U.S. patent number RE34,373 [Application Number 07/704,774] was granted by the patent office on 1993-09-07 for microwave heating apparatus for laboratory analyses.
This patent grant is currently assigned to CEM Corporation. Invention is credited to Michael J. Collins, Dennis P. Manchester.
United States Patent |
RE34,373 |
Collins , et al. |
September 7, 1993 |
Microwave heating apparatus for laboratory analyses
Abstract
A microwave oven particularly suitable for laboratory analytical
use is described. The oven is designed for chemical digestion and
the drying of materials to very low moisture levels. The oven
utilizes a rotating platform to move the material being subjected
to microwave radiation through the oven chamber to ensure uniform
contact of the microwaves with the material. Radiation mixers and
radiation isolators are also used to disperse radiation in the oven
and absorb excess radiation.
Inventors: |
Collins; Michael J. (Charlotte,
NC), Manchester; Dennis P. (Matthews, NC) |
Assignee: |
CEM Corporation (Matthews,
NC)
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Family
ID: |
27363975 |
Appl.
No.: |
07/704,774 |
Filed: |
May 23, 1991 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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31906 |
Mar 30, 1987 |
|
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|
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416011 |
Sep 8, 1982 |
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Reissue of: |
189727 |
May 3, 1988 |
04835354 |
May 30, 1989 |
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Current U.S.
Class: |
219/751; 219/754;
333/22F |
Current CPC
Class: |
B01L
7/00 (20130101); F26B 3/343 (20130101); H05B
6/666 (20130101); G01N 1/44 (20130101); F26B
9/003 (20130101) |
Current International
Class: |
B01L
7/00 (20060101); F26B 3/32 (20060101); F26B
9/00 (20060101); F26B 3/34 (20060101); G01N
1/44 (20060101); H05B 6/80 (20060101); H05B
6/68 (20060101); H05B 006/78 () |
Field of
Search: |
;219/1.55B,1.55R,1.55A,1.55D,1.55F,1.55E ;392/356 ;333/22R,22F
;374/149 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Leung; Philip H.
Attorney, Agent or Firm: Kramer; Raymond F.
Parent Case Text
This is a continuation of application Ser. No. 07/031,906 filed
Mar. 30, 1987 which is a continuation of application Ser. No.
06/416,011 filed Sept. 8, 1982, now both abandoned.
Claims
What is claimed is:
1. An analytical microwave apparatus that is especially suitable
for laboratory heatings to digest or dry small analytical samples,
which comprises a chamber, which retains microwave radiation
therein except for that which exits back out through an opening in
the chamber which is an entrance for such radiation, which chamber
is devoid of any microwave absorbent material except for any
analytical samples being heated, a magnetron located outside of
such chamber, a wave guide located between the chamber and the
magnetron and communicating them through the chamber opening, an
isolator, comprising a magnetic shape in the wave guide, and a
dummy load and a heat sink, in combination, external to such wave
guide, which dummy load absorbs any microwave radiation that exits
from the chamber through the wave guide, a first fan or blower for
directing air past the heat sink to cool it, so that the air can
cool the heat sink continuously over long time periods when the
magnetron is operating at full power and there are no analytical
samples in the chamber, mechanical radiation mixing means for
dispersing throughout the chamber microwave radiation entering the
chamber from the wave guide, a second fan or blower, of variable
speed capability, for cooling the chamber by drawing air through
it, and for removing from the chamber any volatiles that may be
generated by the analytical samples during heating thereof, a
turntable for moving analytical samples in the chamber while they
are being subjected to microwave radiation therein, and
programmable means for controlling power input to the magnetron and
the duration of such power input, so that the analytical samples
may be safely and effectively heated to digest or dry them without
damaging the magnetron and without allowing escaping of microwave
radiation from the analytical microwave apparatus.
2. An analytical microwave apparatus according to claim 1 wherein
the turntable is of synthetic organic polymeric plastic material
which is transparent to microwave radiation.
3. An apparatus according to claim 2 wherein the turntable is of
polypropylene, polycarbonate, polyester or
polytetrafluoroethylene.
4. An apparatus according to claim 2 wherein the turntable is of
variable speed capability.
5. An apparatus according to claim 4 wherein the speed of rotation
of the turntable is adjustable within the range of 1/2 to 20
revolutions per minute.
6. An apparatus according to claim 1 wherein the chamber includes
interior walls of microwave transparent and chemically resistant
material and a door interior wall of such material.
