U.S. patent application number 09/816474 was filed with the patent office on 2001-11-15 for gas flow switching device.
Invention is credited to Gellert, Udo, Mueller, Friedhelm, Steckenborn, Arno.
Application Number | 20010039880 09/816474 |
Document ID | / |
Family ID | 7882179 |
Filed Date | 2001-11-15 |
United States Patent
Application |
20010039880 |
Kind Code |
A1 |
Gellert, Udo ; et
al. |
November 15, 2001 |
Gas flow switching device
Abstract
To switch gas flows between gas sources and gas sinks, a gas
flow switching device includes gas passages, which communicate with
one another and which have connecting points for the gas sources
and the gas sinks. Furthermore, the gas flow switching device has a
device for setting different pressures. To simplify the
construction of the gas flow switching device and to achieve
precisely defined pressure and flow conditions without the need for
calibration, the gas flow switching device has two plates (9, 10),
which are positioned on top of one another and joined together. The
two plates (9, 10) have congruent channels (11) on their respective
sides that face one another. These channels (11) have semicircular
cross sections and form gas passages (4 to 8). In addition, at
their lateral exit points from the plates (9, 10), the channels
(11) form connecting points (12 to 17).
Inventors: |
Gellert, Udo; (Bellheim,
DE) ; Mueller, Friedhelm; (Linkenheim-Hochstetten,
DE) ; Steckenborn, Arno; (Berlin, DE) |
Correspondence
Address: |
SUGHRUE, MION, ZINN, MACPEAK & SEAS, PLLC
2100 PENNSYLVANIA AVENUE, N. W.
WASHINGTON
DC
20037-3213
US
|
Family ID: |
7882179 |
Appl. No.: |
09/816474 |
Filed: |
March 26, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09816474 |
Mar 26, 2001 |
|
|
|
PCT/DE99/03054 |
Sep 23, 1999 |
|
|
|
Current U.S.
Class: |
96/102 ;
96/105 |
Current CPC
Class: |
G01N 30/10 20130101;
G01N 30/465 20130101; G01N 30/32 20130101; G01N 2030/324
20130101 |
Class at
Publication: |
96/102 ;
96/105 |
International
Class: |
B01D 015/08; B01D
053/04 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 24, 1998 |
DE |
19843942.3 |
Claims
What is claimed is:
1. A gas flow switching device for switching gas flows between gas
sources and gas sinks, comprising: two plates, which are positioned
on top of one another and joined together, comprising congruent
channels on sides of the two plates that face each other, wherein
the congruent channels have respective semicircular cross sections,
wherein the congruent channels form gas passages that communicate
with each other, and wherein the congruent channels form connecting
points for the gas sources and the gas sinks at points at which the
congruent channels exit the two plates; and a device for setting
different pressures at predefined ones of the connecting
points.
2. The gas flow switching device of claim 1, wherein the respective
semicircular cross sections at the connecting points are larger
than the respective semicircular cross sections of the gas passages
between the connecting points; and wherein capillaries that lead to
the gas sources and to the gas sinks are inserted in the connecting
points.
3. The gas flow switching device of claim 1, wherein the two plates
comprise monocrystalline silicon, into which the congruent channels
are formed by isotropic etching.
4. The gas flow switching device of claim 3, wherein, in the area
of the congruent channels, the monocrystalline silicon is converted
into porous silicon and subsequently removed by etching.
5. The gas flow switching device of claim 3, wherein the congruent
channels are lined with a silicon dioxide layer.
6. The gas flow switching device of claim 1, wherein, for switching
sample gas flows and carrier gas flows between two chromatographic
separation columns, the congruent channels in the two plates form a
main gas passage, two auxiliary gas passages, and two connecting
gas passages; wherein a respective one of the two auxiliary gas
passages extends along one side of the main gas passage; wherein
the respective one of the two auxiliary gas passages is connected
with the main gas passage via a respective one of the two
connecting gas passages; wherein junction points of the two
connecting gas passages into the main gas passage are arranged
mutually offset along the main gas passage; wherein cross sections
of the two connection gas passages are smaller than a cross section
of the main gas passage and than cross sections of the two
auxiliary gas passages; wherein the cross section of the main gas
passage in an area between the junction points of the two
connecting gas passages is smaller than the cross section of the
main gas passage outside the area; wherein the main gas passage is
series-connected between the two separation columns; and wherein
the two auxiliary gas passages are, on one side, connected to a
carrier gas source via the device for setting different
pressures.
