter (ADC), the output of which is connected to a microprocessor, an indication device, characterized in that an additional photoconductor is installed at the entrance to the cell a receiver, in this case, pyroelectric type photodetectors are used as both photodetectors, which are connected through pre-amplifiers to the inputs of synchronous detectors controlled by a microprocessor, the outputs of synchronous detectors are connected to the inputs of an ADC of a microprocessor, one of the outputs of which is connected to a digital-to-analog converter, which is connected to a pulse-forming circuit current connected to the LED matrix to adjust the radiation intensity of the LEDs of the LED matrix, and three the other outputs of the microprocessor are connected to the inputs of the pulse current generation circuit of the LED matrix to control the LEDs in a certain sequence.

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