U.S. patent application number 13/885178 was filed with the patent office on 2013-12-19 for gas cell for the optical analysis of gases.
This patent application is currently assigned to BRUKER OPTIK GMBH. The applicant listed for this patent is Jens Eichmann, Roland Harig, Sven Krause, Gerhard Matz, Lars Schomann, Yifei Wang. Invention is credited to Jens Eichmann, Roland Harig, Sven Krause, Gerhard Matz, Lars Schomann, Yifei Wang.
Application Number | 20130335734 13/885178 |
Document ID | / |
Family ID | 44992926 |
Filed Date | 2013-12-19 |
United States Patent
Application |
20130335734 |
Kind Code |
A1 |
Krause; Sven ; et
al. |
December 19, 2013 |
Gas cell for the optical analysis of gases
Abstract
A measuring cell for a gas analysis spectrometer has an inner
chamber (23) for a sample gas to be analyzed and an inlet (21) and
an outlet (22) which are connected thereto. A traversing optical
path for a measuring beam (14) is formed in the inner chamber (23).
The measuring cell is tubular, the inlet (21) and the outlet (22)
are arranged at opposite ends, and the inner chamber (23) of the
measuring cell has a cross-sectional shape that is monotonic over
the length of the tube and which has an oval-shape at the start,
which disappears toward the end. That special shape results in fast
gas exchange and thus high dynamics, even with larger measuring
cells, which have high sensitivity due to the long optical paths
thereof. Two characteristics which until now appeared to be
conflicting are thereby combined.
Inventors: |
Krause; Sven; (Hamburg,
DE) ; Wang; Yifei; (Hamburg, DE) ; Schomann;
Lars; (Hamburg, DE) ; Matz; Gerhard;
(Buchholz, DE) ; Harig; Roland; (Hamburg, DE)
; Eichmann; Jens; (Hamburg, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Krause; Sven
Wang; Yifei
Schomann; Lars
Matz; Gerhard
Harig; Roland
Eichmann; Jens |
Hamburg
Hamburg
Hamburg
Buchholz
Hamburg
Hamburg |
|
DE
DE
DE
DE
DE
DE |
|
|
Assignee: |
BRUKER OPTIK GMBH
Ettlingen
DE
|
Family ID: |
44992926 |
Appl. No.: |
13/885178 |
Filed: |
November 18, 2011 |
PCT Filed: |
November 18, 2011 |
PCT NO: |
PCT/EP11/70462 |
371 Date: |
July 18, 2013 |
Current U.S.
Class: |
356/246 |
Current CPC
Class: |
G01N 2201/02 20130101;
G01J 3/0267 20130101; G01N 21/11 20130101; G01N 21/031 20130101;
G01J 3/021 20130101 |
Class at
Publication: |
356/246 |
International
Class: |
G01J 3/02 20060101
G01J003/02 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 19, 2010 |
DE |
10 2010 051 928.6 |
Claims
1-8. (canceled)
9. A flow-through measuring cell for a gas analysis spectrometer,
the measuring cell comprising: a casing defining a tubular inner
chamber for a sample gas to be analyzed, said tubular chamber
having an outer surface, a first end and a second end; an inlet
communicating with said tubular chamber at said outer surface and
proximate said first end, said inlet having a tangential component;
an outlet communicating with said tubular chamber at said outer
surface and proximate said second end, said outlet having an axial
component, wherein said tubular chamber has a substantially oval
cross-sectional shape proximate said inlet, which decreases
monotonically along a length thereof and substantially vanishes
proximate said outlet; a first concave mirror disposed at said
first end of said tubular chamber; a second concave mirror disposed
at said second end of said tubular chamber, wherein said first and
said second mirrors induce a multiply reflected, fanned out optical
path for a measuring beam in said tubular chamber.
10. The measuring cell of claim 9, wherein said inlet is disposed
in said casing of the measuring cell.
11. The measuring cell of claim 9, wherein said axial component of
said outlet is disposed in such a way that said outlet forms an
angle of no more than 30.degree. with a center axis of the
measuring cell.
12. The measuring cell of claim 11, wherein said outlet has a
cross-section that is tapered towards an outside.
13. The measuring cell of claim 9, wherein said tubular chamber has
a substantially circular cross section proximate said outlet.
