Calibration weights ensure scales and other devices offer accurate results in the lab. However, they calibrate different instruments and are made of different materials.
They are given a class based on their accuracy. This article looks at various classes and considerations when choosing one.
What are Calibration Weights?
Calibration weights check a scale’s accuracy. They are commonly certified by industrial laboratories or bodies such as ISO, ANSI, NIST, and ASTM. Each weight often has a precise mass, ensuring the scale at hand is working precisely.
And to ensure they meet international or national standards; they will come with supporting documents. The documents contain information such as cavities, surface finishing, design, tolerance, and material.
Calibration weights can also check the accuracy of balances and weight cells. As you’d expect, the weights are used for different reasons, with those used in scientific labs having a higher benchmark.
Classes of Calibration Weights
Generally, there are three main calibration weight classes. They include NIST, ASTM, and OIML.
The National Institute of Standards and Technology (NIST) class F weights are standard in industrial settings. They verify Class III, Class IIII, and non-designated scales.
They are common in manufacturing and warehouse settings where large products are produced or shipped. Since they are not accurate enough, scientific laboratories don’t use them.
ASTM International is a non-profit organization dealing with the production of voluntary consensus standards. The organization has developed ten weight classes: Class 000 to Class 7.
The Class with the lowest number has less tolerance. Laboratories will often use Class 000 to Class 4. ASTM classes are most common in USA laboratories.
ASTM Class 0 – 000
ASTM Class 0 – 000 is the most preferred class weight in most USA laboratories. This class range is, however, common with metrology laboratories. They are expensive compared to other classes and require a controlled environment for optimal results. Besides, they are high tolerance. A lab technician should handle them with tweezers to avoid damage.
ASTM Class 1
Ideally, semi-micro and analytical balances use readabilities of 0.01 mg and 0.1 mg, respectively. As such, one should use ASTM Class 1 to calibrate devices weighing a fraction of a gram. They are also used as a reference when calibrating high-tolerance weights. Like ASTM Class 0 – 000, they are delicate, and one should use tweezers or gloves when handling them.
ASTM Class 2
This Class is ideal for higher precision balances, ideally, those between 0.01 and 0.001 grams of readabilities. Such balances are standard in chemical compounding and pharmaceutical industries.
ASTM Class 3
The ASTM Class 3 is ideal for balances with readability between 0.1 and 0.001 grams. These scales are standard in commercial laboratories.
ASTM Class 4-6
ASTM 4 thru 6 calibration weights check the accuracy of Class III, IIIL, and unmarked scales. Scales in student laboratories often require this calibration class weight. They are ideal for readability lower than 0.1 grams.
ASTM Class 7
It is the Class with the highest tolerance. It is suitable for Class III, Class III, and non-designated scales. These scales are standard in industrial applications such as shipping, warehouses, and others.
The International Organization of Legal Metrology (OIML) is an international organization dealing with the provision of standards and systems in the metrology industry. They ensure adherence to harmonic and legal procedures, with the lowest Class being the most accurate. OIML classes are E1, E2, F1, F2, M1, M2, and M3. They are popular outside the US.
OIML Class E1
It is the reference class for calibrating other reference calibration weights within a controlled and safe environment. Metrology and calibration laboratories or similar settings should use this class weight. It can calibrate Class I and Class II laboratory balances and analytical balances. The OIML Class E1 is identical to ASTM Class 00.
OIML Class E2
The OIML Class E2 is a reference standard for calibrating other weights. It is ideal for calibrating high-precision balances with readability between 0.1 mg and 0.01 mg. They can calibrate high-precision analytical balances like Class I and Class II. These weights are equivalent to ASTM Class 1 weights.
OIML Class F1
It is appropriate for calibrating balances with readability between 0.01 g and 0.001 g. It is suitable for high-precision top-loading Class I and II balances, making it ideal for digital scales and lab balances. ASTM Class 2 and Class 3 weights are equivalent to this Class.
OIML Class F2
It is appropriate for semi-analytical balances and student use. They should be used in a class setting where minimal calibrations are needed. They are suitable for calibrating Class III, IIIL, or IIII digital weighing scales.
OIML Class M1, M2, M3
These are primarily used in general-purpose industrial, educational, commercial, and technical sectors. They can calibrate Class III, IIIL, or IIII digital scales. Since they are made of stainless steel or cast iron, a little precaution is needed when handling them.
While these are the most common calibration weights, some manufacturers like Troemner have unique Class weights. They are UltraClass Platinum, UltraClass Gold, and UltraClass.
The following classes are no longer in use.
- Class M
- Class T
- Class C
- Class Q
- Class S
- Class P
- Class S1
Factors to consider when choosing calibration weights
Cast iron and stainless steel are often used for OIML Class M1 and M2, as well as NIST Class F and ASTM Class 6 and 7. Cast iron is recommended for weights that are 10 kg or heavier. If the weight is in metric units, it will have gold paint, while avoirdupois units will have silver paint.
Weights between 5 kg and 10 kg can be stainless steel, steel, or iron. For weights between 5g and 5kg, they should be of a material with a hardness of Rockwell B80 and above. All calibration weights should be of wear and corrosion-resistant material.
OIML E1, E2, and F1, as well as ASTM 000 – 4, are of a highly polished material such as stainless steel. As such, one should handle them with care. We recommend using tweezers or gloves.
Weight scale calibration uses classes of different styles, the most common being a leaf, cylindrical, slotted, and grip handle.
Grip handle weights
Grip handle weights are made of cast iron. They are for calibrating large scales, mainly in industries producing large products. Ideally, they fall under NIST (Class 6 and 7) or OIML (M1 and M2). Unlike other weights, one can place them in calibrated buckets during calibration.
Referred to as nesting slab weights, they are appropriate for calibration requiring one to hang test weights. Thanks to their design, you can easily stack and slot them onto calibrated hangers. They fall under the same Class as grip handle weights.
Cylindrical weights are 5kg and hence can be applied in different calibrations. Falling in the Class F weights, they are appropriate for calibrating the bench and other more minor scales.
Leaf weights calibrate high precision and resolution balances. Since they are delicate, one should handle them with gloves and tweezers.
For best results, you should use calibration weights with similar units as those of the scale. For example, consider buying a metric weight if the subject is in metric units.
Avoid buying weights with magnetic material to avoid scale damage and incorrect calibration.
Test load configuration
To avoid errors, use a few calibration weights. For example, if the test point is 20 kg, use one 20kg instead of two 10kg weights.
The application will require calibration weights to come with a certificate. It confirms they are accredited, hence will offer accurate results. Some typical accreditations are:
The International Organization for Standardization (ISO) is a non-governmental organization with 164 national standard body members. The International Electrochemical Commission (IEC) sets standards in the electrochemical field. Together, they created the ISO/IEC 17025 standard that calibration laboratories use for accreditation.
NVLAP is part of NIST, created by the National Voluntary Laboratory Accreditation Program. It offers accreditation to calibration labs. The NIST program follows the ISO/IEC 17025 standard.
Typical calibration weights are made of one cast metal piece. However, weights such as Troemner’s 1g and above UltraClass consist of two parts. The calibration weights consist of a knob and a body.