U.S. patent application number 12/315614 was filed with the patent office on 2009-06-04 for pressure transducer employing a micro-filter and emulating an infinite tube pressure transducer.
This patent application is currently assigned to Kulite Semiconductor Products, Inc.. Invention is credited to Anthony D. Kurtz, Tonghuo Shang.
Application Number | 20090139339 12/315614 |
Document ID | / |
Family ID | 39968324 |
Filed Date | 2009-06-04 |
United States Patent
Application |
20090139339 |
Kind Code |
A1 |
Kurtz; Anthony D. ; et
al. |
June 4, 2009 |
Pressure transducer employing a micro-filter and emulating an
infinite tube pressure transducer
Abstract
A pressure transducer for measuring pressures in high
temperature environments employs a tube which is terminated at one
end by an acoustic micro-filter. The acoustic filter or
micro-filter has a plurality of apertures extending from one end to
the other end, each aperture is of a small diameter as compared to
the diameter of the transducer and the damper operates to absorb
acoustic waves impinging on it with limited or no reflection.
Mounted to the tube is a pressure transducer with a diaphragm flush
with the inner wall of the tube. The tube is mounted in an aperture
in a casing of a gas turbine operating at a high temperature. The
hot gases propagate through the tube where the pressure of the
gases are measured by the transducer coupled to the tube and where
the acoustic filter operates to absorb acoustic waves impinging on
it with little or no reflection, therefore enabling the pressure
transducer to be mainly responsive to high frequency waves
associated with the gas turbine operation.
Inventors: |
Kurtz; Anthony D.; (Saddle
River, NJ) ; Shang; Tonghuo; (Basking Ridge,
NJ) |
Correspondence
Address: |
The Plevy Law Firm;Arthur L. Plevy
10 Rutgers Place
Trenton
NJ
08618
US
|
Assignee: |
Kulite Semiconductor Products,
Inc.
Leonia
NJ
|
Family ID: |
39968324 |
Appl. No.: |
12/315614 |
Filed: |
December 4, 2008 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
11409139 |
Apr 21, 2006 |
7484415 |
|
|
12315614 |
|
|
|
|
Current U.S.
Class: |
73/727 |
Current CPC
Class: |
G01L 19/0007 20130101;
G01L 19/0681 20130101; G01L 19/0636 20130101; G01L 19/0609
20130101 |
Class at
Publication: |
73/727 |
International
Class: |
G01L 9/06 20060101
G01L009/06 |
Claims
1. A pressure transducer assembly for measuring pressure in high
temperature environments, comprising: a tube having a first opened
end and a second opened end, a pressure transducer mounted on a
surface of said tube and extending in to the tube opening to allow
said transducer to measure a pressure applied to said tube via said
first opened end, said pressure obtained from a pressure source to
be monitored, and a micro-filter mounted and positioned in said
second opened end and operative to absorb incoming acoustic waves
associated with said pressure source applied to said first opened
end.
2. The pressure transducer assembly according to claim 1, wherein
said pressure source includes hot gases emanating from a gas
turbine engine.
3. The pressure transducer assembly according to claim 2, wherein
said front opened end of said tube is mounted within an aperture of
a gas turbine engine casing.
4. The pressure transducer assembly according to claim 1, wherein
said micro-filter has a plurality of apertures extending from a
front surface to a back surface, with said filter positioned and
mounted in said second opened end of said tube with said front
surface surrounded by said tube wall, wherein pressure waves in
said tube are directed to said apertures and travel from said front
to back surface of said filter where they are absorbed.
5. The pressure transducer assembly according to claim 4, wherein
said apertures have a tapered opening at said front end with said
opening at said front end having a larger diameter which tapers
into a smaller diameter along a given length of said aperture.
6. The pressure transducer assembly according to claim 5, wherein
said front opening of said apertures are conical in cross
section.
7. The pressure transducer assembly according to claim 4, wherein
said micro-filter is fabricated from glass tubes with said
apertures formed by assembling glass tubes of a given diameter in
to a cylindrical array.
8. The pressure transducer assembly according to claim 7, wherein
said glass tubes have a tapered front opening with a larger
diameter at the front which tapers into a smaller diameter.
9. The pressure transducer assembly according to claim 7, wherein
said glass tubes prior to forming said filter have an outer
diameter between 0.005 inches to 0.020 inches and an inner diameter
between 0.004 and 0.015 inches, depending on the frequency of the
acoustic waves to be absorbed.