7. The gas flow switching device of claim 6, wherein the respective
one of the two auxiliary gas passages is connected to a respective
connecting point for a pressure measuring device via a respective
branching gas passage.
8. The gas flow switching device of claim 1, wherein, for metering
a sample gas, the congruent channels in the two plates form a
carrier gas passage, a sample gas passage, and a connecting gas
passage between the carrier gas passage and the sample gas passage;
wherein, at a branching point of the connecting gas passage from
the sample gas passage, a ratio of a cross section of the
connecting gas passage and a continuation of the sample gas passage
corresponds to a predefined dividing ratio of the sample gas
passage; wherein the carrier gas passage and the sample gas passage
are, on one side, connected to a carrier gas source via the device
for setting different pressures; and wherein a metering device for
injecting a sample gas slug into the carrier gas passage is
arranged between the carrier gas source and the sample gas
passage.
9. The gas flow switching device of claim 1, wherein the sample gas
is metered for gas chromatographic analysis purposes.
10. The gas flow switching device of claim 8, wherein, at the
dividing ratio of the sample gas passage of 50:50, the branching
point of the connecting gas passage from the sample gas passage is
formed as symmetrical fork.
11. The gas flow switching device of claim 1, further comprising:
at least one chromatographic separation column connected thereto,
wherein the device for setting different pressures comprises
electronic pressure regulators, whose set point values are
calculated and set based on geometric data of the gas passages and
the separation columns and as a function of parameters of a flowing
gas, a temperature, and a desired flow rate within the separation
columns.
12. The gas flow switching device of claim 11, wherein the gas flow
switching device, with the calculated set point values being set,
is operated by a sample gas, which does not interact with a
separation phase of the separation columns; wherein a transit time
of the sample gas through the separation columns is measured;
wherein an average inner diameter of the separation columns is
calculated based on the transmit time of the sample gas; and
wherein the set point values for the pressure regulators are
recalculated based on the calculated average inner diameter.
Description
[0001] This is a Continuation of International Application
PCT/DE99/03054, with an international filing date of Sep. 23, 1999,
the disclosure of which is incorporated into this application by
reference.
FIELD OF AND BACKGROUND OF THE INVENTION
[0002] The invention relates to a gas flow switching device for
switching gas flows between gas sources and gas sinks. The gas flow
switching device includes gas passages that communicate with one
another and connecting points for the gas sources and gas sinks.
Furthermore, the gas flow switching device includes a device for
setting different pressures at predefined connecting points.
[0003] German Patent DE 28 06 123 C2 teaches a gas flow switching
device that is used to change gas flow directions in a
chromatographic separation column switching system. Therein,
pressure drops of changing direction are generated between suitable
points in the separation column switching system. To this end, the
known gas flow switching device includes a main gas passage having
two connecting points, which is disposed between two separation
columns. In the vicinity of each of the two connecting points, the
main gas passage is connected with a respective auxiliary gas
passage via a respective connecting gas passage. The two auxiliary
gas passages are connected with a carrier gas source via a device
that comprises several valves for setting different pressures. By
setting different pressure drops between the auxiliary gas passages
themselves and between the auxiliary gas passages and the
connecting points of the main gas passage, a gas sample exiting
from the first separation column may enter the second column or may
be prevented from entering the second column. Therein, the latter
event occurs in the operating mode "cut." In this case, the gas
sample is directed to a downstream detector or to a third
separation column via the corresponding auxiliary gas passage.
Furthermore, the first separation column with the carrier gas may
be backflushed from the carrier gas source. The valves required for
switching the gas flows come into contact with the carrier gas only
but not with the gas sample. Moreover, the valves can be disposed
outside the oven that is typically used to heat the separation
columns.
[0004] For implementing the gas passages, the known gas flow
switching device has a block with a center bore into which the end
pieces of the two separation columns are inserted from both sides.
The main gas passage includes a capillary, which extends coaxially
in the center bore and whose ends project into the end pieces of
the separation columns. The auxiliary gas passages are embodied as
capillaries, which are inserted into the block and which lead into
two spatial halves of the center bore. Therein, the spatial halves
are sealed against one another. The connecting gas passages are
formed by the annular gaps between the end pieces of the separation
columns and by the capillary of the main gas passage that projects
into the separation columns. The multipart construction of the
known gas flow switching device is thus relatively complex. In
addition, the parts of the known gas flow switching device must be
calibrated in relation to one another.
