Calibration weights are the backbone of most laboratories because they help ensure that laboratory balances provide accurate measurement results. However, not all calibration weights are the same.
Calibration weights are assigned a class based on weight accuracy or tolerance. There are different weight class sets, such as OIML, NIST and ASTM. In addition, there are certifications for various weights.
This guide describes the different types of calibration weights. It also consider other factors when purchasing calibration weights, such as style, material, and structure.
Three main calibration weight class systems :
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. OIML class weights are more commonly used outside the US. 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.
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. 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.They used in warehouses and manufacturing settings to calibrate scales used for the shipping or production of large products.
ASTM: ASTM Internation (formerly American Society for Testing and Materials) is a nonprofit non-governmental organization that develops voluntary consensus standards. ASTM class weights are the most common type of calibration weights used in US laboratories. 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.
There are various styles of calibration weights suitable for different applications. Here are the most common ones found in a lab setting:
Many weights have a simple cylindrical shape. These often have a ridge around the top to make them easier to handle.
A grip handle on a cylindrical weight can make it easier to deal with larger weights.
Similarly a pipe handle can help with transporting large weights.
Some weights come with a small tapered section or a knob at the top, making them easier to pick up by hand or with a forceps.
Hooked weights are often used for measuring torque, pressure, or tensile strength.
Disk-shaped slotted weights are useful when you want to adjust a load or force. They can be used alone or slotted onto a stand or hanger.
Leaf weights are thin metal tabs used for very low denominations. A fold in the tab makes it easy to pick up the weight with a forceps.
The materials used to create calibration weights vary depending on the weight and use. Most weights are cast from stainless steel or aluminum, with the latter often being used for smaller weights.
Some heavier and less accurate weights are made from cast iron. Cast iron test weights may be painted gold or silver depending on the units their weight is measured in. In this case, gold represents metric units and silver indicates avoirdupois units.
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