10. The pressure transducer assembly according to claim 1, wherein
the temperature of operation is greater than 700.degree. C.
11. A micro-filter for use in conjunction with a pressure
transducer, said pressure transducer having a diaphragm mounted
flush with an inner wall of a pressure conducting tube, said tube
having a front opened end for receiving a pressure and a back
opened end, said filter in combination therewith, comprising: a
cylindrical member having a front surface and a back surface, a
plurality of apertures of a given diameter found in said member and
directed from said front to said back surface and having a diameter
and length selected to absorb lower frequency acoustic waves
associated with said received pressure, said member adapted to be
mounted at said back opened end of said tube with said apertures
aligned parallel to the axis of said pressure conducting tube.
12. The micro-filter according to claim 11, wherein said
cylindrical member is fabricated from a plurality of glass tubes
each having said given diameter and arrayed in a bundle to form
said member.
13. The micro-filter according to claim 12, wherein said glass
tubes have a larger diameter front opening which tapers along a
partial length of said tube to a smaller diameter.
14. The micro-filter according to claim 13, wherein said glass tube
tapers are conical in cross section.
15. The micro-filter according to claim 13, wherein said glass
tubes are tapered using a conically shaped coring tool.
16. The micro-filter according to claim 11, wherein said opened
front end of said tube is coupled to the inner wall of a gas
turbine casing.
17. The micro-filter according to claim 11, wherein said pressure
transducer is a piezoresistive pressure transducer.
18. The micro-filter according to claim 1, wherein said apertures
have an inner diameter between 0.004 and 0.015 inches.
19. The micro-filter according to claim 12, wherein said glass
tubes have an outer diameter between 0.005 to 0.020 inches and an
inner diameter between 0.004 and 0.015 inches.
20. The micro-filter according to claim 11, wherein the temperature
of operation is greater than 700.degree. C.
Description
FIELD OF THE INVENTION
[0001] This invention relates to pressure transducers and more
particularly to a pressure transducer incorporating a micro-filter
replacing the prior art infinite tube.
BACKGROUND OF THE INVENTION
[0002] Aerodynamic engineers have long desired to measure high
frequency flow and pressure disturbances in gas turbine engines and
aircraft wings. The capability was made possible with extremely
compact pressure transducers fabricated from micro-machined
silicon. The frequencies of concern were for example, in the tens
of thousands of kilohertz (kHz). As such, Kulite Semiconductor
Products, Inc., the assignee herein, has developed many transducers
which operate to measure such pressure disturbances in gas turbine
engines and aircraft wings. Such devices are the subject matter of
various patents that describe their operation and fabrication. See,
for example, U.S. Pat. No. 6,612,178 entitled "Leadless Metal Media
Protected Pressure Sensor" issued on Sep. 2, 2003 to A. D. Kurtz et
al. and assigned to the assignee herein. See also, U.S. Pat. No.
6,363,792 entitled "Ultra High Temperature Transducer Structure"
issued on Apr. 2, 2002 to A. D. Kurtz et al. and assigned to the
assignee herein. In any event, as will be explained, there are
certain situations where mounting of the transducer becomes
extremely difficult.
[0003] For example, in order to determine the pressure and high
frequency flow in gas turbines, a recessed pipe is attached to the
combustion chamber that allows the hot gasses within the chamber to
cool before reaching the sensor. While the pipe does successfully
cool the gases, it also reduces measurement of bandwidth because of
the generation of harmonic frequencies. Similar to blowing air over
an open bottle top, the air inside the recessed pipe will be
compressed by the air jet back out of the recess. In essence, the
air inside the bottle acts as a spring. The oscillations of the air
inside the recess results in a resonant frequency similar to that
of an organ pipe. Such vibrations make measuring the pressure and
flow of the gases within the combustion chamber of the gas turbine
difficult. As will be explained, in the prior art, a long, curled
tube (or "infinite tube") of decreasing diameter has been used to
remove such resonances. However, such a solution requires many feet
of tubing and very accurate coiling of the tube. Further
difficulties associated with the prior art include the lack of
access for mounting such transducers in a turbine case, as well as
problems which involve discriminating against low and high
frequencies.
[0004] An alternative mechanism that overcomes one or more of these
problems is desirable.