[0005] European Patent EP 0 386 033 B1 teaches a further gas flow
switching device, which is used for a valve-less metering of a gas
sample for gas chromatographic analysis purposes. For this purpose,
a carrier gas passage and a sample gas passage, which communicate
with one another via a connecting gas passage, are connected to a
carrier gas source via a device for setting different pressures.
Therein, a metering device is disposed between the carrier gas
source and the sample gas passage for injecting a sample gas slug
into the carrier gas flow. The sample gas passage has the form of a
tubular chamber. The carrier gas passage includes two interior
tubes of different diameters, which penetrate the chamber and whose
ends are pushed into one another so as to form an annular gap. This
annular gap represents the connecting gas passage between the
sample gas passage and the carrier gas passage. By adjusting
different pressures in the carrier gas passage and in the sample
gas passage, the sample gas from the sample gas passage is
prevented from entering the carrier gas passage at the location of
the annular gap. Alternatively, the sample gas from the sample gas
passage may be specifically channeled into the carrier gas flow
flowing through the carrier gas passage. In this known gas flow
switching device too, the multipart construction is comparatively
complex.
OBJECTS OF THE INVENTION
[0006] It is an object of the invention to simplify the
construction of a gas flow switching device, wherein precisely
defined pressure and flow conditions are to be achieved without the
need for calibration.
SUMMARY OF THE INVENTION
[0007] This and other objects of the invention are achieved by a
gas flow switching device for switching gas flows between gas
sources and gas sinks. The gas flow switching device according to
the invention includes two plates, which are positioned on top of
one another and which are joined together. These plates have
congruent channels on those sides of the two plates that face each
other. The congruent channels have semicircular cross sections and
form gas passages that communicate with each other. Furthermore,
the congruent channels form connecting points for the gas sources
and the gas sinks at points at which the congruent channels exit
the two plates. In addition, the gas flow switching device
according to the invention includes a device for setting different
pressures at predefined ones of the connecting points.
[0008] The channels are produced in the plates with great
technological precision. Therefore, the desired pressure and flow
conditions, for which the geometries of the gas passages are
calculated, can actually be obtained in practice. In contrast to
the parts of the known gas flow switching devices, the plates of
the gas flow switching device according to the present invention
are comparatively easy to calibrate, and the joining of the plates
is done automatically or semi-automatically. Finally, the planar
structure of the gas flow switching device according to the
invention is highly compact. Very small dimensions are obtained,
particularly if the gas passages are produced
micromechanically.
[0009] The gas sources and gas sinks are preferably connected with
the gas passages of the gas flow switching device via capillaries.
To this end, the cross sections of the channels at the connecting
points are larger than the cross sections of the channels in the
area of the gas passages in between the connecting points, and the
capillaries, together with their ends, are inserted into the
connecting points. Therein, the cross sections of the areas of the
gas passages located directly behind the connecting points
correspond to the inner cross sections of the capillaries to
prevent the creation of flow impediments.
[0010] The channels in the plates may principally be made in
various ways, e.g., by means of a laser. The plates are preferably
made of monocrystalline silicon in which the channels are formed by
isotropic etching. This is done, for instance, by means of a
mixture of hydrofluoric acid and nitric acid. Alternatively, in the
area of the channels, the monocrystalline silicon may be converted
into porous silicon and subsequently removed by etching. The
etching process in the porous silicon is isotropic, so that the
channels formed therein have the desired semicircular cross
sections. The channels may be lined with a silicon dioxide layer in
order to protect them against the flowing gas.
[0011] To switch sample gas and carrier gas flows between two
chromatographic separation columns, as it is known from the
aforementioned German Patent DE 28 06 123 C2, the channels in the
plates of the gas flow switching device according to the invention
form a main gas passage, two auxiliary gas passages and two
connecting gas passages. Moreover, a respective auxiliary gas
passage extends on either side of the main gas passage. Each of the
two auxiliary gas passages is connected to the main gas passage via
one of the connecting gas passages. The junction points of the
connecting gas passages to the main gas passage are mutually offset
along the main gas passage. The cross sections of the connecting
gas passages are smaller than the cross sections of the main gas
passage and the cross sections of the auxiliary gas passages. The
cross section of the main gas passage in the area between the
junction points of the connecting gas passages is smaller than the
cross section outside that area. In a generally known manner, the
main gas passage is series-connected to the two separation columns
between these columns, and the auxiliary gas passages on one side
are connected to a carrier gas source via the device for setting
different pressures. In order to measure the different pressures or
pressure drops between the auxiliary gas passages, which are
required for switching the gas flows between the separation
columns, each of the auxiliary gas passages is advantageously
connected to connecting points for pressure measuring devices via
branching gas passages.