7. An apparatus according to claim 1 wherein the programmable means
for controlling power input to the magnetron and for controlling
the duration of such power input includes a microprocessor.
8. An apparatus according to claim 7 which comprises a digital
panel readout which provides visible information about the program
in operation in the apparatus. .Iadd.
9. An analytical microwave apparatus that is suitable for
laboratory heatings to digest or dry analytical samples, which
comprises a chamber, which retains microwave radiation therein
except for that which exits back out through an opening in the
chamber, which is an entrance for such radiation, which chamber is
devoid of any microwave absorbent material except for any
analytical samples being heated, a magnetron located outside of
such chamber, a wave guide located between the chamber and the
magnetron and communicating them through the chamber opening, an
isolator, comprising a magnetic shape in the wave guide, and a
dummy load and a heat sink, in combination, external to such wave
guide, which dummy load absorbs any microwave radiation that exits
from the chamber through the wave guide, means for directing air
past the heat sink to cool it, so that the air can cool the heat
sink continuously over long time periods when the magnetron is
operating at full power and there is no analytical sample in the
chamber, mechanical radiation mixing means for dispersing
throughout the chamber microwave radiation entering the chamber
from the wave guide, a turntable for moving an analytical sample in
the chamber while it is being subjected to microwave radiation
therein, and programmable means for controlling power input to the
magnetron and the duration of such power input, so that analytical
samples may be safely and effectively heated to digest or dry them
without damaging the magnetron and without allowing escaping of
microwave radiation from the analytical microwave apparatus.
.Iaddend. .Iadd.
10. An analytical microwave apparatus according to claim 9 wherein
the means for directing air past the heat sink to cool it is a
blower. .Iaddend. .Iadd.11. An analytical microwave apparatus
according to claim 9 wherein the means for directing air past the
heat sink to cool it is a fan. .Iaddend.
Description
This invention relates to an apparatus for the microwave drying and
chemical digestion of materials for laboratory purposes. More
particularly, the invention is directed to a microwave oven
particularly suitable for analytical laboratory usage such as for
the rapid drying of material to extremely low moisture levels and
the rapid chemical digestion of materials as is desirable for
analytical and laboratory purposes.
BACKGROUND OF THE INVENTION
Microwave ovens have come into common usage primarily for the
heating and cooking of food for human consumption. Various other
uses have also been developed. For instance, the microwave oven has
been found to be particularly suitable for the rapid drying of
various substances and determination of volatile content such as is
set forth in U.S. Pat. No. 3,909,598. Further, it is known that
microwave radiation greatly enhances the chemical digestion of
various materials either as a result of the molecular stimulation
caused by microwave radiation and/or the heating effect. Methods
which use microwave radiation in acid digestion are also known.
Various developments in microwave oven technology have included the
ability to program the oven to control the power input/output of
the magnetron as well as the length of time that the magnetron will
run. However, because most of these designs have been directed to
ovens which maximize the energy input into the substance being
heated such as is required for cooking, such ovens are not suitably
designed for laboratory usage.
In laboratory usage, many analytical methods require the reduction
or elimination of moisture in a sample, or at least, reduction to a
very low near complete dryness. This makes conventional microwave
ovens unsuitable for many analytical methods. As moisture level in
a sample subjected to microwave oven heating is reduced to low
levels, excessive radiation energy is reflected from the oven as it
can no longer be absorbed by the sample. The loss of the polar
absorbing material in the sample causes the microwaves to be
reflected back to the magnetron. Such reflected radiation quickly
damages the magnetron.
Further, due to the characteristics of microwave radiation, it is
diffucult even under ideal mechanical and size configurations based
on wave length, to thoroughly disperse the radiation throughout an
oven to provide uniform heating. Typical microwave ovens are best
suitable for the placement of relatively large packages to be
heated within the oven thereby taking up a relatively large
percentage of the oven capacity. With analytical laboratory usage,
normally relatively small samples are used, thus taking up a very
small percentage of the oven cavity. When small samples are used,
excess radiation has inadequate polar material to absorb the energy
and the wave energy is thus reflected back to the source i.e., the
magnetron. Also, hot spots frequently develop in the sample which
may destroy part of the sample. For most cooking purposes,
localized hot spots are of little significance because they in turn
aid in convection and conductive heating of the rest of the package
notwithstanding the localized heating to higher temperatures in
various portions of the package. Such irregular heating and the
development of hot spots is particularly unsuitable for laboratory
usage.
