As we discussed in our previous blogs weights are divided into different accuracy classes based on different characteristics and areas of applications.

But still, there is common confusion about the basic differences between E1 & E2 accuracy class weights, both accuracy class weights are almost identical in the case of design, shape, type, and material of construction, and that confuses the user which class to prefer for their use.

The fundamental difference between both the classes is the maximum permissible Errors of these weights, E1 class weights have lesser errors as compared to E2 Accuracy class weights, which means mass values of E1 accuracy class weights are more closer to the ‘Zero value’ as compared to E2 class and permissible Error band of E2 class weights is larger in comparison of E1 class weights.

We can understand this by an example of 1mg weight, maximum permissible error for 1mg weight in E1 accuracy class is ±0.003 mg, which means the mass value of 1mg E1 class weight will fall within the range of 0.997 mg to 1.003 mg so there are total seven possible mass values, while the error limit of the same weight in E2 accuracy class is ±0.006 mg and the mass values will fall within the range of 0.994 mg to 1.006 mg making it Thirteen possible mass values.

We can conclude this by saying that E1 accuracy class weights are two to three times better than the E2 accuracy class weights in terms of their accuracy and maximum permissible errors.

Now there is an interesting question, can an E2 class weight be considered as an E1 class weight if the mass value of the weight falls within the range of E1 accuracy-

And the answer is YES, as per the OIML Recommendation OIML-R-111-1 

“Class E2 weights or weight sets shall be accompanied by a calibration certificate (see 15.2.2.2). They may be used as class E1 weights if they comply with the requirements for surface roughness, magnetic susceptibility, and magnetization for class E1 weights and if their calibration certificate gives the appropriate data as specified in 15.2.2.1.”

Now the most important question is which accuracy class is to prefer should we use E1 class weights as they are more precise or go with E2 accuracy class as they are comparably cheaper and handling is a little bit easier as compared to E1 class weights as E1 weights are more precise and chances of going out of accuracy due to normal wear and tear are higher. 

Selection of the E1 or E2 accuracy class weight depends on the requirement of applications, by the basic definition E1 class weights are also called primary reference standards that are used as a bridge to establish tractability between national prototype and other accuracy class weights. In simple words, E1 class weights primarily used for the calibration of E1 and E2 class weights that are required by different industries.

In cases where weights are being used for the verification of analytical weighing balance, the selection of weights depends on two points

Depending on the accuracy of weighing balance, as per the guideline given in OIML R-76 or other USP guidelines selection can be made either E1 or E2 class, ie. For ultra-microbalance balance, E1 can be preferred over E2 and for semi microbalance E2 class can be preferred over E1 class weights.  

On the basis of SOP, the selection of accuracy class of weight based on the desired accuracy as per the SOP of the user ie. If SOP confirms the desired accuracy of weight 0.001%, then selection of the class can be made on this basis. For example for a 200g weight considering criteria of 0.001% the error required in weight should be greater than 0.002 g