Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
2000-02-17
2002-09-17
Mai, Huy (Department: 2873)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
C356S437000
Reexamination Certificate
active
06452182
ABSTRACT:
DESCRIPTION
The invention relates to a photometer using the non-dispersive infrared spectroscopy method, NDIR method for short, having a measuring cell, an infrared radiator with radiator modulation, the measuring cell consisting of a measurement and comparison chamber, and having at least one optopneumatic detector, in accordance with the preamble of Patent Claim
1
.
The output characteristic of absorption photometers, which also include the NDIR photometer, obeys the Lambert-Beer law. The desired linear relationship between concentration and output current requires electronic measures forlinearization. In addition to pure absorption, however, there is also extinction along the beam path through the cell. To this extent, the measuring range is limited in general by a maximum product of cell length and concentration. Here, pure extinction is the non-selective general attenuation of radiation by gases or solids. Extinction too effects attenuation of the original signal and therefore simulates absorption. To this extent, the cell lengths cannot be selected arbitrarily.
Moreover, optopneumatic photometers are known whose gas-filled detectors can be connected in series, with the result that it is possible to determine two or more components simultaneously with only one measuring cell. However, this procedure fails in the case of very different measuring ranges, because of the abovementioned problem with the output characteristic. This is the case, for example, with the more or less routine analysis of combustion gases. The aim here is frequently to determine the generally low CO concentration (100 ppm) and the high CO
2
concentration (15% by volume). It is usually necessary to construct two beam paths with different cell lengths in order to solve this problem.
Furthermore, there is known from DE 44 19 458 a method for measuring the purity of CO
2
in which the measurement of natural CO
2
is limited to the absorption band f the isotope
13
CO
2
. In this case, the method is tuned and configured exclusively such that only purity measurements of this one measuring component can be carried out.
The problem is thus that it is mostly necessary to use a plurality of measuring cells in the case of a desired measurement of a plurality of components, since the at least two components to be measured occur in different measuring ranges. One example of this is the already known method of using NDIR spectroscopy to measure the ratio of
12
CO
2
and
13
CO
2
in separate beam paths. Since the concentrations to be measured differ by approximately 1:100, each channel is provided with a dedicated measuring cell differing in length. The different lengths are selected in order to render it possible to linearize the output characteristics, which sag in accordance with the Lambert-Beer law. Both cells are charged in parallel with measuring gas. As a result of the measurement, the individual components, and also the quotient of
13
CO
2
/
12
CO
2
are output. Owing to the fact that the cells can be charged with measuring gas in a fashion which is not exactly simultaneous, when a quotient is formed during an online measurement of, for example, respiratory air unavoidable dynamic errors occur which cause large dynamic deviations when conducting online measurement of a proband. Added to this, again, is the overall problem, already outlined, of handling isotope ratios of an at least chemically identical gas component.
Moreover, the two measurement results must, again, be further computed in order to obtain some degree of mutual correspondence, since it is, after all, one and the same measuring gas which is concerned, even if it consists of a plurality of components. The reason why this is so critical is that the NDIR method is an absorption method. That is to say, the higher the concentration in the measuring gas of the component actually to be measured, the higher the specific absorption inside the gas. That is to say, given high concentrations all that remains is a small residual signal which reaches the detector. The remaining radiation intensity is, however, decisive, again, for producing the measuring effect, since the detectors depend in this gas-filled form on the optopneumatic effect. That is to say, with ever larger concentrations the residual signal paradoxically becomes ever smaller, and thus also ever less accurate. By contrast, low concentrations can be measured accurately because owing to the low concentration the specific absorption inside the measuring cell is also correspondingly low, with the result that a relatively high light signal remains to excite the detector. This problem, which essentially reflects the Lambert-Beer law is important in measuring a plurality of components.
It is therefore the object of the invention to render it possible to measure a plurality of components with high accuracy and the smallest possible outlay on apparatus.
According to the invention, the object adopted is achieved by means of the characterizing features of Patent Claim
1
in the case of an NDIR photometer of the generic type.
Further advantageous refinements of the invention are reproduced in the dependent claims.
The aim of this invention is to measure a plurality of components without the disadvantages of the measurement techniques of this type which are currently employed. By contrast therewith, it is to be possible for this measurement to be implemented simply and cost-effectively. The aim in this case is to use only a single measuring cell in order to achieve the same dynamic characteristic for the various measuring components. It is essential here to use a plurality of detectors which are connected in series and measure the individual gas components selectively. The gas components and/or the correspondingly selected absorption bands possible in this case must be selected here such that each detector exercises maximum absorption for the measuring component which it is to measure, and must be correspondingly transparent for the component which is to be detected in the downstream detector. Since the detectors enclose only relatively low gas volumes, the extinctions effected thereby from one detector to the other are negligible or at least known, and therefore capable of compensation.
The embodiment according to the invention assumes that the measuring component is present in its natural isotope abundance. Thus, it is known that natural CO
2
consists of approximately 98.9%
12
CO
2
and a fraction of approximately 1.1%
13
CO
2
. Similar relationships hold for other gases such as CO, CH
4
and others. The isotope ratio is sufficiently constant for most technical processes, with the result that it is possible, for example, to measure
13
CO
2
instead of
12
CO
2
. If there is thus a change in the composition of CO
2
, it is a proportional, representative change in the largely constant low fraction of
13
CO
2
, as well. However, it is important that the concentration present here is approximately 100 times lower than when CO
2
overall or
12
CO
2
is measured. Consequently, absorption as such in the measuring cell is so low, again, that as large as possible a residual light signal reaches the detector. This means, therefore, that when the detector measures CO
2
represented by
13
CO
12
it measures in a clearly more favourable branch of the Lambert-Beer law. A second beam path with a second cell for a second measuring component. The photometer according to the invention operates optimally for those gas components in the case of which, for example, carbon is contained as chemical ligand in the molecules. It is then therefore possible the way according to the invention of the representative measurement of
13
CO
2
as a representative for CO
2
also to be applied generally to other molecules such as, for example, CO or CH
4
and others. In this way, the transmittance ratios are then selected such that the corresponding isotope absorption bands are shifted with respect to those of the basic element. Only thus is it possible to implement this mode of procedure in general. Thus, for example, in
Fabinski Walter
Liedtke Thomas
Moede Michael
Vogt Siegfried
Zochbauer Michael
ABB Patent GmbH
Mai Huy
Rickin Michael M.
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