[0012] To meter a sample gas, particularly for gas chromatographic
analysis purposes as disclosed in the German laid-open publication
DE 37 35 814 A1, the channels in the plates of the gas flow
switching device according to the invention form a carrier gas
passage, a sample gas passage, as well as a connecting gas passage
between the carrier gas passage and the sample gas passage. At the
branch of the connecting gas passage from the sample gas passage,
the cross section ratio of the connecting gas passage and the
continuation of the sample gas passage corresponds to a predefined
dividing ratio of the sample gas flow. In a generally known manner,
the carrier gas passage and the sample gas passage are, on one
side, connected to a carrier gas source via the device for setting
different pressures. A metering unit for injecting a sample gas
slug into the carrier gas flow is disposed between the carrier gas
source and the sample gas passage. By determining the cross section
of the connecting gas passage and the cross section of the
continuation of the sample gas passage as a function of the
adjusted division of the sample gas flow, discrimination between
differently sized gas molecules is prevented when a portion of the
sample gas is diverted from the sample gas passage into the
connecting gas passage. Large molecules, e.g., molecules of the
sample gas, are not as easily deflected as smaller molecules, e.g.,
molecules of the carrier gas. Consequently, if the branching fork
is asymmetrical, either the larger or the smaller molecules of the
sample gas would be more likely to reach the connecting gas passage
and, subsequently, the carrier gas passage. This would distort the
measurements in the subsequent gas chromatographic analysis. In a
preferred 50:50 split of the sample flow, the branching fork of the
connecting gas passage from the sample gas passage is
symmetrical.
[0013] As previously mentioned, the gas passages in the gas flow
switching device according to the invention can be formed with
great accuracy by means of micromechanical production methods, so
that the geometry of the gas passages after production is very
precisely known. This is advantageously exploited in that the gas
flow switching device according to the invention, together with at
least one chromatographic column connected thereto, includes the
device for setting different pressures, wherein the device has
electronic pressure regulators. The set point values of these
pressure regulators are calculated and set based on geometric data
of the gas passages and the separation column. In addition, these
set point values are calculated and set as a function of the gas
flow parameters and as a function of the temperature and the
desired flow rate in the separation column. This eliminates the
previously required basic pressure calibration by means of
adjustable needle valves.
[0014] Since, as a rule, the inner diameter of the separation
column is not exactly known due to manufacturing tolerances and due
to the coating of the column with a liquid separation phase, the
gas flow switching device is preferably operated with a test gas or
sample gas, when the calculated set point values are set at the
pressure regulators. Therein, the transit time (retention time) of
the sample gas through the separation column is measured and, based
thereon, the average inside diameter of the column is calculated.
The sample gas is e.g. air, which does not interact with the
separation phase of the column. Based on the thus determined
average inner diameter of the separation column, the set point
values for the pressure regulators are then recalculated and
reset.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The invention and further advantageous refinements of the
invention according to the features of the dependent claims are
explained in more detail below with the aid of diagrammatic,
exemplary embodiments in the drawings, in which:
[0016] FIG. 1 shows a first embodiment of the gas flow switching
device according to the invention for switching sample and carrier
gas flows between two chromatographic separation columns;
[0017] FIG. 2 shows a second embodiment of the gas flow switching
device according to the invention for metering a sample gas for gas
chromatographic analysis purposes;
[0018] FIGS. 3 to 18 show a preferred embodiment of forming gas
passages in the gas flow switching device according to the
invention in several successive manufacturing steps; and
[0019] FIG. 19 is a block diagram of an embodiment of the gas flow
switching device according to the invention including a device for
setting different pressures, which has electronic pressure
regulators.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0020] FIG. 1 shows two, only partly depicted chromatographic
capillary separation columns 1 and 2, which are connected to a gas
flow switching device 3. The gas flow switching device 3 is
configured either to direct a gas sample exiting from the one
separation column, e.g., separation column 1, to the other
separation column 2, or to prevent the gas sample from entering
into the other separation column 2 and to divert it. To this end,
the two separation columns 1 and 2 are connected with one another
via a main gas passage 4, which communicates with two auxiliary gas
passages 7 and 8 via two connecting gas passages 5 and 6.
[0021] To form the gas passages 4 to 8, two plates 9 and 10, which
are positioned on top of one another and joined together, have
congruent channels 11 with respective semicircular cross sections.
