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How to Differentiate Calibration Weights Among Several Classes?

 How to Differentiate Calibration Weights Among Several Classes?

The most common question users do face is


“How to identify to which accuracy class my weight belongs to”….By just a visual examination.

 Most of the time a user has to work with previous or old weights or old reference manufacturer’s data and sometimes they don’t get any documented proof at all for the identification of accuracy class of their weights.

This creates an issue & misinterpretation of the data when going for periodic calibration of their weighing balances or weights itself.

In this blog we will discuss about different Accuracy classes and their design characteristics.

 Broadly OIML (International Organization for Legal Metrology: an International organization that is responsible for drafting the guidelines and recommendations for weighing and measuring Instruments) Weights are classified in to 3 classes, on the basis of their Tolerances of maximum permissible error.

E, F & M (E being highest accuracy class followed by F & M respectively).

Each class is further subdivided in to E1, E2, F1, F2, M1, and M2 & M3

(E1 being highest accuracy class followed byF1, F2, M1, M2 & M3 respectively).


So among these 7 different accuracy classes sometimes it becomes complex to identify which class a weight belongs to, as all look like nearly identical to general users.


Here we are going to point out ‘Design wise differences’, we hope this will help our valuable users to find out which accuracy their weights belongs to by just visual inspection, this will also help in their future purchases too.

 OIML weights can be differentiated on the basis of following parameters.


  • Construction
  • Type
  • Marking
  • Maximum permissible Error


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

  -  Clause 7.1.1, page no. 14, OIML R-111-1(E):2004

 “Class F weights may consist of one or more pieces manufactured from the same material. Class F weights from 1 g to 50 kg may have an adjusting cavity; the cavity shall be closed either by means of a lifting knob or by any other suitable device.”     

 - Abstract from Clause 7.2, page no. 14, OIML R-111-1(E):2004  


“Class M1, M2 and M3 weights from 1 g to 10 g shall be solid, without an adjusting cavity. Class M1, M2 and M3 weights from 100 g to 50 kg shall have an adjusting cavity. However, the adjusting cavity is optional for class M1 and M2 weights from 20 g to 200 g that are made of stainless steel. Class M1, M2 and M3 weights from 100 g to 50 kg of the cylindrical type shall have an adjusting cavity coaxial with the vertical axis of the weight, opening on the upper face of the knob and including a widening of the diameter at the entrance. “

 -Abstract from Clause 7.3, page no. 15, OIML R-111-1(E):2004  


Weights in practical are of two types with adjustment and without adjustment, different classes in OIML have different configuration of weights with adjustment and non-adjustable weights.

Here we need to understand pros and cons of both type of weights, non-adjustable weights are recommended where highest accuracy is desired, only cons of these weights are once they goes out of accuracy with normal wear and tear in regular use, we have to discard such weights, life span of these type of weights are short as compared to adjustable weights and a lot of things depends on handling of the weights. (Please go through our guideline on how to handle weights).

Weights with adjustment are of medium accuracy, but they have a long life, as when they go out of accuracy they can be readjusted to their desired accuracy.


Weights or a set of weights of different OIML classes have different types, on that basis we can their class.

E Class weights are Non Adjustable type, if you have weight (s) that has an adjusting cavity it cannot belong to E Class.


F class weight(s) greater than 1g may have an closed to air (not visible from outside) adjusting cavity, weights from 1mg to 500mg are single piece wire or sheet (non-adjustable).

M class weight(s) greater than 10g may have an open to air ( visible on top) adjusting cavity, weights from 1mg to 500mg are single piece sheet (non-adjustable).



Marking on the weights is required to identify the nominal value and accuracy class of the weighs, weights may also have user marking to identify the weight characteristic as per their calibration certificate.

Mainly marking on the weights consist of following details

Nominal value

Accuracy class

Unit of mass

User identification marking

Marking for nominal value and its unit may be the best and reliable way to differentiate between E & F & M class weights. Point wise differences are-


E Class-

No marking for nominal value or its unit in on the weights for E1 & E2 class Weights.


F class-

In F1 class only nominal value (without it’s unit) shall be marked on the top of the weight.

In F2 class nominal value (without it’s unit) along with “F” shall be marked on the top of the weight.


M Class-

In M1 class weights nominal value with it’s unit ‘g’ or ‘kg’ and a “M” or “M1” shall be marked on the top of the weight.