SUMMARY OF THE INVENTION
[0005] A micro-machined filter operates in conjunction with a
transducer to absorb incoming acoustic waves and can be installed
in close proximity to the transducer, thereby eliminating the long
curled tube or the so called infinite tube.
[0006] A pressure transducer assembly for measuring pressure in
high temperature environments comprises: a tube having a first
opened end and a second opened end, a pressure transducer mounted
on a surface of the tube and extending in to the tube opening to
allow the transducer to measure a pressure applied to the tube via
the first opened end, the pressure obtained from a pressure source
to be monitored. A micro-filter mounted and positioned in the
second opened end is operative to absorb incoming acoustic waves
associated with the pressure source coupled to the front opened end
and operative to apply a pressure thereto. The first opened end of
said tube has a larger diameter which tapers to a smaller diameter
front opening, the front towards the back opened end along a given
tube length.
BRIEF DESCRIPTION OF THE FIGURES
[0007] FIG. 1 depicts a prior art technique of mounting a pressure
transducer to a turbine casing designated as a flush mount
condition;
[0008] FIG. 2 depicts an alternate method of mounting a pressure
transducer to a turbine casing using a elongated tube;
[0009] FIG. 3 depicts still another technique of mounting a
pressure transducer to a turbine casing using a coiled tube or
infinite tube array;
[0010] FIG. 4 depicts a pressure transducer mounted to a turbine
casing employing the micro-damper according to an embodiment of the
present invention;
[0011] FIG. 5 shows a pressure transducer utilizing a micro-damper
or filter according to an embodiment of the present invention;
[0012] FIG. 6 shows a front view of a micro-damper according to an
embodiment of the present invention;
[0013] FIG. 7 shows an enlarged view of the array of pores or
apertures in the micro-damper of FIG. 6.
[0014] FIG. 8 shows a micro-damper having tapered apertures.
DETAILED DESCRIPTION OF THE INVENTION
[0015] Referring to FIG. 1, there is shown a prior art technique
depicting a typical installation which is employed in the
aerodynamics industry. In FIG. 1, reference numeral 15 represents a
gas turbine casing. The gas turbine casing is typically found in a
gas turbine engine. The operation of such an engine is attendant
with extremely high temperatures which are directed to the casings.
There is shown a transducer 10 which is mounted on a housing 16.
Housing 16 is threaded and essentially threads into a threaded
aperture which is formed in the turbine casing 15. Located remote
from the transducer 10 is a sensing diaphragm 12. The sensing
diaphragm 12 as seen in FIG. 1 is responsive to the pressure
created by the hot air gases associated with the turbine which
therefore causes the diaphragm 12 to deflect and produces a
pressure response from the transducer which is coupled thereto via
the tube cavity 17. The cable 11 directs the output from the
transducer 16 to various monitoring equipment as is well known. As
indicated, FIG. 1 shows a typical installation with the transducer
10 installed on the gas turbine case 15. The close coupling of the
transducer sensing diaphragm 12 which is flush mounted with the
inner wall surface 18 of the turbine case gives a relatively good
measurement frequency. The upper limit of the frequency being the
sensor resonant frequency, is typically in the hundreths of KHz.
The diaphragm 12 basically is flush with the inner wall 18 of the
turbine casing. However, there are certain situations where the
flush mount is not possible.
[0016] Referring to FIG. 2, there is shown a more typical
installation where the transducer 20 must be recessed some distance
away from the turbine case 24. The reasons for such a recess
include lack of access, but most often this is due to the extremely
high turbine gas temperature pressure compressor temperatures which
can be as high as 2000.degree. C. These temperatures require use of
the elongated tube 23. In modern aircraft engines the compressor
air temperature reaches about 700.degree. C. and the combustor gas
temperature can be as high as about 20000.degree. C. The latter
temperature is beyond the capabilities of even the most advanced
piezoresistive transducers. Thus, the measurement of pressure at
these high temperatures involves a recessed installation with the
transducer and pressure source separated by a pipe or tube 23 so
that the transducer is located in a somewhat cooler ambient area.
As seen in FIG. 2, the pipe 23 extends from the gas turbine case 24
and now accommodates the transducer 20, the cable 21 and the
diaphragm 22. The diaphragm 22 is remote from the casing 24 (as
compared to FIG. 1). One example of transducer 20 is the high
temperature miniature IS pressure transducer manufactured and sold
by Kulite Semiconductor Products, Inc., as the XCEL-072 series.