THE INVENTION
In accordance with the invention, a microwave oven particularly
suitable for analytical laboratory usage is provided comprising an
enclosure forming a chamber for the retention of microwave
radiation, said enclosure having means communicating therewith for
dispersing microwave radiation throughout said chamber, means for
absorbing excess microwave radiation, means for removing volatiles
from said chamber and means for moving items in the said chamber
while being subjected to microwave radiation, said microwaves being
directed into said chamber from a magnetron, said magnetron power
input, duration of power input, volatile removal means and moving
means in said chamber being controlled to interact with each other
as selectively programmed prior to operation. The preferred
apparatus utilizes a turntable positioned in said chamber which
preferably forms an oven and said turntable slowly rotates in the
chamber during the application of microwave energy.
DETAILS OF THE INVENTION
The present apparatus provides the ability to reduce the moisture
content of a sample for analytical purposes to less than one tenth
of a milligram of water without danger of damaging the magnetron of
the microwave oven. Samples of extremely small size, on the order
of less than one gram up to the capacity of the oven, can be
effectively utilized while providing substantial improvements in
even heating. The oven utilizes a rotatable turntable for placement
of the sample which results in the sample being passed throughout
the oven as it is radiated. This avoids a fixed location which may
result in excessive or inadequate radiation. In addition, the
microwave oven utilizes radiation dispersing means and radiation
isolator means. These modifications result in a microwave oven
which is totally transparent to microwave radiation and can be
safely operated with a zero load, i.e., empty oven cavity, without
danger of magnetron damage.
BRIEF DESCRIPTION OF THE DRAWING
The invention will be more readily described by reference to the
drawing, wherein:
FIG. 1 is a partially cutaway perspective view of the analytical
microwave apparatus of the present invention;
FIG. 2 is a cutaway view of the interior sidewalls of the microwave
apparatus of the invention; and
FIG. 3 is an elevational view of control panel parts identified by
numerals 26, 28 and 30 of FIG. 1, and a block diagram of associated
circuitry for controlling the invented apparatus.
The apparatus of the present invention comprises a microwave oven
10 specifically adapted for the purpose of reducing volatile
content to a very small residual. A conventional microwave oven 10
is modified to provide a rotatable turntable 12 which is
transparent to microwave radiation. Polypropylene, Teflon,
polycarbonates, polyester and the like plastics are preferably used
for the turntable 12 and axle 14. Turntable 12 is fixed to an axle
14 which extends through the floor 16 of microwave oven 10. Axle 14
is driven by motor 18 which can be of variable speed or of fixed
geared ratio to provide the desired turning speed. It is preferable
to have a variable speed turntable because it has been found that
certain processes for which the present invention is particularly
suited are responsive to different rotational speeds. Thus a speed
which is best suited for the chemical digestion or drying operation
can be selected to give the most desired result.
The microwave oven may be of conventional design having the
modifying improvements set forth herein added thereto along with
proper radiation shielding. The modifications particularly needed
for the present contemplated uses are a radiation mixer 34 to mix
and disperse the radiation. Various radiation mixers are known and
have been described in the art. Normally they are rotating fan-like
machines which reflect the radiation. Such mixers reduce the
production of hot spots which could decompose or destroy part of
the sample being tested. The radiation mixer 34 is positioned
between the magnetron 24 and the chamber forming the oven to
thereby disperse the radiation as it enters the oven chamber.
The internal chamber of microwave oven 10 comprises floor 16,
sidewalls 20, ceiling (not shown) and door 22. These internal parts
can be coated with chemically resistant finishes 42, such as
ceramic, Teflon, epoxy and the like to increase their life for use
with corrosive materials. Door 22 is provided with safety switches
which ensure that magnetron 24 cannot be operated without the door
being in the closed and sealed position. Such safety devices are
well known and required by Federal regulations to avoid the loss of
radiation from the oven.
Magnetron 24 is located preferably outside of the oven chamber and
uses a wave guide 25 to direct the microwave radiation into the
oven proper. Alternatively, magnetron 24 can be positioned within
the oven chamber itself to provide correspondingly good results.
However, the dimensional size of the oven is thereby further
limited to a factor of the wave frequency and thus the utilization
of the wave guide to direct the radiation into the oven proper is
normally preferred.