14. The measuring cell of claim 9, wherein said cross sectional
shape of said tubular chamber at said inlet and said outlet differ
but have a substantially same surface area.
15. The measuring cell of claim 9, further comprising an additional
element which is connected to an outlet end and has a cross-section
progression, which is inverse with respect to said tubular chamber.
Description
[0001] The invention concerns a measuring cell for a gas analysis
spectrometer with an inner chamber for a gas to be analyzed, an
inlet and an outlet, wherein a traversing optical path for a
measuring beam is formed in the inner chamber.
[0002] The optical analysis of gases is applied widely in various
areas of technology. Special requirements are demanded by exhaust
gas measurement applications for internal-combustion engines. Due
to increasingly strict exhaust gas regulations, not only is a high
level of sensitivity required to achieve a low detection threshold,
but also a high time resolution to ensure a sufficiently good
dynamic response of the measurement, in particular, with respect to
non-stationary operating states of internal-combustion engines.
This results in a conflict of objectives between detection
sensitivity and the time resolution of the system. In such
measuring cells for optical gas analysis devices, the detection
sensitivity depends on the optical path length that the measuring
beam travels through the gas to be analyzed in the measuring cell.
This path length, in turn, depends on the inner chamber volume of
the measuring cell and the guidance of the measuring beam. However,
the time resolution that is decisive for the dynamic response
directly depends on the time needed to replace the gas to be
analyzed in the measuring cell. It is important that the gas is
replaced in its entirety. Increasing the volume of the measuring
cell therefore has the disadvantage that, while other parameters
remain constant, the time required to completely replace the gas
increases, causing the time resolution and therefore the dynamic
response to decrease correspondingly.
[0003] Various approaches for increasing the quality of the
measurement are known from prior art. In many measuring cells,
attempts are made to increase the detection sensitivity for a
constant cell volume by optimizing the optical path. U.S. Pat. No.
5,440,143 A1 describes attaching a special mirror system onto an
otherwise standard measuring cell with a square cross-section,
which produces a multiply folded and therefore extended optical
path for the measuring beam. Disposing multiple measuring cells one
behind the other so that the measuring beam is first guided through
a first measuring cell and then through another, is known from US
2007/0182965 AI. A universal measuring cell for adapting the length
of the optical path is known from JP 10/062,335 A, wherein the cell
is constituted as two telescopic partial bodies.
[0004] An alternative approach has tried to influence the flow of
sample gas within the measuring cell (DE 103 18 786 A). In such a
measuring cell, however, relatively large "dead zones" are formed,
which increase the exchange time and worsen the dynamic response.
As FIG. 7 schematically shows, in a measuring cell (9) according to
prior art, swirling (91) of sample gas in the measuring cell causes
formation of dead zones in which molecules of the sample gas can
dwell for a comparatively long time, preventing fast exchange. As
the concentration of the supplied sample gas (90) changes, the
previous concentration is still partly present so that the new
concentration value can only be correctly determined once the gas
in the dead zones has also been exchanged. The resulting time delay
causes carryover (concentration carryover), which in turn results
in a long response time of the measuring cell and therefore of the
entire measuring system.
[0005] The object of the invention is to create an improved
measuring cell with a better dynamic response.
[0006] The inventive solution is a measuring cell with the
characteristics of the independent claim. Advantageous embodiments
are the subject of the dependent claims.
[0007] In a measuring cell for a gas analysis spectrometer with an
inner chamber for a gas to be analyzed (sample gas) and an inlet
and an outlet connected to it, an optical path traversing the inner
chamber is formed for a measuring beam, wherein according to the
invention, the measuring cell is constituted as a tube with the
inlet and the outlet at opposite ends, and its inner chamber has a
cross-sectional shape, which extends monotonically over the length
of the tube, with an ovality at the start, which disappears toward
the end.
[0008] Some of the concepts and terms used are explained below:
[0009] Inlet describes a facility through which sample gas can flow
into the inner chamber of the measuring cell. Correspondingly,
outlet describes a facility through which it flows out.
[0010] The beginning of the measuring cell describes the region
where the inlet is positioned. Correspondingly, the end of the
region is that which leads to the outlet.
[0011] Monotonic means a change that occurs in one direction only.
An ovality that decreases monotonically along the length of the
tube therefore means that at no point does the ovality increase
along the length of the tube, not even intermittently.