Such transducers are about 0.075 inches (0.075'') in diameter. The
pipe 23 while shown not to scale in FIG. 2, is essentially an organ
pipe with a finite length. This reduces the measurement frequency
due to organ pipe harmonic frequencies, defined as f=c/4 L, where f
is equal to the frequency, c is equal to the velocity of the hot
air and L is the length of the pipe as for example pipe 23. As is
known, the recess pipe reduces the measurement frequency based on
pipe harmonic frequencies as defined by the above equation and high
harmonics. For example, a one inch long organ pipe-filled with air
has a resonant frequency of about 3.3 kHz representing an acoustic
wave bouncing back and forth between the organ pipe ends. The
usable frequency is even less, by about a factor of 5, to about 60
Hz, which is too low for most gas turbine applications. To overcome
this limitation, a technique known as infinite tube pressure
transducer solves the organ pipe frequency limitation.
[0017] Referring to FIG. 3, a transducer 30 (as above) is installed
on the side wall of the so called infinite tube 35. The transducer
is installed some distance D away from the hot gas inlet, typically
a distance of about one to six inches, with longer distances if the
gas temperature is higher. The tube 35 is coupled to the gas
turbine case 32 via an aperture. The hot air enters the tube at
inlet 35a and the tube has an end which essentially is coiled as
indicated by reference numeral 36. The acoustic waves as generated
from the source, enter the tube 35 and travel to the transducer
location with little or no attenuation. Because there is basically
no reflection off the far end, the transducer measurement will not
be contaminated with organ pipe harmonics and thus will measure
static and dynamic pressure to higher frequencies than achievable
in installations such as those depicted in FIG. 1 and FIG. 2. This
arrangement allows the transducer to be positioned in a cooler
location, therefore, allowing pressure measurements at very high
gas temperatures. In situations where dynamic pressure at the
transducer location is attenuated, calibration curves can be used
to correct the measurement data. The infinite tube which typically
can be 30 to 1000 feet long, is packaged into a cylindrical bundle
as indicated by reference numeral 36 to the size of about 2 to 3
inches in diameter, and 3 to 5 inches long. As one can ascertain,
this cylindrical bundle is rather large, compared for example, to
the dimensions of the transducer 30 associated with the deflection
diaphragm 31. For example, a typical IS transducer as indicated
above, is about 0.375 inches in length and has a diameter of about
0.075 inches. Thus, as one can ascertain, the infinite tube package
is quite large compared to the size of the transducer. The infinite
tube package is also cumbersome to handle in practice. For example,
slight kinks in the tube cause undesirable acoustic reflections.
Therefore, great care must be taken in coiling the tube into a
cylindrical bundle. It is thus preferable to use small diameter
tubes for ease of packing and low weight. However, better
performance results if the sensing diaphragm is as close to the
tube's inner wall as possible, thus avoiding sharp edges and
cavities. The edges and cavities are sources of acoustic
reflection. For this reason, larger diameter tubes or tubes of oval
cross section are more desirable so that small diameter (e.g.,
0.075 inch) transducers can be used. Practically, the infinite tube
diameter is a compromise between these two constraints, and is
typically about 0.125 inches in diameter. Thus, even with optimum
packaging, typical infinite tube transducers are size and weight
limited. They are prone to damage by shock and vibration typically
found in gas turbine test environments. It is well known that
instrumentation engineers prefer not to use these transducers
whenever an alternative method is available. Because of these
limitations, the infinite tube transducer is used by few of the
world's gas turbine manufacturers. For example, of the particular
uses of infinite tubes, reference is made to two pending
applications, entitled "Low-Pass Filter Semiconductor Structures
for Use in Transducers for Measuring Low Dynamic Pressures in the
Presence of High Static Pressures" by A. D. Kurtz et al. and
assigned to the assignee herein, and "Improved Pressure Transducer
for Measuring Low Dynamic Pressures in the Presence of High Static
Pressures" also by A. D Kurtz and assigned to the assignee herein.
The above-identified applications describe infinite tube
transducers and essentially the characteristics and operation of
such tubes in frequency responsive applications.