These channels 11 are formed on those sides of the plates 9 and 10
that face one another. The channels 11 form the gas passages 4 to 8
and, at their lateral exit points from the plates 9 and 10,
connecting points 12 to 17 of the gas passages 4 to 8. For
clarity's sake, the two plates 9 and 10 are shown separated from
one another. At the connecting points 12 to 17, the cross sections
of the channels 11 are larger than the cross sections in the area
of the gas passages 4 to 8 there between. These cross sections of
the channel 11 at the connecting points 12 to 17 correspond to the
outside cross sections of the capillary separation columns 1 and 2
and further capillaries 18 to 21. The further capillaries 18 to 21
are inserted into the connecting points 12 to 17 and bonded there.
The cross sections of the areas of the gas passages 4 to 8 located
immediately behind the connecting points 12 to 17 correspond
approximately to the inner cross sections of the capillaries 1, 2,
18 to 21, so that no unnecessary flow impediments are created.
[0022] As shown in FIG. 1, the auxiliary gas passages 7 and 8
extend on either side of the main gas passage 4 and parallel
thereto. The junction points 22 and 23 of the two connecting gas
passages 5 and 6 to the main gas passage 4 are mutually offset
along the main gas passage 4. Therein, the cross section of the
main gas passage 4 in the area between the junction points 22 and
23 is smaller than in the areas between the junction points 22, 23
and the connecting points 12, 13 for the separation columns 1 or 2.
The cross sections of the connecting gas passages 5 and 6 either
correspond to the cross section of the main gas passage 4 in the
area between the junction points 22 and 23 or they are smaller than
the cross section of the main gas passage 4, as shown here.
[0023] At the connecting points 14 and 16, the auxiliary gas
passages 7 and 8 are connected to a carrier gas source 25 via the
capillaries 18 and 20 and via a device 24 for setting different
pressures in the auxiliary gas passages 7 and 8. The device 24
includes a pressure regulator 26, the input of which is connected
to the carrier gas source 25 and the output of which is connected
to the two capillaries 18 and 20 via a controllable switching valve
27. Furthermore, respective needle valves 28 and 29 are inserted
between the output of the pressure regulator 26 and the two
capillaries 18 and 20.
[0024] The connecting points 15 and 17 of the auxiliary gas
passages 7 and 8 can be connected to a monitoring detector or to a
gas chromatographic detector, which is located downstream from the
separation column 2, via capillaries 19 and 21. The monitor
detector and the gas chromatographic detector are not shown. In a
manner known from German Patent DE 2806 123 C2, the capillaries 19
and 21 have further needle valves 30 and 31 arranged therein.
[0025] In the depicted embodiment of the gas flow switching device
according to the invention, the auxiliary gas passages 7 and 8 are
each connected via respective branching gas passages 32 and 33,
which have connecting points 34 and 35 for pressure measuring
devices, in this case a differential pressure gauge 36.
[0026] For the following functional description it is assumed that
a sample gas flow 37 is driven though the separation column 1. The
pressure conditions in the auxiliary gas passages 7 and 8 are
adjusted by means of the differential pressure gauge 36. This is
accomplished by first setting the pressure in the pressure
regulator 26 to a value above the pressure value that would be
established based on the sample gas flow 37 flowing through the
series-connected separation columns 1 and 2. The needle valves 28
and 29 are adjusted so as to establish a pressure drop between the
auxiliary gas passages 7 and 8, which has, depending on the
position of the switching valve 27, a different direction of
action. If the pressure in the auxiliary gas passage 7 is greater
than in the auxiliary gas passage 8, a pressure drop is created in
the main gas passage 4, which acts from the connecting point 12 of
the separation column 1 in the direction toward the connecting
point 13 of the separation column 2. As a consequence, the sample
gas flow 37 exiting from the separation column 1 flows through the
main gas passage 4 and then enters the separation column 2. Since
the pressure in the two auxiliary gas passages 7 and 8 is greater
than in the main gas passage 4, no sample gas components can pass
from the main gas passage 4 into the auxiliary gas passages 7 or 8.
Instead, small amounts of the carrier gas reach the main gas
passage 4 from the auxiliary gas passages 7 and 8. This does not
affect the gas analysis, however, due to the neutral
characteristics of the carrier gas.
[0027] If, by means of the switching valve 27, the direction of the
pressure drop between the auxiliary gas passages 7 and 8 and thus
the direction of the pressure drop within the main gas passage 4
between the connecting points 12 and 13 are reversed, the sample
gas flow 37 exiting from the separation column 1 is diverted into
the auxiliary gas passage 7 via the connecting gas passage 5.