[0012] The inventive measuring cell has a shape that is optimally
adapted to the formation of a vortex at the inlet of the sample gas
and the transformation of the vortex as it moves toward the outlet
and in such a way that the flow of gas that moves from the inlet to
the outlet fills the entire cell volume along a direct path. The
emphasis here is on a direct path, i.e. secondary curls or other
fluidic figures do not have to be formed to exchange the gas in
remote zones (dead zones). Indeed, the inventive shape avoids the
existence of such dead zones, resulting in particularly fast gas
exchange due to the exchange along a direct path.
[0013] The invention has recognized that the dynamic response of
the measuring cell can be improved not only with a particularly
small size of the cell volume but, in contrast to previous attempts
in prior art, also with a larger size of measuring cell having a
special shape. This special shape is provided by the ovality on the
inlet side, which disappears toward the outlet. As has already been
mentioned, this special shape allows a particularly fast exchange
of gas and produces the desired improvement in the dynamic
response. This invention therefore no longer relies on an
especially small size of measuring cell, enabling the measuring
cell to be larger and therefore more robust. This lengthens the
optical path for the measuring beam and these good optical
conditions improve detectability of the measuring cell. The
invention therefore achieves a combination of advantages with
respect to improved dynamic response and improved detectability. It
achieves this in a surprisingly simple way, namely solely by
ingenious shaping of the measuring cell. There is no example of
this in prior art.
[0014] To reliably achieve favorable vortex formation even as the
sample gas flows in, the inlets are preferably disposed in the tube
casing. Disposing them thereby in the region of the start of the
tube has the advantage, compared to positioning on the start end
face, that reliable and fluidicly advantageous main vortex
formation can be achieved. This particularly applies when the
inlets are disposed diametrically opposite each other, and offset
with respect to the central axis of the tube shape. This not only
applies if two inlets are provided but also if more than two inlets
are provided: in this case, they should be disposed in such a way
that the sample gas initially flows into the tube tangentially.
With this configuration, the inflowing sample gas can be induced to
swirl. This results in stabilization of the flow and ensures the
desired penetration of the entire inner chamber volume with the
main vortex.
[0015] The outlets for the exiting sample gas are preferably
constituted with an axial component. This is understood to mean
that the outlets have an angle of maximum 30.degree. with respect
to the tube axis. Disposing them on the casing allows the mirror
for the measuring beam to be disposed in the center. In this way,
the end region can be optimally used for generating the optical
path for the measuring beam. Furthermore, this outlet configuration
has the advantage that unimpeded exit of the gas can be achieved
due to the considerable tangential component. The outlets are
preferably tapered. This is understood to mean that at their start,
i.e. in the region of their entry, they have the largest
cross-section, which successively tapers the further it is from the
inner chamber. It has been shown that a particularly good discharge
characteristic from the inner chamber into the outlet of the sample
gas can be achieved in this way, particularly with respect to the
paucity or absence of reflections and the vortex or antivortex
caused by them.
[0016] Preferably, the ovality in the region of the outlet
disappears completely. This is not absolutely necessary, a slight
ovality (compared with the inlet) can remain. Preferably, the shape
of the tube of the measuring cell in the region of the outlet is
circular. Advantageously, it is already circular at some distance
(up to 1/3 of the total length of the tube) from the position where
the outlet is disposed. In this connection, the cross-sections
preferably have substantially equal surface areas despite being
different in shape, wherein by "substantially" a deviation of no
more than 15%, preferably 10% is understood.
[0017] In most cases, the outlet will be disposed in the end region
of the tube. However, this is not absolutely necessary. Therefore,
an additional element can be provided in addition to the tube,
which has a cross-sectional shape that is inverse with respect to
the tube. It is disposed in such a way that the non-oval side of
the measuring body (i.e. its end) is connected to the
correspondingly shaped beginning of the additional element, and the
additional element changes to become oval along the length of the
tube. This intermediate element therefore provides a sort of
continuation of the original measuring length. This is especially
suitable for the detection of sample gases in especially low
concentrations.
[0018] The invention is explained below using the included drawing,
which shows an advantageous embodiment.
