Communications: electrical – Condition responsive indicating system – Specific condition
Reexamination Certificate
1999-12-21
2001-02-20
Hofsass, Jeffery A. (Department: 2736)
Communications: electrical
Condition responsive indicating system
Specific condition
C340S551000, C340S514000, C324S260000
Reexamination Certificate
active
06191698
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates in general to a magnetic testing apparatus more specifically it relates to a self controlled magnetic stray field threshold tester which gives warning signals when the external magnetic stray-field of an air freight consignement exceeds a predetermined value in a predetermined distance from the surface of air freight packages containing magnetized material. With air traffic safety involved such an apparatus must include means for self controlling its functioning.
Previously navigation in air traffic was mainly performed by the aid of a magnetic compass. Since air freight consignements containing magnetized materials may disturb the indication of the magnetic compass such shipments are classified as “Dangerous Goods”. Although a magnetic compass is no longer of great importance in commercial air traffic, the maximum permitted distortion of the geomagnetic field in a distance of 15 ft (about 4.6 m) from any point of the surface of a consignement will not be changed. This maximum distortion should not exceed 0.418 A/cm −(0,0053 Oersted)-.
Measuring such a small stray field requires a hardware and personal expenditure that normally is beyond the means of the consignor and the carrier. Furthermore low-field measurements must be performed in an environment free from other magnetic disturbancies such as iron structures etc. In consequence consigments containing magnetized material quite often are not tested at all. On the other hand, the tonnage of magnetized materials, (i.a. strong permanent magnets), shipped by air has increased by more than a factor 100 during the past 15 years. Under those circumstances many shipments have been found with high external strayfields which even may disturb the function of other air plane equipment.
Thus there exists obviously an essential demand for a simple and inexpensive apparatus which enables also less experienced personnel in normal environment to determine quickly whether a package containing magnetized materialsatifies the above rules.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an apparatus for determining whether the stray field enumating from a given package containing magnetized material is within the permitted range of field strengths. The apparatus should be robust, easy to use and self testing its safe and proper functioning.
The present invention is based on the discovery that for testing, whether an air freight consignement containing magnetized material meets the above rules, a much higher threshold value of the magnetic field strength can be used at a shorter distance than previously stated by the above rules. Thus the distance of 4.6 m as given by the rules is reduced according to the invention to about 10 to 50 cm.
A distance of 20 cm has proved to be a good practical compromise. Here the threshold field is about 30 times higher than the geomagnetic field. Geo-magnetic influences thus will not irritate the user. Although a shorter distance, 10 cm for instance, would even be better in this respect, the correlation to the permisseable stray field in 4.6 m distance becomes much poorer when packages with different arrangements of the magnetized materials have to be considered.
Since magnetized materials —(permanent magnets)— of greatly varying shapes, sizes and arrangements, sometimes additionally with shielding materials are being shipped, the maximum permitted field strength at such reduced distances can not be calculated mathematically. Thus, it has been established by extensive practical tests that, for instance, a field strength threshold value of 10 A/cm, (12,6 Oersted) at a distance of 20 cm will guarantee that the prescribed maximum field strength value at a distance of 4.6 m are not exceeded.
Preferred embodiments of the invention comprise a magnetic field sensor, providing an output signal which is a function of the strength of the magnetic field at the location of the sensor; indicating or alarm device(s) for warning whether the value of the sensed magnetic field strength is above a predetermined threshold value or not; an electronic circuit coupling said sensor to said alarm device(s), said circuit actuating said alarm device(s) if and when the sensed magnetic field strength is above said threshold value; a housing containing said circuit and said sensor; and nonmagnetc distance or spacing defining means, which when brought in contact with a point on the surface of the package (consignement) establishes a predetermined distance between said point and said sensor. A “self-test” switch or button energizes an internal coil, located around or near the sensor element with a low electric dc current creating a magnetic field at the location of the sensor element. This magnetic test field being equal or ½ of the permitted threshold field. In order to test proper functioning in both field directions means for reversing said current are provided.
One or more of the following features are further preferred:
The magnetic field sensor comprises a Hall probe or similar device
The housing contains or supports the magnetic field sensor, the alarm device(s) and switch for activating apparatus;
The circuit provides an output signal independent of the polarity of the input signal of the magnetic field sensor;
The circuit comprises an amplifier;
A rechargeable battery energizes the circuit;
Recharging means to be connected to the mains is included or attached;
The circuit produces one warning signal when half the predetermined threshold value is reached and another when it is exceeded.
REFERENCES:
patent: 4458198 (1984-07-01), Schweitzer
patent: 4675607 (1987-06-01), Golker
patent: 5309097 (1994-05-01), Bill
patent: 5608319 (1997-03-01), Maurer
Hennig Gerhard
Voigt Martin
Helfgott & Karas, P C.
Hofsass Jeffery A.
La Anh
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