Weighing scales – Miscellaneous
Reexamination Certificate
2000-09-20
2003-04-22
Gibson, Randy W. (Department: 2841)
Weighing scales
Miscellaneous
C073S001130, C029S090010, C148S210000, C148S206000, C148S225000
Reexamination Certificate
active
06552280
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to precision weights of the kind that are used
1. to calibrate and test weighing instruments and other kinds of measuring apparatus,
2. to calibrate and test other weights of a lower class of accuracy,
3. to weigh things on a weighing instrument, and/or
4. to perform measurements other than weighing with an apparatus employing at least one weight, e.g., to measure force, pressure, and other physical quantities.
Within the present context, the term “weight” means the physical embodiment of a defined quantity of mass in a solid, durable artifact such as a compact body of metal. Generally, the quantity of mass represented by a weight is known within a certain tolerance band, also called “maximum permissible error” (MPE). Based on their maximum permissible errors, weights are divided into accuracy classes according to official standards.
In the U.S., a commonly but not exclusively used standard for weights is ASTM. Standard E 617, “Standard Specification for Laboratory Weights and Precision Mass Standards”. It covers weights from 1 milligram to 5000 kilograms and divides the weights into 8 classes according to a tolerance table. For example, a one-kilogram weight of the most accurate ASTM class (ASTM Class 0) has a tolerance of ±1.3 milligrams, while a one-kilogram weight in the least accurate class (ASTM Class 7) has a tolerance of ±470 milligrams.
On a worldwide basis, the authoritative standard for weights is OIML Recommendation 111, “Weights of classes E
1
, E
2
, F
1
, F
2
, M
1
, M
2
, M
3
”, published by Organisation Internationale de Métrologie Légale (OIML). It covers weights from 1 milligram to 50 kilograms and divides the weights into 7 classes according to a tolerance table. For example, a one-kilogram weight of the most accurate OIML class (OIML Class E
1
) has a tolerance of ±0.5 milligrams, while a one-kilogram weight in the least accurate class (OIML Class M
3
) has a tolerance of ±500 milligrams.
In addition to defining a class structure based on mass tolerances, both standards have requirements for the physical properties of weights, i.e., shape, construction, material, density, surface finish, provisions for fine adjustment, and identification markings.
Generally, the requirements on physical characteristics are more demanding for the more accurate weight classes. For example, a weight of ASTM Class 0 or one of the OIML Classes E
1
, E
2
:
must consist of a single, solid piece of material and cannot have a cavity where material could be added or removed for a calibration adjustment;
must be adjusted by abrasion, grinding, electrolysis, or other appropriate surface-removal process (i.e., mass can only be removed, never added);
must have a density of nominally 8000 kg/m
3
with only a narrow tolerance band (whereby materials other than steel are virtually excluded);
must have a smooth surface finish (of a “glossy appearance” according to OIML R 111) and no significant scratches or other surface defects;
must be corrosion-resistant;
must have a hardness and resistance to wear that is similar or better than that of austenitic stainless steel; and
must be practically non-magnetic (within a very small tolerance of magnetic susceptibility).
Austenitic stainless steels of the AISI (American Iron and Steel Institute) designations 316, 317, 318 or 321 or similar alloys are among the preferred materials for precision weights of the type that the present invention relates to. State-of-the-art weights made of one of these materials will meet the required material properties of density, corrosion resistance, hardness, and low magnetic susceptablilty, albeit that the resistance to wear will be only the minimum required, i.e., not better than that of austenitic stainless steel. Resistance to wear and corrosion, however, is tied directly to the permanence of the mass value of a weight, which is one of the most desirable properties, particularly with respect to weights in the highest classes of accuracy. The loss of mass due to surface wear over the course of a calibration interval of, e.g., one or two years, should be small in comparison to the maximum permissible error for the respective weight, in order to minimize the risk that weights that are used in critical applications could run out of tolerance at any time between calibrations. Also, as mentioned above, weights of the highest classes of accuracy can never be restored to a higher mass value, because adjustments can only be made by removing mass from the weight. Thus, a high-precision weight that is found to be below tolerance because of wear has to be either discarded or reclassified as a lower-class weight with a wider tolerance.
OBJECT OF THE INVENTION
It is therefore the object of the present invention, to provide a precision weight that meets all requirements of applicable standards but represents an improvement over state-of-the-art weights by offering a greater permanence of mass through greater surface hardness, greater wear and scratch resistance, and greater corrosion resistance than weights of the previously available state-of-the-art.
SUMMARY OF THE INVENTION
In accordance with the present invention, the foregoing objective can be met by providing a precision weight of austenitic stainless steel with a hardened surface layer. In a typical application, the weight may be of a type that meets a standard specification for precision weights, such as OIML Class E
1
, E
2
, F
1
, F
2
, or one of the ASTM Classes 0 to 3, but the present invention applies equally to weights that may be made to other standards or to custom specifications.
In preferred embodiments of the invention, the weight is made from a type of austenitic stainless steel that contains chromium and one or more transition metal elements such as nickel and/or manganese and at least one additional transition element such as molybdenum for increased corrosion resistance, and is free of ferrite as well as martensite. Examples of stainless steels with these preferred characteristics are UNS-S3xxxx and UNS-S2xxxx, where xxxx designates a 4-digit number, e.g., UNS-S30403, UNS-S31603, UNS-S32100, UNS-S28200. In the aforementioned Unified Numbering System (UNS) the S stands for stainless materials, the 3 for standard chromium-nickel austenitic steel and 2 for chromium-magnanese austenitic steel. The weight is formed from the raw material, e.g., by machining the prescribed shape from bar stock. Through a mechanical, chemical and/or electrochemical method of removing surface material from the raw piece, the weight is finished and adjusted so that its surface quality and mass value are already at least within preliminary tolerance limits before the weight is provided with the hardened surface layer.
Further in preferred embodiments of the invention, the hardened surface layer is the result of a heat treatment under a gas atmosphere containing, e.g., methane, ethane, ammonia or ethylene. Heat treatments of this kind are known, for example, under the technical terms “case hardening”, “carbiding”, “nitriding”, “carbonitriding”, “nitrocarbiding”, “diffusion coating”, and “diffusion layering”. The heat treatment produces a diffusion layer of increased carbon and/or nitrogen concentration extending to a certain depth from the surface of the weight. The diffusing atoms should have smaller diameters than iron and they should preferably be interstitial to the crystal structure of the austenitic steel.
Preferred for a weight according to the invention is a heat treatment as described above, wherein the process temperature is lower than 350° Celsius, in order to prevent the formation of intermetallic compounds and/or precipitates which could cause corrosion and/or oxidation problems.
One proprietary heat treating process that falls in the aforementioned category is known under the trade names “Kolsterizing™” and/or “Kolsterisieren™” and/or “Kolsterisation™” and/or “Hardcor™”. This process is owned by Hardiff BV, Surface Treatment Technology, 7333 PA Apeldoorn, Netherlands. A description is
Gibson Randy W.
Mettler-Toledo GmbH
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