[0019] The drawings show:
[0020] FIG. 1 A schematic representation of a measuring device with
an inventive gas cell;
[0021] FIG. 2 A representation of the gas cell showing the beam
path;
[0022] FIG. 3 A view from above onto the gas cell without its inlet
and outlet;
[0023] FIG. 4 A sectional view of the gas cell;
[0024] FIG. 5 An alternative embodiment of the gas cell;
[0025] FIG. 6 An exploded view of the gas cell according to FIG. 2;
and
[0026] FIG. 7 A conventional gas cell.
[0027] The invention is explained using the example of an FTIR
spectrometer. FTIR stands for Fourier transform infrared
spectroscopy. Such devices are known from prior art and will
therefore be only briefly explained with reference to FIG. 1.
[0028] An infrared light beam 10 (IR beam) from a source 11 for
infrared radiation is focused onto an obliquely disposed beam
splitter 12 of an interferometer, which is collectively designated
by reference numeral 1. The IR beam 10 is divided into two
components 10a and 10b, of which component 10a is reflected by the
beam splitter 12 to a fixed mirror 13a, and component 10b is
allowed to pass through to a movable mirror 13b, whose distance
from the beam splitter 12 can be altered (symbolized by the dashed
double-headed arrow in FIG. 1). The partial beams 10a, 10b
reflected back by the mirrors 13a, 13b, interfere at beam splitter
12 and are together radiated as IR measuring beam 14 into a gas
cell 2.
[0029] The gas cell 2 is the actual measuring cell. Conventionally,
it is constituted in the shape of a cell or vessel (cf. FIG. 7). It
has an elongated basic body 20 with an inlet 21 at one end and an
outlet 22 at the other end. The gas to be analyzed flows through
the inlet 21 into the basic body, fills the latter and flows out
again through the outlet 22. While the gas dwells in the basic body
20, the gas is irradiated by the measuring beam 14. Depending on
the composition and concentration of the gas in the gas cell 2,
different components of the spectrum of the measuring beam 14 will
be absorbed and the remaining component that is allowed to pass
through (transmitted) is projected onto a detector 15.
[0030] Detector 15 is an MCT semiconductor detector, which converts
the change in photon intensity into an electrical quantity.
However, a photodiode, a bolometer or the like can also be used.
The signal measured by detector 15 is guided to an analog/digital
converter 16. The interferogram 18 can be displayed on a suitable
display device. Then, what is now a digital signal is processed by
a transformation element 17 by means of fast Fourier transform
(FFT). It is constituted to generate a spectral representation 19
from the interferogram provided by the analog/digital converter 16
in a known way and to display it.
[0031] The functional and structural configuration of gas cell 2 is
shown in FIGS. 2 to 6. As FIG. 2 most clearly shows, the gas cell
has an elongated, round hollow basic body 20 with a double-entry
inlet 21 at one end and a double-entry outlet 22 at its other end.
The basic body has a cavity 23, which is delimited by a casing 27.
According to a core element of the invention, the cross-section of
the cavity 23 in the basic body 20 is not constant but changes
continually from inlet 21 to outlet 22. According to the invention,
the shape of the cross-section of the cavity 23 has been chosen
such that the cross-section is oval at inlet 21 and this ovality is
increasingly reduced toward outlet 22, until it practically
disappears completely in the region of outlet 22, i.e. there, the
cross-section is practically circular. This permits use of a round
mirror 32 in the outlet region to reflect the measuring beam 14 and
a polygonal mirror 31 in the region of the inlet cross-section. The
mirrors 31, 32 have the same radius of curvature.
[0032] The inlets 21 are disposed on the basic body 20,
diametrically opposite along the longer axis of the oval, with a
small offset in opposite directions (less than one tenth of the
size of the width of the basic body 20 in this region) relative to
the center axis 24 of the basic body 20. In this way, it is ensured
that the sample gas flowing in quickly fills the oval-shaped
cross-section. An intended asymmetry is achieved by this offset
with which the flow in the cavity 23 takes a preferred direction so
that a defined vortex can form, which ensures fast mixture at the
beginning and during continued flow of the sample gas toward outlet
22. Because of the tapered cross-sectional shape, the vortex along
the path to the outlet 22 gradually turns into a circular vortex
and its peripheral speed slowly decreases. At the outlet end, the
outlets are disposed diametrically opposite and oriented in such a
way that they are tangential to the direction of flow (symbolized
by arrow 5) from inlet 21 to outlet 22 and form an angle .alpha. of
approx. 25.degree. with respect to the center axis 24. In this way,
the sample gas can exit the gas cell 2 via the outlets 22 in a way
that is favorable to the flow.
