WO2003014712A1

WO2003014712A1 - Method and device for quantitative gas analysis

Info

Publication number: WO2003014712A1
Application number: PCT/EP2002/007841
Authority: WO
Inventors: Frank Eder; Herbert Rosenstiel
Original assignee: Testo Gmbh & Co.
Priority date: 2001-08-06
Filing date: 2002-07-15
Publication date: 2003-02-20
Also published as: DE10138508A1

Abstract

The invention relates to a method and device for quantitative gas analysis. The invention is characterized by the provision of a switch device by means of which the direction of the power supplied to the source of radiation is alternated, thereby substantially avoiding mass transfers in the spiral-wound filament of the radiation source.

Description

description

Method and device for quantitative gas analysis

The invention relates to a method and a device for quantitative gas analysis.

In many applications, there is a need to measure gases, be it in the area of emission monitoring to comply with or check legal limit values, to assess indoor air quality, to check biological activities, etc. For example, the C0 ₂ concentration plays a role in the assessment indoor air plays a crucial role and is therefore used as an indicator.

There are a variety of different measuring methods and measuring arrangements for measuring gases. For example, infrared-optical gas sensors are used to measure the C0 ₂ concentration, since carbon dioxide only triggers a chemical reaction to a limited extent under measuring conditions. Since there are no cross-sensitivities in C0 ₂ measurements with infrared gas sensors, these gas sensors are particularly well suited for measuring the concentration of carbon dioxide. Cross-sensitivities are understood to be the influences of other gases that are not to be examined, but which may be contained in the measuring medium.

Infrared optical gas sensors are designed to measure the concentration of organic and inorganic gases in a sample atmosphere using infrared radiation. They typically work using the absorption spectroscopy method. Multi-atomic, non-elementary gases have the property of absorbing the infrared radiation emitted by the gas sensors, thereby creating an absorption spectrum which is characteristic of the gas to be measured in each case. Every gas has its

RFSTΔTIfil INftSt DPIF sorption maximum, which is about 4.27 μ in the case of carbon dioxide. For a given absorption wavelength, the attenuation of the infrared radiation emitted by the measuring medium is a measure of the concentration of the gas to be examined. This physical property of the gases to be measured is used in the infrared gas analysis.

Figure 1 shows the structure of an arrangement for quantitative gas analysis. The measuring arrangement 1 has a sensor device consisting of a radiation source 2, for example an incandescent lamp, and a detector device 3. Furthermore, a measuring chamber 4 is provided, in which the measuring medium 5 to be analyzed is contained. The radiation source 2 and the detector device 3 are fixed to the measuring chamber 4 in a defined orientation, so that the measuring radiation 6, 6 'emitted by the radiation source 2 passes through the measuring chamber at least once and after it has emerged (X = d) from the measuring chamber 4 of the detector device 3 can be detected.

The spectral location of the radiation 6 with the intensity Ie emanating from the radiation source 2 is selected such that the radiation can be at least partially absorbed by the measuring medium 5 in the measuring chamber. The radiation passes through the measuring chamber of thickness d, a portion of the injected radiation 6 being absorbed by the measurement medium 5, which causes the intensity Ie of the injected radiation 6 to be attenuated. The radiation 6 ′ of intensity Ia <Ie emerging from the measuring chamber 4 then hits the detector device 3 and is measured there. The ratio of emerging radiation intensity Ia to incoming radiation intensity defines the permeability D = Ia / le - or also transmission - of the measuring medium 5 in the measuring chamber 4. The radiation component absorbed in the measuring medium is the absorption A = 1-D. In the known measuring arrangements for optical gas analysis, the radiation required for the measurement is usually generated by means of incandescent lamps. It is essential for the optical measurement that the radiation emitted by the radiation source has a defined, stable intensity. To stabilize this radiation power, a regulating device is provided, by means of which the current or the voltage of the radiation source is regulated. However, this is only possible in DC voltage operation with a justifiable amount of circuitry. In the measuring arrangements known today, the radiation source is therefore supplied with a direct voltage or a direct current.