There, the sample gas flow 37 is transported by the carrier gas
from the carrier gas source 25 in the direction of the capillaries
19. The separation column 2 is supplied with the carrier gas from
the carrier gas source 25 via the capillaries 20, the auxiliary gas
passage 8 and the connecting gas passage 6. This creates a slight
backflow of carrier gas in the main gas passage 4, which, together
with the sample gas flow 37 exiting from the separation column 1,
reaches the connecting gas passage 5 and, from there, the auxiliary
gas passage 7.
[0028] If no sample gas flow 37 is introduced into the separation
column 1, the separation column I can be backflushed with the
carrier gas from the carrier gas source 25. This is accomplished in
that the pressure regulator 26 sets a pressure in the gas passages
4 to 8 and thus a pressure at the ends of the separation columns 1
and 2, which are inserted into the connecting points 12 and 13,
that is greater than the pressures at the opposite ends of the two
separation columns 1 and 2. The separation column 2 continues to be
supplied with carrier gas from the carrier gas source 25 via the
capillaries 20, the auxiliary gas passage 8 and the connecting
passage 6.
[0029] FIG. 2 shows a portion of a chromatographic capillary column
38, which is connected to a gas flow switching device 39. The gas
flow switching device 39 is configured to inject, at a predefined
instant, a gas sample into a carrier gas flow flowing through the
separation column 38. For this purpose, the separation column 38 is
connected to one end of a carrier gas passage 40, which
communicates with a sample gas passage 42 via a connecting gas
passage 41. The other end of the carrier gas passage 40 and the
sample gas passage 42 are connected to a carrier gas source 46 via
capillaries 43 and 44 and via a device 45 for setting different
pressures in the carrier gas passage 40 and the sample gas passage
42. In the course of the capillary 44 between the sample gas
passage 42 and the device 45, a metering unit 47 is arranged for
injecting a sample gas slug into the carrier gas flow.
[0030] To form the gas passages 40, 41 and 42, two plates 48 and 49
are positioned on top of one another and joined together. Congruent
channels 50, which have respective semi-circular cross sections,
are formed on those sides of the two plates that face one another.
These channels form the gas passages 40, 41 and 42 and, at their
lateral exit points from plates 48 and 49, connecting points 51 to
54 of the gas passages 40 to 42. For clarity's sake, the two plates
48 and 49 are shown separated from one another. At the connecting
points 51 to 54, the cross sections of the channels 50 are larger
than in the area of the gas passages 40 to 42 there between. These
cross sections correspond to the outer cross sections of the
capillary column 38 and of the capillaries 43, 44 and 63, which are
inserted into the connection points 51 to 54. There, they are
bonded.
[0031] As shown in FIG. 2, the connecting gas passage 41 branches
off, at an obtuse angle 55, from the segment of the sample gas
passage 42 coming from the connecting point 54. At a junction 56,
the sample gas passage 42 continues at the same angle in another
direction, so that the connecting gas passage 41 and the
continuation 57 of the sample gas passage 42 form a symmetrical
branching fork. The connecting gas passage 41 and the continuation
57 of the sample gas passage 42 have identical cross sections.
[0032] The device 45 for setting different pressures in the carrier
gas passage 40 and in the sample gas passage 42 includes a pressure
regulator 58, the input of which is connected to the carrier gas
source 46 and the output of which is connected to the capillary 44
and, via a solenoid valve 59, to the capillary 43.
[0033] The metering unit 47, which is arranged in the course of
capillary 44, has a metering valve 60 of known design. In its first
position, which is indicated by solid lines, the metering valve
guides a sample gas flow from a line 61 into a metering volume 62
and simultaneously connects the sample gas path 42 directly to the
carrier gas source 46 via the device 45. In a second position,
which is indicated by a dashed line, the metering volume 62 is
switched directly to the capillary 44, so that the content of the
metering volume 62 is transferred into the sample gas passage 42 by
the carrier gas flowing through capillary 44.