[0033] The beam guidance with the IR source 11 and the detector 15
and the installation location with reference to the gas cell 2 are
shown in FIG. 4. The measuring cell 2 represented in the embodiment
is 16 cm long and has a 7.5-cm diameter. A floor-sided pot 4 is
provided beneath the actual gas cell 2, in which the IR source 11,
the detector 15, and the interferometer 1 are disposed. The IR
source and detector can also be disposed externally, in which case
corresponding access openings for the inflow and outflow
(represented by a dashed line) would have to be provided. They
radiate through openings located at the edge of the polygonal
mirror 31 (see reference figure 35 in FIG. 3). Taking into
consideration this surface intended for the beam entry and exit,
the polygonal mirror 31 forms an envelope that is elliptical. The
gas cell 2 is closed at its top end by a cover 26. Further, the
round mirror 32 is disposed on the inside of the cover 26 so that
it faces the polygonal mirror 31. The round mirror 32 is configured
as a double mirror comprising two parallel concave mirrors 32a,
32b. Their radius of curvature is identical and dimensioned such
that their focal points are located on the surface of the opposite
mirror 31. Mirror 31 is also concave, wherein its focal point is
aimed exactly onto the center of the two concave mirrors 32a, b.
This results in a multiply reflected, fanned out light path for the
measuring beam 14, which forms a stationary beam pattern in the two
concave mirrors 32a, b, and a beam pattern on the polygonal mirror
31 that moves slightly each time it reflects back and forth. In
this way, both mirrors 31, 32 are illuminated fully for the
measurement. All the input light is reflected from one mirror 31,
32 to the other 32, 31, so that there is practically no loss. The
fanning out with multiple reflection produces a light path that is
a multiple of the actual overall length of the gas cell 2 (see
FIGS. 2 and 4).
[0034] Several advantages are achieved in this way. On the one
hand, sample gas flowing in at inlet 21 is immediately caught by
measuring beam 14, which results in a very fast response time. The
sample gas is measured before it even has time to mix with the old
gas still present in gas cell 2. As a result, changes to the
composition and/or concentration in the sample gas are visible
practically immediately. The invention has also recognized that the
claimed cross-sectional transition shape not only provides
advantages in terms of minimizing the internal volume of the gas
cell 2 but is also provides favorable conditions for flow. When the
sample gas flows in, a vortex is formed, which more or less fills
the entire cross-section in the inlet area, and changes shape along
its path to the outlet such that it acquires an increasingly
circular cross-section. The invention takes advantage of the
behavior of the measuring gas vortex by adapting the
cross-sectional shape of the gas cell precisely to this change in
shape, thus having a cross-section along the entire length of the
gas cell that is entirely filled by the flow. This effectively
reduces the "dead zones," which are critical to the response and
precision. Because the gas exchange in the gas cell is faster than
the measurement of an interferogram, a maximum dynamic response is
achieved.
[0035] The long light path results in a high level of sensitivity.
The light fan produced between the polygonal mirror 31 and the
circular mirror 32 is optimally adapted to the cross-sectional
shape of the inner chamber. This results in practically the entire
inner chamber being irradiated and, because of the complete filling
with the flow described above, quickly being filled with the
entering sample gas (without formation of the disturbing dead zones
known from prior art). The wide fanning in conjunction with the
flow pattern produced by the special shape ensures a fast response.
In this way, the inventive gas cell can provide two essential
advantages at once.
[0036] To further increase the sensitivity while maintaining the
advantageous dynamic properties, an alternative embodiment is
possible. It has an additional element 6, which is directly
connected to the gas cell 2. In this case, the cover 26 of the gas
cell 2 is eliminated so that, together with the additional element,
a large uniform cavity 23' is produced. The shape of the cavity in
the additional element 6 is inverse, i.e. circular where it forms a
connection with casing 27 of the gas cell 2 and oval at the outside
end. The additional element 6 is preferably constructed identically
and connected to the coverless gas cell 2 in a "back-to-back"
configuration. Inlet 21 is located at the base of gas cell 2 and
inlet 22' is located at the other end at the additional element 6.
With this configuration, the sensitivity can be almost doubled,
wherein the advantageous shape of the gas cell 2 is retained due to
the mirrored shape of the cavity of the additional element 6.
* * * * *