In direct voltage operation of the radiation source, however, undesirable effects occur, such as the well-known Soret

Effect and the electromigration effect, in the foreground, which change the optical properties of the radiation source with increasing radiation duration:

The Soret effect or the Soret 'see effect is based on a large temperature gradient in the longitudinal direction of the axis of the filament, which leads to mass transfer of the material of the filament. This leads to an increasing inhomogeneity of the surface of the incandescent filament, which manifests itself in a more or less serrated or jagged surface structure. The resulting reduction in cross-section can lead to a break in the incandescent ice or to a change in the optical properties under extreme conditions, for example mechanical shocks or vibrations, preferably in the regions with a smaller cross-section.

The same change in the wire surface can be observed in electromigration as in the Soret effect. However, this effect is caused by a high direct current through the filament, the mass transport being supported by the electrical field located in the longitudinal direction of the filament with a one-sided voltage gradient. Also a change in the surface occurs here. The resulting reduction in cross-section of the glow plug produces a local increase in resistance at the points with a smaller cross-section, and thus also a local temperature increase in the glow plug, which further supports the process of reducing the cross-section of the surface. Under extreme conditions, this also leads to the breaking of the incandescent ice or to a change in the optical properties.

A change in the optical properties automatically also results in a change in the optical imaging properties and the spectral radiation distribution of the radiation source. As a result, the radiation emitted by the radiation source no longer has the predetermined radiation intensity Ie that is required for the measurement, as a result of which the entire measuring system is undesirably misaligned. The measuring system must therefore be readjusted regularly or the radiation source must be replaced. However, this is not always possible, especially with very long measurements. In addition, the constant adjustment of the measuring arrangement is time-consuming and therefore cost-intensive. In the event of a broken glow plug, the entire radiation source must be replaced, which also makes the measuring process significantly more expensive.

The present invention is therefore based on the object of providing a method and a measuring arrangement for optical gas analysis by means of which the optical imaging properties of the radiation source can be kept largely constant.

The method-related problem is solved according to the invention by a measuring method with the features of claim 1, the arrangement-related problem by a measuring arrangement with the features of claim 7. Accordingly, it is provided:

A method for quantitative optical gas analysis by means of a measuring arrangement having a radiation source, in which the radiation source has a current and / or

Voltage signal is supplied, in which the radiation source generates measurement radiation for gas analysis of a sample atmosphere as a function of the supplied current and / or voltage signal, the sign of the supplied current or voltage signal being changed (claim 1);

A measuring arrangement for quantitative optical gas analysis with a closed measuring chamber, in which a sample atmosphere to be measured is introduced, with a sensor device having a radiation source and a detector device, in which the radiation source supplies measuring radiation depending on a current and / or voltage signal supplied to it Gas analysis of the sample atmosphere is produced, in which the measuring radiation emitted by the radiation source can be detected by the detector device after passing through the measuring chamber, with a switching device which alternately signs the current and / or voltage signal supplied to the radiation source changes (claim 7).

The idea underlying the present invention is that the current direction of the current supplied to the radiation source is alternately changed. As a result, the mass transports, which are caused by the effects mentioned at the outset and which in extreme cases even render the radiation source inoperable, are largely avoided. A change in the optical imaging properties and the spectral radiation distribution is thereby minimized, as a result of which the functionality of the radiation source and thus also of the entire measuring arrangement is retained. In addition to a small effort for the adjustment of the radiation source increased the lifetime of the radiation source is also advantageous.

The sign of the current or voltage signal supplied to the radiation source is advantageously changed in a clock-controlled manner, for which purpose a specially provided clock line must be provided, which is connected to a control device and a clock generator. This method is particularly advantageous in continuous measurement mode. In the case of very short measurements, a clock-controlled switchover is often not necessary and, moreover, is too complex. Here, for example, the sign of the current supplied to the radiation source is changed each time the radiation source is switched on or also each time a new measuring process is carried out.

