Updated: Jun 29, 2022
Calibration Weights or Laboratory Weights or Standard Weights are major components for any labs. They help and ensure that lab balances are giving accurate readings by keeping weighing balances calibrated on time to time basis. However, they are not all made the same. Calibration weights are assigned a class which is based on the accuracy or tolerance of the weight. There are different sets of classes of weights, including ASTM, OIML and NIST Standards. In addition, there are certifications granted to different weights.
In this blog, we explain the different classes of calibration weights and the certificates that can help you purchase these weights.
Classes of Weights
There are three main calibration weight class systems that you’ll come across the world:
NIST: National Institute of Standards and Technology (NIST) Class F weights are often used in industrial settings to verify Class III, Class IIII, and non-designated scales. They may also be used in warehouses and manufacturing settings to calibrate scales used for the shipping or production of large products. NIST Class F weights aren’t typically used in laboratories as they are not accurate enough to verify the scales used for most laboratory applications.
ASTM: ASTM International (formerly American Society for Testing and Materials) is a nonprofit non-governmental organization that develops voluntary consensus standards. It has developed 10 calibration weight classes as guided by document ASTM E 617: ASTM Class 000 thru ASTM Class 7. The higher the class number, the higher the level of tolerance (and less accurate) the weight will be. Most laboratory applications require ASTM weights of Class 4 or below. ASTM class weights are the most common type of calibration weights used in US laboratories.
OIML: The Organisation Internationale de Métrologie Légale or International Organization of Legal Metrology (OIML) is an intergovernmental organization that provides standards and systems with the goal of harmonizing legal metrology procedures. From lowest (most accurate) to highest tolerance, the OIML classes are E1, E2, F1, F2, M1, M2, and M3. Most laboratory applications require OIML weights of Class F2 or below. OIML class weights are more commonly used outside the US.
Certifications of Weights
While you search for calibration weights, very often you might come across mentions of certifications. Indeed, depending on your application, you may require that your calibration weights come with a certificate confirming that the lab testing the weights is accredited. Here are the terms you’ll commonly see and what they’re referring to.
ISO/IEC 17025: The International Organization for Standardization (ISO) is an independent non-governmental organization comprising a membership of 164 national standards bodies. The International Electrochemical Commission (IEC) is an international standards organization for electrotechnical fields. Together with other liaison organizations, they created the ISO/IEC 17025 standards. This is the international reference to be used by testing and calibration laboratories that want to display their ability to provide reliable results.
NIST/NVLAP: NVLAP refers to a calibration program developed by the National Voluntary Laboratory Accreditation Program (NVLAP) at the (NIST). It provides a means to assess calibration labs for competency, offering accreditation in various fields. It follows the international standard ISO/IEC 17025 (discussed above) when accrediting calibration laboratories.