Calibration and Test Weights Information
Calibration and test weights are standardized masses certified by international, national, or industrial laboratories, such as NIST, ANSI, ASTM, or ISO. Each weight has a precise mass that makes it suitable for calibrating scales to ensure subsequent weight measurements are accurate. Documents often accompany calibration and test weights to guarantee they meet the standard body’s specifications for properties such as tolerance, material, surface finishing, design, cavities, and adjustments. Units of measurement are typically based on the kilogram, but other units are designated.
Calibration and test weights calibrate scales, balances, weight cells and other masses or weights. Weights are often accredited for certain applications, with those used in scientific settings held to higher benchmarks than those used for commercial or industrial uses, such as materials testing, material handling equipment load evaluation, pressure generation on deadweight testers, and cable testing.
Design tip: Weight measurements can vary due to density-buoyancy factors and local gravitational variations.
Class designation indicates the uncertainty or adjustment tolerances for reference weights. Balances, weight scales, force instruments, forces sensors, load cells, and weight modules should be calibrated using a reference weight or mass standard with a class tolerance greater than the readability or resolution of the sensor or instrument.
For calibration of an instrument with a resolution or readability to 0.001 grams, a reference weight near the maximum capacity of the scale or balance having a class tolerance of 0.0009 or smaller should be selected. Selection of a class tolerance of one-third or less of the scale's readability will avoid the requirement to apply correction factors supplied with the calibration weight.
Calibration weights are available as individual weights or as weight sets. Weight sets contain several different masses that are packaged in a case. Many sets include gloves, weight tweezers, and a weigh handle or fork, for handling weights without unintentionally affecting their mass. Forks or handles grasp knobs or other features on the weight.
- Bar/rod: typically used as ballast, counter, or balancing weights.
- Block/brick: used as ballast, counter, test, or balancing weight, and for calibrating high capacity scales
- Cylinder: cylindrically-shaped reference weights are useful for scale calibration
- Grip handle: the weight has an integrated handle for easy carrying, which is useful for high mass weights
- Hook: an integral hook for hanging from a scale or weight cart; range in size from low to high mass sizes
- Knob: a knob machined into the weight provides easy handling with a weigh fork or weigh handle; commonly made from brass or stainless steel
- Leaf: weights resemble a leaf or thin metal tab; common as low mass calibration weights.
- Plate: typically used as ballast, counter, test, or balancing weights.
- Ring: weight resembles a ring, toroid, or doughnut; useful as inertial, test, counter, and balancing reference masses
- Slotted: disc-shaped weights with a thin slot from the center to the circumference for cumulative assembly on a rod; used in mechanical and hardness testing, creep and stress rupture testers, and other material testers.
- Stackable: weights feature mating indentations and protrusions that prevent sliding when weights are stacked
- Wire: low mass weights for calibration
The density of a reference weight varies according to its material of construction due to air buoyancy effects. Calibration should be completed with a material of similar density as commonly weighed objects, or completed in a vacuum chamber. Magnetic fields present in the calibration site can misrepresent the masses of ferrous weights.
Common materials used for reference weights include: aluminum, cast iron, stone, lead, molybdenum, steel, stainless steel, and tungsten.
|Laboratory reference: high precision for calibrating other measurements||OIML class E1; ASTM class 0; UltraClass|
|Precision balances: to calibrate balances with a readability range of 0.1 mg to 0.01 mg, and class.||OIML class E2; ASTM class 1|
|High accuracy balances: standard for analytic work and calibrated scales with a readability as low as .1 g to .001g.||OIML class F1, class F2; ASTM class 2|
|Top loading balances: for calibrating balances with intermediate accuracy and readability as low as .1 g to .01 g.||ASTM class 3|
|Industrial and commercial scales and balances: for commercial and postal scales, legal-for-trade scales, and Class IIIL or IV scales.||OIML class M1, class M2, class M3; ASTM class 4, class 5, class 6; NIST class 6|
Within this classification system, weights can also have a Type I and Type II designation. ASTM standards for scale calibration require reference weights of single-piece, solid construction (Type I) to prevent modifications from being made to the weight. Buoyancy impacts mass comparison as well, so density of the reference weight is important. Density is more easily evaluated by single-piece, solid reference weights.
Type II weights include a screw, knob, ring, sealed plug, or feature that allows material adjustments to be made to the weight.
Detailed descriptions of ASTM, OIML, and NIST calibration weight classes and tolerances can be found under the Class & Tolerance section on the Calibration and Test Weights SpecSearch page.
The majority of precision weights are available in SI or metric and avoirdupois units. ANSI/ASTM standard weights would use pounds and ounces, while OIML certified weights would use kilograms and grams. Specialized units also exist, such as grains, troy ounces, or pennyweights.
To verify scale accuracy, a reference weight of similar mass to commonly-weighed objects should be used. To calibrate a scale, a reference weight near the maximum capacity of the scale should be selected.
References weights should be certified by a national laboratory. All weight standards are traceable to the international kilogram at the Bureau International des Poids et Measures (BIPM) in Serves, France.
A calibration certificate recognizes the difference in a weight compared to the national lab standard. This would be the NIST in the United States. Calibration certificates provide uncertainty values, as well as other parameters pertaining to the accuracy of the weight, such as density, testing conditions, and mass corrections. Scales and balances that need to be calibrated to industrial standards should be calibrated with certified weights.
Traceability reports provide NIST traceability, but evaluations are not as thorough as a calibration certificate. They are primarily for applications that require precision, but not industrial standardization.
Calibration weight / mass standard: Weights and masses are certified and traceable to national laboratory standards, such as NIST, ANSI/ASTM, or OIML ISO. Weights calibrate scales, balances, weigh cells, and other masses.
Counterweight/counterpoise: Masses counterbalance another mass in linear or rotary motion systems.
Test/load test weight: Masses supply a calculated load for mechanical, materials, and load testing and scientific applications.