In a particularly advantageous embodiment, the switching device is designed as a full bridge and has at least four controllable switches for this purpose. The controllable switches can be switched on and off via a control device. The controllable switches of the full bridge are typically designed as MOSFETs, but other switches such as bipolar transistors, JFETs or even a relay would also be conceivable here.

In the operation according to the invention, two switches of the full bridge are switched on, while the other two are switched off. The switches that are switched on and off are arranged crosswise. In an advantageous further development, the switches which are arranged crosswise are switched on and off alternately. These switches are typically switched on and / or off fully synchronously.

The radiation source is supplied with a current which is generated by a current source via the full bridge circuit. The current source is typically designed as a resistor or as a MOSFET.

The sample atmosphere contained in the measuring chamber consists of C0 ₂ or a gas containing C0 ₂ . However, the method and the arrangement according to the invention are of course also suitable for the gas analysis of other gases.

Advantageous refinements and developments can be found in the subclaims and in the description with reference to the drawing.

The invention is explained in more detail below on the basis of the exemplary embodiment indicated in the figures of the drawing. It shows:

Figure 1 shows the structure of a generally known arrangements for quantitative gas analysis;

Figure 2 shows a circuit arrangement according to the invention, which provides an alternating current for the radiation source of a measuring arrangement.

Identical or functionally identical features and signals have been provided with the same reference symbols in both figures of the drawing.

A circuit arrangement according to the invention is designated by reference number 10 in FIG. The circuit 10 is arranged between a first connection 11 with a first supply potential VDD, for example the positive supply potential, and a second connection 12 with a second supply potential VSS, for example the potential of the reference ground or a negative supply potential. Furthermore, a current source 13, for example a resistor or a MOSFET, is provided, which is between the circuit 10 and the second connection 12 is arranged and which supplies the circuit 10 with a direct current II.

In the present exemplary embodiment, the circuit 10 according to the invention has four controllable switches 14-17 arranged to one another to form a full bridge 21. The radiation source 2 is connected as a load between the two outputs or the taps 18, 19 of the switches 14-17. Conventional transistors, for. B. MOSFETs or bipolar transistors are used, but other configurations would also be conceivable. The controllable switches 14-17 each have a control connection via which they can be switched on and off. The controllable switches 14-17 or their control connections are controlled via control signals VS1-VS4, which are provided by a control device 20. A microprocessor, microcontroller or alternatively also a logic circuit can be used as control device 20, for example.

The method according to the invention is explained in more detail below with reference to the circuit in FIG. 2. It is assumed here that the arrow directions of the current II in FIG. 2 denote a positive current, while a negative current is denoted by a current counter to the arrow direction:

When switched off, all switches 14-17 are switched off; there is therefore no current II flowing through the radiation source 2. In the operating mode, the switches 14, 17 and the switches 15, 16, which are arranged crosswise, are alternately switched on and off. The two switches 14, 17 and the two switches 15, 16 are each switched on as synchronously as possible. First, the two switches 14, 17 are closed, the switches 15, 16 remain open. A positive current II thus flows through the radiation source. Then the two switches 14, 17 are opened and, in parallel, the other two switches 15, 16 are closed, so that the negative current -II now flows through the radiation source 2. The currents II, -II flowing through the radiation source 2 have the same magnitude, but differ in their signs. The switches 14-17 are controlled via the control device in such a way that the switches 14-17 switch as abruptly as possible and thus the sign of the current II changes as smoothly as possible. Since the radiation source 2 is supplied with an identical current II in terms of amount, the radiation 6 emitted by the radiation source 2 is also identical. The change of sign of the current II therefore has almost no influence on the emitted radiation 6.

While the method just described is designed for continuous measurement operation, the method according to the invention is modified in the case of very short measurement processes in such a way that the change of sign of the current II or the switching of the switches 14-17 takes place when the measurement arrangement 1 is switched off.

