Thermal measuring and testing – Thermal gravimetric analysis
Reexamination Certificate
1999-09-17
2001-10-16
Gibson, Randy W. (Department: 2859)
Thermal measuring and testing
Thermal gravimetric analysis
C374S014000, C177S245000, C177S145000, C073S076000
Reexamination Certificate
active
06302577
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to measuring sample weights during sample heating. In particular, the present invention promotes accurate weight measurements by employing a microwave permeable air shield that substantially reduces disruptive convection currents within a heating cavity. The invention is also a method of using this apparatus to accurately measure the weight of a sample that is undergoing microwave heating.
BACKGROUND OF THE INVENTION
There are analytical procedures for high-speed, quantitative analysis of various substances (e.g., agricultural commodities, foodstuffs, dairy products, paints, coatings, paper products, and tobacco) that require the volatilization of moisture or solvents from the substances. These procedures often use microwave energy to heat a sample to remove various volatiles, such as solvents or moisture. Thereafter, moisture, solids, or other residuals and losses can be determined. To achieve these weight measurements rapidly and accurately, the sample is not removed from the balance, but rather weighed in place after each succeeding step. Preferably, the weight of the substance is sensed or measured repeatedly during the microwave heating while volatile species are still being driven from the heated sample. Such procedures not only require sensitive analytical balances, but the capability to measure weight while the sample is hot.
Those having ordinary skill in the art will understand that, when placed in cool environments, hot substances generate convection currents that cause air to flow around the heated substances. These air currents can interfere with the ability of analytical balances to obtain accurate weight measurements. For example, analytical balances should be accurate within plus or minus 0.10 milligram to facilitate completion of the desired analytical procedures. At these kinds of error margins, convection currents can substantially affect the measured results and impair procedural reproducibility.
U.S. Pat. No. 4,291,775 (hereinafter the Collins '775 patent), which is commonly assigned with this application, addresses the problem of disruptive convection currents and is incorporated entirely herein by reference. More specifically, the Collins '775 patent describes a method and apparatus for improving the weighing accuracy of sensitive automatic balances when weighing heated substances. The Collins '775 patent accomplishes this by introducing an air barrier shield to cover the balance plate without contacting or touching the automatic balance. This tends to reduce the convection currents that can interfere with the sensitive balance and hinder the achievement of accurate sample weight measurements. In other words, eliminating convection air currents reduces movement of the balance and, thereby, fluctuations in the measurement of sample weight being sensed. The Collins '775 patent teaches that the air barrier is permeable to microwave radiation and capable of absorbing moisture and other volatilized substances while being substantially impermeable to air currents. In particular, the Collins '775 patent teaches that glass fiber matting or padding is particularly useful for forming the air barrier enclosure. U.S. Pat. No. 4,291,775 is hereby entirely incorporated by reference.
Although the method and apparatus disclosed by the Collins '775 patent is useful and effective, its employment of a portable air barrier can be somewhat awkward. In particular, an operator using the apparatus must place sample material on a balance plate, which is positioned within the microwave oven cavity. Then, the operator must position the barrier over the balance plate and the sample material such that the barrier does not come in contact with the balance plate and the sample material. Because the air barrier is not affixed to the microwave oven, however, the operator must position the barrier such that it rests upon the floor of the microwave oven. Consequently, heating of the sample material is delayed while the operator manipulates the portable barrier.
Moreover, even though the barrier disclosed by the Collins '775 patent is permeable to microwave radiation and absorbent of moisture and solid vapors, its confining structure, whether a box, cone, or dome, tends to capture and condense more volatiles than can be transported through the barrier for re-volatilization. This increases the concentration of condensates under the portable barrier, thereby slowing the vaporization rate of the sample material and, thus, the length of the test procedure.
OBJECT AND SUMMARY OF THE INVENTION
Therefore, it is an object of this invention to improve upon the apparatus design of the Collins '775 patent by including an air shield that is removably secured to the inside of the heating cavity such that the air shield does not contact the analytical balance when the air shield is fastened to the heating cavity. By securing the air shield to the interior of the microwave cavity, a laboratory technician need not manipulate a movable barrier that must rest on the cavity floor, yet be placed so that the barrier substantially surrounds both the balance plate (e.g., a balance stem) and the sample to be heated.
Accordingly, in one aspect, the invention is an apparatus that facilitates accurate sample weight measurements during sample heating by positioning within a heating cavity a microwave permeable air shield that substantially reduces the convection currents that disrupt accurate sample weight measurements.
In another aspect, the invention is an apparatus that promotes the removal of volatile species from a sample by using an air shield that is an integral part of the heating cavity to condense, transport, and re-volatilize the volatile species freed from the sample.
In another aspect, the invention is an apparatus that promotes microwave heating of a sample by focusing microwave energy upon a sample via the advantageous positioning of a microwave permeable air shield.
In yet another aspect, the invention is a method of exhausting from the cavity volatiles freed from a sample during heating while arresting the development of disruptive convection currents.
REFERENCES:
patent: 4142403 (1979-03-01), Lohnes et al.
patent: 4291775 (1981-09-01), Collins
patent: 4316384 (1982-02-01), Pommer et al.
patent: 4438500 (1984-03-01), Collins et al.
patent: 4441002 (1984-04-01), Teich et al.
patent: 4457632 (1984-07-01), Collins et al.
patent: 4485284 (1984-11-01), Pakulis
patent: 4554132 (1985-11-01), Collins
patent: 4566312 (1986-01-01), Collins et al.
patent: 4651285 (1987-03-01), Collins et al.
patent: 4681996 (1987-07-01), Collins et al.
patent: 4753889 (1988-06-01), Collins
patent: 5349138 (1994-09-01), Dong
patent: 5402672 (1995-04-01), Bradford
patent: 3814959 (1989-11-01), None
Jennings William Edward
Moser Cindy Rushing
CEM Corporation
Gibson Randy W.
Gilchrist Jason P.
Summa & Allan P.A.
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