[0034] The gas passages 40, 41 and 42 and the capillaries 38, 43,
44 and 63, which may have valves or restrictors installed therein,
are dimensioned such that, if the solenoid valve 59 is open, the
pressure in the carrier gas passage 40 is greater than that in the
sample gas passage 42. As a result, no sample gas from the sample
gas passage 42 can reach the carrier gas passage 40 and thus the
separation column 38 via the connecting gas passage 41. If the
solenoid valve 59 is closed, a reverse pressure drop results in the
connecting gas passage 41 in the direction from the sample gas
passage 42 to the carrier gas passage 40. Consequently, the sample
gas injected into the sample gas passage 42 via the metering unit
47 is diverted from the sample gas passage 42 into the connecting
gas passage 41. From there, the sample gas reaches the separation
column 38 via the carrier gas passage 40. A valve 64 in the
capillary 63 is used to adjust the dividing ratio of the sample gas
flow to 50:50. The symmetrical embodiment of the connecting gas
passage 41 and of the continuation 57 of the sample gas passage 42
at the point of the fork 56 prevents any discrimination of the
differently sized gas molecules when a portion of the sample gas is
diverted from the sample gas passage 42 into the connecting gas
passage 41. An asymmetrical embodiment of the branching fork is
also possible. In this case, the ratio of the cross sections of the
connecting passage 41 and the continuation 57 of the sample gas
passage 42 corresponds to the dividing ratio of the sample gas
flow.
[0035] An preferred embodiment of forming the channels 11, 50 in
the plates 9, 10 and 48, 49, respectively, will now be described in
greater detail with reference to FIGS. 3 to 18.
[0036] FIG. 3, by way of example, shows a longitudinal section
through plate 9, which extends along the main gas passage 4 to be
formed in the plate 9. The plate 9 is made of monocrystalline
silicon, which on its top and bottom side is provided with a
silicon carbide layer 70 and 71, respectively.
[0037] In a next step, which is illustrated in FIG. 4, saw markings
are defined on the underside of the plate 9 by means of a etching
mask 72 and by means of etching the silicon carbide layer 71 at
points that are not covered by the etching mask 72.
[0038] As shown in FIG. 5, by means of an etching mask 73 and by
subsequent etching along a strip, which extends in the area between
the junction points 22 and 23 of the main gas passage 4 to be
formed, the thickness of the silicon carbide layer 70 on the upper
side of the plate 9 is subsequently reduced by about one-third FIG.
6 shows how, by means of a mask 74 and by subsequent etching of the
silicon carbide layer 70, the thickness of the silicon carbide
layer 70 on the upper side of the plate 9 is reduced in two narrow
strips by about two thirds. These strips extend along the main gas
passage 4 in the areas between the junction point 22 and the
connecting point 12 as well as between the junction point 23 and
the connecting point 13.
[0039] In a next process step shown in FIG. 7, the upper silicon
carbide layer 70 is covered by a mask 75 with the exception of
narrow strips in the areas of the connecting points 12 and 13 of
the main gas passage 4 to be formed. Subsequently, the
monocrystalline silicon of the plate 9 is exposed by etching away
the silicon carbide 70 in the areas where it is not covered.
[0040] At the exposed locations, the monocrystalline silicon is
then converted into porous silicon 76 up to a depth of 80 .mu.m, as
shown in FIG. 8. This conversion takes place in an isotropic
etching process. Starting from the narrow strips in which the
monocrystalline silicon is not covered, the isotropic etching
process advances under the silicon carbide layer 70 in horizontal
direction to the same degree as it progresses in depth. Thus, the
areas with the porous silicon have the shape of a respective half
cylinder.
[0041] By thinning the upper silicon carbide layer 70, as shown in
FIG. 9, the monocrystalline silicon is exposed in the area of the
main gas passage 4 between the connecting point 12 and the junction
point 22 and in the area between the connecting point 13 and the
junction point 23. These areas were defined in the process step
according to FIG. 6.
[0042] As shown in FIG. 10, the monocrystalline silicon of the
plate 9 is subsequently converted into the porous silicon 76 up to
a width and depth of 145 .mu.m along the narrow strips that are not
covered by the remaining silicon carbide layer 70. The areas that
were previously converted into the porous silicon 76 in the process
step according to FIG. 8 are further widened and deepened to 225
.mu.m.
[0043] According to FIG. 11, by further thinning the silicon
carbide layer 70, the monocrystalline silicon is exposed in the
area of the main gas passage 4 between the junction points 22 and
23, which was defined in the process step according to FIG. 5.
[0044] According to FIG. 12, the exposed silicon is converted into
the porous silicon 76 up to a width and depth of 15 .mu.m. The
areas previously converted into the porous silicon are further
widened and deepened by this amount.
[0045] The remaining parts of the silicon carbide layers 70 and 71
are then removed, as shown in FIG. 13.