[0018] Referring now to FIG. 4, there is shown schematically an
apparatus according to an exemplary embodiment of the present
invention which essentially eliminates the infinite tube bundle
depicted in FIG. 3. According to an aspect of the present
invention, a micro-filter 45 essentially mimics the effects and
benefits associated with the infinite tube structure illustrated in
FIG. 3. The micro-filter 45 operates to absorb acoustic waves
impinging on it with limited or no reflection. One way of achieving
a micro-filter is to use a wafer of silicon with micro-pores etched
from the wafer. The micro-pores are small in diameter so as to
maximize viscous damping effects. Because the acoustic waves are
likely to bounce off solid surfaces, the micro-filter surface
facing the flow should be as small as possible and of course
include pores or apertures in the surface. As shown in FIG. 4, a
tube 46 is coupled to the turbine casing and allows hot air to
enter the front opening 42a. The transducer 40 is remotely located
from the turbine casing and is placed on the surface of the tube 46
with the sensing diaphragm 41. Following and terminating the
infinite tube at the back opening 42b is the micro-filter 45. The
micro-filter replaces the very long tube as indicated in FIG. 3,
achieving substantial size and weight reductions. A number of
unanticipated benefits are derived from this construction.
[0019] Referring to FIG. 5, there is shown a cross-sectional view
of a transducer assembly utilizing a micro-filter 60. The
micro-filter 60 can be fabricated by a number of techniques,
including, for example, use of a silicon wafer having suitable
pores etched therein. The micro-filter or damper can be a
compilation of micro-glass tubes each of about 0.5 inches in
length. One can use an additional porous silicon wafer to increase
damping. A single glass tube array or a single porous silicon wafer
can alternatively be used. As one can see, the pressure transducer
assembly has a housing 61, which housing contains the pressure
transducer 62 with the diaphragm end of the transducer 68 located
within the cavity 65 of the housing assembly 61. The cavity 65
extends from one opened end to the other opened end, where the
other opened end is terminated with the micro-damper 60 and has an
end cap 66. A mounting arrangement indicated generally as reference
numeral 67 is shown for mounting the entire unit to a suitable
structure, and includes fitting screws and/or other well know
connectors. Such mounting arrangement is well known and further
description is omitted herein for brevity.
[0020] Referring to FIG. 6, there is shown a cross-sectional view
of the micro-filter or damper 60 depicted in FIG. 5. As one can
ascertain there are a plurality of small apertures 81 which extend
from one end of the damper 80 to the other end. FIG. 7 shows an
enlarged view of the apertures in FIG. 6. As one will understand,
the micro-filter 60 can be fabricated from silicon and one can etch
apertures shown in FIG. 6 and FIG. 7 into the silicon by
conventional etching techniques. The etching of silicon and
formation of apertures in silicon is well known. The acoustic
damper 60 is fabricated by the packing small diameter glass tubes
which basically are stacked within an outer shell or housing
depicted by reference numeral 80 of FIG. 6. The glass tubes can
have an inner diameter of about 0.004 inches with an outer diameter
of about 0.005 inches. The outer diameter can vary between about
0.005 inches and 0.02 inches with the inner diameter varying
between about 0.004 inches and 0.015 inches. The tubes are about
0.5 inches in length. The variation of diameters is a function of
the frequencies to be accommodated. Thus, the glass tube matrix
array as shown in FIGS. 6 and 7 illustrate configuration(s)
employed with the glass tubes abutting against one another. The
glass tubes are conventionally joined together under heat and one
then extrudes the bundle to produce the array. A wafer of silicon
can be utilized with the apertures directed from a first to a
second surface of the silicon, or both devices can be employed
together. In any event, the present invention has many advantages
which are not accommodated by the prior art techniques. For
example, the size and weight of the unit, in contrast to the unit
of FIG. 3, are greatly reduced by at least one order of magnitude.
The device shown in FIG. 5 is easier to handle and less susceptive
to shock and vibration damage and represents a more viable device
for wide spread use in gas turbine testing or laboratory research.