The manner in which the gas analysis is carried out on the basis of the emitted measuring radiation 6 is generally known and is therefore not explained further.

A preferred circuit 10 was described with reference to FIG. 2, by means of which the sign of the current II supplied to the radiation source 2 is reversed according to the invention. However, the method according to the invention cannot, of course, be implemented exclusively with an arrangement shown in FIG. 2; rather, other advantageous arrangements for reversing the sign of the current II supplied to the radiation source 2 would also be conceivable here. LIST OF REFERENCE NUMBERS

1 measuring arrangement 2 radiation source

3 detector device

4 measuring chambers

5 measuring medium

6 incoming radiation 6 'emerging radiation

10 circuit arrangement

11, 12 connections of the supply voltage

13 power source

14 - 17 controllable switches 18, 19 taps

20 control device

21 full bridge (connection)

d thickness of the measuring chamber II current through the radiation source

Ia emerging radiation intensity

Ie entering radiation intensity

VS1-VS4 control signals

VSS, VDD supply potentials

Claims

claims

1. Method for quantitative optical gas analysis by means of a measuring arrangement having a radiation source (2)

(1), in which the radiation source (2) is supplied with a current and / or voltage signal (II), - in which the radiation source (2) is a function of the supplied current and / or voltage signal (II)

Measuring radiation (6) for gas analysis of a sample atmosphere (5) is generated, the sign of the supplied current or voltage signal (II) being changed.

2. The method of claim 1, that the sign of the supplied current or voltage signal (II) is changed in a clock-controlled manner.

3. The method according to claim 1, so that the sign of the supplied current or voltage signal (II) is changed each time the radiation source is switched on.

4. The method according to any one of the preceding claims, characterized in that the measuring arrangement (.1) has a full bridge circuit (21), the controllable switch (14-17) via a control device (20) in a first and a second configuration are controlled, in the first configuration at least the two switches (15, 17) of the full bridge (21) directly connected to a current source (13) are switched off and thus no current (II) is supplied to the radiation source (2), in the second configuration, two first switches (15, 16) of the full bridge (21), which are arranged crosswise, are switched off, and two other, second switches (14, 17) of the full bridge (21), which are arranged crosswise , be switched on.

5. The method according to claim 4, so that the first and second switches (14, 17; 15, 16) are switched on and off alternately in the second configuration.

6. The method according to any one of claims 4 or 5, d a d u r c h g e k e nn z e i c h n e t that the switching on and / or off of the first and / or second switches (14, 17; 15, 16) takes place synchronously.

7. Measuring arrangement for quantitative optical gas analysis

with a closed measuring chamber (4) in which a sample atmosphere (5) to be measured is introduced,

With a sensor device having a radiation source (2) and a detector device (3), in which the radiation source (2), depending on a current and / or voltage signal (II) supplied to it, a measuring radiation (6) for gas analysis of the sample atmosphere (5 ) in which the measuring radiation (6) emitted by the radiation source (2) can be detected by the detector device (3) after passing through the measuring chamber (4),

with a switching device (10) which alternately changes the sign of the current and / or voltage signal (II) supplied to the radiation source (2).

8. Measuring arrangement according to claim 7, since you draw rchgeke nn, that the switching device (10) has at least four full-bridge circuit (21), which can be controlled via a control device (20) and has switches (14-17), which is arranged between the connections (11, 12) of a supply voltage source (VDD, VSS) and which a current source (13) for generating a current signal (II) for the radiation source (2) is connected, the radiation source (2) being arranged between the two outputs (18, 19) of the full-bridge circuit (21).

9. Measuring arrangement according to claim 8, so that the controllable switches (14-17) are designed as MOSFETs.

10. Measuring arrangement according to one of claims 8 to 9, since you rchgek characterized that the sample atmosphere (5) has a C0 ₂ containing gas.

11. Measuring arrangement according to one of claims 8 to 10, so that the current source is designed as a resistor or as a MOSFET.