[0046] In a next process step, which is illustrated in FIG. 14, the
areas in the plate 9 that were converted to the porous silicon 76
are etched away to create the channel 11 in the plate 9, wherein
the channel 11 has differently sized semicircular cross sections.
In the area of the subsequent connecting points 12 and 13, the
semicircular cross sections have an inner radius of 240 .mu.m. In
the area between the connecting point 12 and the junction point 22
as well as in the area between the connecting point 13 and the
junction point 23, the semicircular cross sections have an inner
radius of 160 .mu.m. Finally, in the area between the subsequent
junction points 22 and 23, the semicircular cross sections have an
inner radius of 15 .mu.m.
[0047] According to FIG. 15, the channel 11 is lined with a silicon
dioxide layer 77.
[0048] FIG. 16 shows the plate 9 after the process step indicated
in FIG. 15, together with the plate 10, which is produced by means
of the same process. The plates 9 and 10 are joined and adjusted so
that the sides that include the channels 11 are facing one
another.
[0049] As shown in FIG. 17, the joined plates 9 and 10 are tempered
at 1000.degree. C. to connect them to one unit, which includes the
gas passages formed therein such as, as illustrated here, the main
gas passage 4. The connecting points 12 and 13 of the main gas
passage 4 have a respective diameter of 480 .mu.m. The areas
between the connecting point 12 and the junction point 22 and
between the connecting point 13 and the junction point 23 have a
respective diameter of 320 .mu.m. Finally, the areas between the
junction points 22 and 23 have a respective diameter of 30
.mu.m.
[0050] In a last process step, which is illustrated in FIG. 18, the
two joined plates 9 and 10 are sawed at the saw marks, which were
defined in the process step illustrated in FIG. 4.
[0051] FIG. 19 shows a block diagram of the preferred embodiment of
the gas flow switching device according to the invention depicted
in FIG. 1. On one side, the separation column 1 is connected to an
injection unit 80, from which a gas sample 81 that is to be
analyzed is guided through the separation column 1 by means of a
carrier gas. To this end, the injection unit 80 is connected with a
carrier gas source 83 via an electronic pressure regulator 82. At
its other end, the separation column 1 is connected to a unit 84,
which includes the plates 9 and 10 depicted in FIG. 1, together
with the gas passages 4 to 8 formed therein. The auxiliary gas
passages 7 and 8 of the unit 84 are connected to the carrier gas
source 83 via a device 85 for setting different pressures in the
auxiliary gas passages 7 and 8. Furthermore, the separation column
2, together with a downstream detector 86, is connected to unit
84.
[0052] In the embodiment shown here, the device 85 includes two
electronic pressure regulators 87 and 88. The pressure regulator 87
connects the auxiliary gas passage 7 to the carrier gas source 83
and the pressure regulator 88 connects the auxiliary gas passage 8
to the carrier gas source 83. Due to the highly precise
manufacturing data of the gas passages 4 to 8 in the unit 84, the
pressures in the auxiliary gas passages 7 and 8 can be adjusted via
the set points of the pressure regulators 87 and 88 without the
need for calibration. To this end, all flows and pressures are
calculated based on the known geometrical data of the gas passages
4 to 8 and based on the geometries of the separation columns 1 and
2. Therein, the appropriate flow rates in the separation columns 1
and 2 as a function of the gas type and of the operating
temperature are taken into account. Furthermore, the
compressibility of the gas is taken into account too. The
calculated pressures are supplied as set point values to the
pressure regulators 82, 87 and 88.
[0053] In a next step, a sample gas, e.g., air, which practically
does not interact with the separation phases of the separation
columns 1 and 2, is guided through the separation columns 1 and 2
via the injection device 80. The transit time of the sample gas
through the separation columns 1 and 2 is measured, and from the
measured transit time the average inner diameter of the separation
columns 1 and 2 is calculated. By means of the average inner
diameters of the separation columns 1 and 2 thus determined, the
set point values for the pressure regulators 82, 87 and 88 are
recalculated and finally set, so that the gas flow switching device
is thereby calibrated. This eliminates a time-consuming calibration
of needle valves for adjusting the pressure.
[0054] The above description of the preferred embodiments has been
given by way of example. From the disclosure given, those skilled
in the art will not only understand the present invention and its
attendant advantages, but will also find apparent various changes
and modifications to the structures disclosed. It is sought,
therefore, to cover all such changes and modifications as fall
within the spirit and scope of the invention, as defined by the
appended claims, and equivalents thereof.
* * * * *