As discussed in previous blogs, weights are categorized into different accuracy classes based on their characteristics and areas of application. However, there is often confusion about the fundamental differences between E1 and E2 accuracy class weights. Both classes appear nearly identical in design, shape, type, and material, which can lead to uncertainty about which class to choose.

The key difference lies in the maximum permissible errors of these weights. E1 class weights have smaller permissible errors compared to E2 class weights. This means that the mass values of E1 class weights are closer to the nominal value ("zero value") than those of E2 class weights. The permissible error band for E2 weights is larger, making them less precise than E1 weights.

For example, consider a 1mg weight. The maximum permissible error for a 1mg weight in the E1 class is ±0.003 mg, meaning the mass value will fall within the range of 0.997 mg to 1.003 mg, providing a total of seven possible mass values. In contrast, the permissible error for the same weight in the E2 class is ±0.006 mg, with the mass value falling within the range of 0.994 mg to 1.006 mg, resulting in thirteen possible mass values. Thus, E1 accuracy class weights are approximately two to three times more precise than E2 class weights.

Difference Between E1 and E2 Accuracy Class Weights?

Can E2 Class Weights Be Used as E1 Class Weights?

Yes, according to the OIML Recommendation OIML-R-111-1, E2 class weights can be considered as E1 class weights if they meet certain criteria:

• They comply with requirements for surface roughness, magnetic susceptibility, and magnetization specified for E1 weights.
• They are accompanied by a calibration certificate that provides the appropriate data.

Which Accuracy Class to Choose: E1 or E2?

The choice between E1 and E2 accuracy class weights depends on the application requirements. Here are some key considerations:

1.Purpose and Usage:

• E1 class weights are primary reference standards used to establish traceability between national prototypes and other accuracy class weights. They are typically used for calibrating E1 and E2 class weights in various industries.
• E2 class weights are often used for the verification of analytical balances and other applications where slightly lower precision is acceptable.

2.Weighing Balance Accuracy:

• The selection depends on the accuracy of the weighing balance. For ultra-micro balances, E1 weights are preferred due to their higher precision. For semi-micro balances, E2 weights are generally sufficient.
• Guidelines such as OIML R-76 or USP standards can help determine the appropriate class based on the balance's specifications.

3.Standard Operating Procedure (SOP):

• The accuracy class should align with the requirements specified in the user’s SOP. For instance, if the SOP demands an accuracy of 0.001%, the weights must meet this criterion. For a 200g weight, an error of less than 0.002g would be required, which may dictate the selection of E1 weights over E2.

Additional Considerations:

• Cost and Durability: E2 class weights are generally more affordable and easier to handle compared to E1 weights. E1 weights are more precise but are prone to wear and tear, which can affect their accuracy over time.
• Application Requirements: In cases where the highest precision is not critical, E2 class weights can be a cost-effective and practical choice. However, for calibration of ultra-sensitive instruments, E1 weights are indispensable.

By understanding these differences and aligning the selection of weights with application-specific requirements, users can ensure optimal accuracy and reliability in their processes.