Weighing Scale Calibration
Different types of balances have their own specific number of resolution and accuracy
classes. It is important to know the accuracy class because it will help you determine the
tolerance limits of the balance.
To easily understand, see the below table which is based on table 1 of NABL doc 129.
Actual scale interval – the symbol is ‘d’ which is commonly known as the resolution,
readability or the smallest change that we can observe. As per OIML definition, it is the
difference of two consecutive indicated values.
Verification Scale Interval– uses the symbol ‘e’– which is defined under OIML as “Value,
expressed in units of mass, used for the classification and verification of an instrument.”
Verification Scale Interval ‘e’ is a scale interval used for verification purposes, hence the
term “verification’ with it.
This means that during verification, in order for us to determine the classification and
tolerance of our balance, we need the value of ‘e’, the verification scale interval, which is
based on the actual scale interval, the ‘d’, or the resolution.
You can understand more the use of the ‘e’ by referring to below table.
If you know the ‘e’, you can determine:
1. The minimum capacity of the balance, some balance specifications, minimum range
is not given, therefore, we need to calculate in order to determine the minimum
range where the balance can perform its best.
2. The accuracy class of the balance
3. The mpe or the tolerance limit of the balance
Usually, e = d on class III and above balances.
How to Calculate the Verification Scale Interval ‘e”?
Below are ways to calculate the value of ‘e’.
Determine the resolution ‘d’, then;
Find the e according to the table below
How to Choose the Appropriate Standard Weights Used for
Balance Calibration or Verification?
Before we perform the calibration or verification, it is important to determine first the
suitability of standard weights that we will use for the balance.
Here is a simple formula to use as per OIML guide.
“They shall not have an error greater than 1/3 of the maximum permissible error of the
instrument for the applied load“.
To make it more simple, here is a summarized guide from UKAS Lab 14.
For example: A 500 g capacity balance with a resolution of 1 g, a class M3 standard
weights is appropriate or has enough accuracy to be used for calibration or verification.
How to Determine the Tolerance Limit or MPE of the Balance?
Now that we know the value of ‘e’, and the accuracy class of the balance, we can easily
calculate the mpe that we need.
By using the tables below, we can now determine the tolerance of our balance.
But another value, the ‘m’ value is now needed. ‘m’ is the value of ‘Mass’ but expressed in
“Verification Scale Intervals ‘n’. To calculate ‘m’, below is the formula:
After we have determined m, we can now choose the multiplier for our ‘e’ (first column) and
then calculate our mpe or tolerance limit, see below table.