The device further extends the high frequency pressure measurement
capability in extremely high temperature and high vibration
environments. Still further, larger diameter coupling tubes can be
used without significant size and weight boundaries because the
very long infinite tube is eliminated and a compact micro-filter
(as for example 60 depicted in FIG. 5) is employed. The pressure
transducer 62, for example, having a 0.075 inch diameter housing as
seen in FIG. 5, is flush mounted to the tube 65 inside wall with
little or no step cavity. This is depicted in FIG. 5 wherein the
diaphragm portion 68 of the transducer 62 is flush with the inner
wall of the internal tube cavity 65. The larger diameter reduces
viscous damping as pressure waves travel to the transducer along
the tube. Both factors lead to better pressure measurements, in
terms of both accuracy as well as frequency range. The pressure
measurements using such a technique will include both static and
dynamic pressure when a piezoresistive pressure transducer is
employed for transducer 62. Also, as new high temperature
piezoresistive transducers are developed, the distance between the
transducer and the hot gases can be reduced, thus allowing pressure
measurements to be made with better accuracy and higher frequency.
While it is clear that the above noted damper operates at
substantially reduced organ pipe resonance, the use of the glass
tube embodiment as for example shown in FIG. 7 experiences certain
problems. While the damper depicted in FIGS. 6 and 7 operates
favorably, it has one particular problem, in that based on the
large surface area between tubes, acoustic waves can be reflected
by this configuration and hence, the reflected acoustic waves
produce undesirable damping, which is not optimal for certain
applications. The optimal result is to have a damper which has a
zero impedance operating in an acoustic channel for replacement of
the infinite tube pressure transducers. In the embodiment depicted
in FIGS. 6 and 7, the acoustic wave will impinge on the flat
surface that exists between the glass tubes. Thus, even if the
cylindrical surface contains a large number of holes through which
some of the sound waves can pass, the remaining flat surface which
basically is provided by the area between the tubes operates as an
acoustic reflecting surface.
[0021] In FIG. 8 there is shown again a bundle of glass tubes which
basically form a cylindrical member 80 which is approximately a
quarter of an inch in diameter D and which contains approximately
20 to 40 through holes, each of which is about 10 mils in diameter.
In regard to this configuration, one then utilizes a conically
shaped diamond impregnated coring tool. This coring tool operates
to enlarge each hole on the surface on which the acoustic wave will
impinge. In one exemplary configuration depicted in FIG. 8, a
conical tube has a diameter D1 of 0.100 inches tapering down to D2
0.025 inches over a length of 0.150 inches. Each hole is enlarged
with the tube and essentially the resulting structure presents a
zero acoustic impedance. With the input acoustic wave directed
towards the conical openings 91 and 92, there is very little area
for which the wave to be reflected. Hence, the wave is absorbed and
enters each of the conical apertures 91 and 92 and travels down the
respective tube 90 which operates to again damp. While the
embodiment shown in FIGS. 6 and 7 operates, it does not operate as
efficiently as the embodiment depicted in FIG. 8. The embodiment
depicted in FIG. 8 eliminates resonances due to reflections off the
front surface of the damper. For example, in regard to the damper
shown in FIGS. 6 and 7 utilizing an overall cylindrical diameter of
0.25 inches and having 31 holes in the glass cylindrical member
each hole having a diameter of 12 mils, this allows an open area
equal to 7.14%. This open area is small and therefore there is a
large amount of reflection from the front surface of the damper
causing less than optimum performance. In regard to the
configuration shown in FIG. 8 utilizing the same number of holes,
the initial diameter due to the coring now is 38 mils and reduces
to a diameter of 12 mils. This creates a conical hole entrance
which basically results in a zero impedance structure. As there is
very little surface area for which the acoustic wave to reflect
from, the acoustic wave enters the conical apertures and is
absorbed within the conical tube. It is noted that the conical
apertures taper from a front opening towards the back during a
predetermined portion of the tube. This taper can be changed or
varied. In any event, as indicated above, the taper extends about
0.15 to 0.25 inches along the length of the tube. Each tube is
typically 0.5 inches in length. It is of course understood that the
length can vary as well as the diameter of the apertures can vary
dependent upon the frequencies to be accommodated. While the use of
micro-machined silicon can be employed as the micro-filter, other
materials can be used, such as micro-machined glass or
micro-machined silicon carbide. Thus, the acoustic damper can be
employed and fabricated utilizing many different materials.
[0022] It will be apparent to those skilled in the art that
modifications and variations may be made in the apparatus and
process of the present invention without departing from the spirit
or scope of the invention. It is intended that the present
invention cover the modification and variations of this invention
provided they come within the scope of the appended claims and
their equivalents.
* * * * *