The oven magnetron 10 is preferably of standard manufacture having
a power output of 500 to 800 watts and a frequency within
government approved ranges. The operator can control the power of
the magnetron within the limits of 0 to 100 percent power input in
one percentage point increments. The United States Federal
Communication Commission has designated the frequencies of 915 and
2450 megahertz as suitable for microwave oven magnetrons. Larger or
smaller magnetrons could be used with corresponding limitations in
versatility. Larger magnetrons are generally unnecessary with 600
watt magnetrons being found to be quite suitable for analytical
usage of the present invention.
The magnetron is conventionally located exterior of the oven
chamber, but it could in fact be in the oven chamber. In such
instances, a wave guide forms the oven chamber with the magnetron
being positioned at one end. However, conventional usage has
indicated a preference for mounting the magnetron exterior of the
oven chamber and utilizing a wave guide 25 to direct the radiant
energy into the chamber. In all instances, adequate radiation
shielding is provided to avoid leakage of radiation from the
instrument as is required by government regulations. Additionally,
safety interlocks are provided to eliminate the possibility of the
magnetron being activated when the oven or chamber door is not
completely closed.
Microwave oven 10 is controlled by programmable microprocessor or
mechanical timers, power control switches and the like devices
which provide the ability to vary the microwave intensity during
drying or heating cycles as well as the time of such cycles.
Microwave oven 10 is thus provided with control panel 26, numerical
input panel 28 and digital readout panel 30. In the operation of
the oven 10, control panel 26 provides for setting the program,
entering a new program, resetting the program, starting and
stopping the program. Numerical input panel 28 provides for
selectively choosing numerical input for the operation of the
microwave oven. Digital panel readout 30 provides visible numerical
reading of the program in operation including the percentage power
of the magnetron, the time of the program and the stage of the
program.
In addition to the radiation mixer 34, the oven is equipped with
radiation absorbing material or isolators 32. Radiation absorbing
materials will couple with the radiation being emitted in the oven
thus preventing decomposition of the sample due to excess radiation
particularly as the amount of polar substances decrease in the
sample thus limiting the amount of absorption in the sample. By
having a radiation coupling material present, the life of the
magnetron is greatly increased. Further, such coupling material
additionally helps in preventing leakage of radiation from the
oven.
It has been suggested that coupling materials such as water be
used, although any other polar substance could be used. The polar
substance is circulated through the oven in radiation transparent
tubing. The amount of coupling material used in this way can then
be readily regulated and adjusted to the desired volume. In one
method of use, a loop of radiation transparent tubing is conveyed
through the oven flooring or sidewalls from a reservoir. The
temperature rise created by the absorption of radiation will effect
the circulation of the water through the tubing from the reservoir.
While this method can be used to protect the magnetron, it has the
particular disadvantage that the coupling material due to its mass
competes for the radiation energy in the oven and can in effect
greatly hinder the heating of a small sample. The undesirable
effects of radiation being absorbed by the coupling agent become
even more pronounced in attempting to heat a small sample to near
total dryness. The preferential absorption of the radiation by the
larger mass of the coupling material will often defeat the attempts
to dry the sample.
Rather than utilizing a coupling material such as water or other
polar substance, a radiation isolator 32 can be used. An isolator
is a device which preferentially absorbs reflected radiation and
prevents damaging reflection back to the magnetron.
In the present invention, the radiation isolator 32 is located
between the magnetron 24 and the oven chamber such as in the wave
guide 25. Radiation isolators 32 are commercially available devices
which comprise magnetic shapes coupled wit heat sinks. The
isolators are designed to permit originating microwaves emitted
from the magnetron 24 to pass through the isolator unaffected.
Reflected waves, however, are absorbed by the isolator and
converted to heat energy. The isolator 32 in fact has a propensity
to attract reflected radiation and thus will actually tend to draw
the reflected radiation out of the oven chamber.
Isolator 32 converts the reflected radiation into heat which is
dissipated through isolator heat exchanger 33. Heat exchanger duct
35 communicates with heat exchanger 33 and fan 38 which draws off
the produced heat. Fan 38 and isolator heat exchanger 33 are
selected to be of sufficient capacity to absorb the full capacity
of magnetron 24 reflected energy with a zero oven load for
prolonged, indefinite operation. This ability to operate the oven
with a zero load enables the present invention to rapidly and
effectively concentrate the microwave energy on any given sample
size, even those with very low polar material present. Total drying
can be effected without damage to the magnetron and without use of
competing radiation absorbers being present in the oven. The total
microwave transparency of the oven itself further prevents the
heating of the oven itself which can be detrimental to continuous
sample testing.
