The International organization of legal metrology (Original French: Organisation Internationale de Métrologie Légale – OIML) is an intergovernmental organization, that is responsible to provide technical recommendations documents for various legal metrology procedures that facilitate international trade.

OIML ensures that the certification of measuring devices such as weighing devices (Analytical weights & Weighing Balances), taxi meters, and speedometers, in one country is compatible with certification in another country.  

Since its establishment OIML has developed a number of guidelines for various measuring instruments and OIML R-111-1 is a recommendation that provides all technical requirements that to be followed for laboratory weights.   

OIML R-111 provides recommendations related to material, shape & design and calibration requirements so that a Calibration Weight must fulfil its desired result of accurate measurement. 

As per the guideline here are few most important requirements that need to be fulfilled at the time of manufacturing, calibration or at the time of purchase from the user end.

Maximum Permissible Error: 

Maximum permissible error (MPE) is a band of lower and upper limit that a weight can have in a particular accuracy class, MPE is a combination of mass value and its associated uncertainty, sum of mass value and its uncertainty shall not exceed the limit of MPE.

For maximum permissible error for analytical weights according to their accuracy class please refer to the table given below.

Shape: 

OIML has set some design standard for all the analytical weights, that needs to be followed from manufactures 

1. Calibration weights below 1g (1mg to 500mg) may be flat polygonal sheet or wire type having triangle, rectangular and pentagon shape or a shape of one segment, two segment and five segments.
2. Weights greater than 1g from a set shall have the same shape.
3. Laboratory weights of 1g to 50 kg may have cylindrical or slightly tapered conical body; these analytical weights may also be provided with lifting knobs.
4. Analytical weights of 5kg to 50 kg may have a shape suitable for their method of handling.
5. Class M analytical weights with nominal values from 5 kg to 50 kg may also have the shape of rectangular parallelepipeds with rounded edges and a rigid handle.

Construction: 

E1 Class weights and Class E2 weights from 1mg to 50kg shall be solid and shall have no cavity open to the atmosphere. They shall consist of a single piece of material.

Class F1 weights and Class F2 weights may consist of one or more pieces manufactured from the same material. Class F1 weights and Class F2 from 1g to 50kg may have an adjusting cavity.

Class M1, M2 and M3 weights from 1g to 10g shall be solid, without an adjusting cavity. For class M1, M2 and M3 weights from 20 g to 50 g an adjusting cavity is optional. Class M1, M2 and M3 weights from 100g to 50kg shall have an adjusting cavity. However, the adjusting cavity is optional for class M1 and M2 weights from 20g to 200g that are made of stainless steel.

Material:

Lab weights shall be corrosion resistant. The quality of the material shall be such that the change in the mass of the weights shall be negligible in relation to the maximum errors permitted in their accuracy class.

Class E1 weights and Class E2 weights equal to or greater than 1 g, the hardness of this material and its resistance to wear shall be similar to or better than that of austenitic stainless steel.

For Class F1 weights and Class F2 weights surface of weights greater than or equal to 1 g may be treated with a suitable metallic coating in order to improve their corrosion resistance and hardness. For weights greater than or equal to 1 g, the hardness and brittleness of the materials used shall be at least equal to that of drawn brass.

Class M weights less than 1 g shall be made of material that is sufficiently resistant to corrosion and oxidation.

Cylindrical class M1 weights below 5 kg and class M2 and M3 weights below 100 g shall be made of brass or a material whose hardness and resistance to corrosion is similar or better than that of brass. Other cylindrical class M1, M2 and M3 weights of 50kg or less shall be made of grey cast iron or of another material whose brittleness and resistance to corrosion is similar or better than that of grey cast iron.

Magnetism:

Magnetism is an important aspect that needs to be checked at the time of procurement, because it can affect the daily weighing routine drastically, manufacturers using cheap material to cut the cost and bypassing the magnetic properties is common nowadays.

The magnetization, M, expressed in terms of the polarization, ?0M, should not exceed the values given here.

Surface conditions:

Under normal conditions of use, the surface qualities shall be such that any alteration of the mass of the analytical weights is negligible with respect to the maximum permissible error. The surface of the weights (including the base and corners) shall be smooth and the edges shall be rounded.

The surface of class E and F lab weights shall not be porous and shall present a glossy appearance when visually examined. A visual examination may suffice except in case of doubt or dispute.