In addition to the noted isolators and radiation mixers, exhausting
means 36 is provided for the removal of vapors from the oven. Air
intake panel 40 is provided to draw a flow of air through the oven.
The control of air flow through the oven can be important
particularly in drying processes wherein large quantities of
volatiles are being expelled. For chemical digestion uses, high
volumes of air may be desirable to remove fumes which are given off
in the digestion. Thus, a programmable or variable speed exhaust
fan is provided, the speed of which can be regulated by the power
supplied to the fan motor. Alternatively, the fan speed can be
simply controlled by manual setting by the operator. The speed of
the fan and corresponding air flow can be critical to the desirable
operation of the invention. When large volumes of volatiles need to
be removed, high air flows greatly enhance volatilization. When
light power-like materials are being treated, low air flow rates
will prevent the loss of sample material from the sample plate.
In the operation of the oven, the operator selects the magnetron
power to be utilized, the time of operation and the volume of air
flow through the chamber. In addition, the speed of the rotation of
the turntable 12 within the desired ranges of about one-half
revolution per minute to about twenty revolutions per minute can be
optionally selected. For most usages, however, the turntable
rotation can be preset with about one revolution per minute being
adequate for most purposes to avoid the development of hot spots in
the sample being radiated.
The preferred method of controlling the apparatus is by means of a
microprocessor 44, wherein the operator programs the sequence of
operation directly into a reprogrammable processor which is part of
the oven unit. Alternatively, the same functions could be
controlled by timer means and variable resistor control of the
magnetron and other variables described. These in turn could
provide for visual readout of both the set time and the lapsed time
as well as the power selected.
The use of a microprocessor however provides for further
versatility, such as the ability to preset the instrument for more
than one setting of time and power usage. Microprocessors presently
available for such usage have a high degree of reliability and may
have a longer life expectancy than mechanical controls. Further,
simple microprocessors provide increased flexibility for
programming at a competitive cost.
Many uses of the present invention repetitive testing of numerous
nearly identical samples. Typically, such testing involves
determination of solids, moisture level, volatile content etc. Such
determinations involve initial weighing of the sample, heating the
sample, reweighing the heated or dried sample and then calculating
the percent change, weight difference etc. The weighing can
conveniently be done with an electric balance. When an electric
balance is used, the total analytical test can be automated by
using the present invention in conjunction with an electric balance
and a programmable calculator such as a Hewlett Packard HP97S or
the like. Such programmable calculators can receive the electrical
weight signal from the electric balance, store the information and
use the stored weight in a subsequent calculation after heating and
reweighing the sample. In this manner, numerous samples can be
tested at the same time or in sequence with a plurality of samples
being subjected to microwave heating at a given time.
Alternatively, when the oven of the present invention utilizes a
microprocessor to control its functions, the electric balance can
be directly interfaced with the microwave oven through its
microprocessor using suitable transistor-transistor logic (TTL)
interface means. The oven microprocessor can then store the weight
signals and perform the desired calculations to give weight loss
information as desired.
Using the programmable capacity of the preferred apparatus of the
present invention, the oven can be preset to go through a
sequential power change during drying for specified time periods.
In a typical drying sequence to a minimum moisture value of a
sample being dried, it may be desirable to preprogram the apparatus
of the present invention to provide for a three-stage drying
sequence. Such a sequence may be desirable particularly with
samples which may otherwise rapidly deteriorate under prolonged
microwave heating. Typically such a sequence would follow the
program wherein stage 1 utilizes 100 percent magnetron power for a
time period of two minutes, followed by a stage 2 at fifty percent
power for three minutes, followed by stage 3 at 25 percent power
for three minutes. Such preprogrammed sequences of three stages is
readily provided by operator selection thereby providing initial
high heating to bring a sample up to temperature at which volatiles
begin to be removed from the sample, subsequently followed by
lesser heating as the sample loses volatiles and concluding with
lower heating to avoid degradation of the sample. Such sequences
enable repetitive treatment of sample after sample without constant
operator attention.
On completion of one series of tests, the operator can merely
change the program to suit another series of tests as may be
desired for different samples.
While the invention has been described more particularly with
reference to the preferred embodiments of the present invention, it
is recognized that variations can be made which are readily
apparent to those skilled in the art. It is intended to claim the
invention broadly in terms of its full novelty and unobviousness
not being limited by the particular expressed mode or examples
specifically described.
* * * * *