Before considering using a balance or perform a weighing scale verification, one primary requirement is to know the tolerance limit. The user should determine the tolerance limit in consideration of their process, but this is not always the case.

As per my experience, most users rely on the manufacturer’s specifications as the basis of their tolerance limit. But one problem I see is that it does not show the tolerance limit or accuracy rating in their specifications (user manual).

How can you verify the balance if the tolerance is not known or given in its specification?

Determining the tolerance of the balance is a bit tricky if you do not know certain terms and parameters dedicated to a weighing scale.

In this post, I will share with you how to check the accuracy of the weighing scale when its accuracy or tolerance limit is not given. I will share the calculations here based on the OIML R 76-1 2006 Guide.

I will discuss the following topics:

- Types of Balances and Its Accuracy Class
- What is the Verification Scale Interval ‘e’?
- How to Calculate the Value of the Verification Scale Interval ‘e”?
- How to Choose the Appropriate Standard Weights Used for Balance Calibration or Verification?
- How to Perform Verification of Weighing Instruments Before Starting Calibration

I hope this post will help you learn how to use digital weighing scale specifications to calculate tolerance limits and other important parameters in order to perform properly a balance verification and calibration.

**Types of Balances and Its Accuracy Class **

Before we perform balance calibration and verification, it is important to familiarize ourselves with the different types of non-automatic weighing scales.

There are so many kinds of scales, and honestly , It is sometimes confusing if you are not familiar with it.

If you are familiar with the balances, you can easily determine the accuracy class and therefore its MPE (Maximum Permissible Error) which is our goal in this post.

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.

Observe that the higher the Accuracy Class, the more decimal places before the significant digit. For example, during the calibration of analytical balance, you easily know that it has 4 decimal places, and it falls under Class I accuracy class.

It is easier to determine the types of balance when you know its accuracy class and number of decimal places and the other way around.

Every type of balance has its own accuracy class and therefore, a tolerance limit or an mpe (maximum permissible error). This is comparable to a pressure gauge where its accuracy class or grade gives you a direct accuracy rating.

But in a balance or weighing machine, it is different, the accuracy class is only a part of the requirement in order to calculate the mpe or tolerance limit. Therefore, it is important to familiarize ourselves first with the accuracy class of a balance.

Below are the symbol used for each accuracy class marked on different weighing scales as per OIML R76-1:

Now that we have determined the different types of Non-Automatic Weighing Scales (NAWS) with its accuracy class, we are now ready to for the next requirement, to determine the “Verification Scale Interval – e”

**What is the Verification Scale Interval ‘e’?**

If you noticed the specifications of most balances in their user manual, you will see a letter ‘e’ alongside with letter ‘d’.

My first time exposed to the ‘e’ and ‘d’ term makes me confused on their use, what I do is just ignore the ‘e’ term. But now that I understand their difference, I see that the ‘e’ term has more use than the ‘d’ term.

Let us see the definition of verification scale interval ‘e’ and the actual scale interval ‘d’ to understand better.

**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.

The table we just seen above shows the requirements for every accuracy class. If you know the ‘e’, you can determine:

- 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.
- The accuracy class of the balance
- The mpe or the tolerance limit of the balance

Once you have determined the ‘e’ value, you can now calculate its tolerance limit then perform a verification.

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;

For example:

balance resolution ‘d’ =0.001 g

e =10(d) = 10 (0.001)**e=0.01 g**

Now that you have calculated the value of ‘e’. You can now calculate and determine the tolerance limit or the *mpe* of the balance. But before we will determine the *mpe*, let us first learn how to choose the suitable standard weights to be used for our verification or calibration.

**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*“.

The error that is referred to here is the measurement uncertainty value. The expanded uncertainty found in the calibration certificate of the standard mass. This means that the measurement uncertainty of the standard weights to be used should be within the limits if the *mpe* of the balance will be divided by 3. See below the relationship/equation.

.**Example:**

Balance mpe: 0.01 grams

0.01/3 = 0.0033 grams.**>> **This means that the measurement uncertainty of the standard weights should not exceed 0.0033 grams.

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 Interval**s** ‘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.

**Example 1:**

Given the specifications above, you will notice that accuracy or tolerance limit is not given. Below are the specs we need.

**Max = 520 gd= 0.001 g**

Verification Scale Interval ‘e’ and Accuracy Class is not given, but based on its resolution, this is a Class II balance which is a precision balance with high accuracy based on the table above (Types of Balances).

First, let us calculate ‘e’

Formula:

e = 10d; where d = 0.001 g

= 10 (0.001)**e = 0.01 g**

Now that we have the value of e, we need to refer to the table above:

We need now the accuracy class and the Verification scale intervals ‘n’

n = Max/e = m

= 520/0.01**n = 52000**

Now, based on the table below, we can see where the value of m =n= 52000 will fall, which is under class II with mpe = 1.5e

mpe=1.5(0.01)** mpe= +/- 0.015 grams = tolerance limit**

We can also calculate the minimum range, based on the given formula above where,

Min =20e

=20 (0.01)**Minimum range = 0.2 grams**

Now that we have calculated the tolerance, it is now time to perform the balance verification.

**Example 2**, where value of ‘e’ is provided

Refer to the photo below:

The above specifications are easier to calculate because the accuracy class and ‘e’ are already given. We have,

Given:

Accuracy Class = III

Max = 60 kg

Min= 0.4 kg

d=0.02 kg

e=0.02 kg; where e=d in a class III balance

Calculate:

1. The value of n, where n= max/e

= 60/0.02

** n = 3000**

- The mpe, where mpe=1.5e (see below table)

=1.5(0.02)

**mpe = 0.03 kg**

**How to Perform Verification of Weighing Instruments Before Starting Calibration**?

Before calibration of balance is performed, the initial task is to perform first the verification of balance then afterwards an adjustment if needed. This makes it different to other instruments where calibration is first performed before verification.

One reason is that there is no point in performing a balance calibration if you know in your results that the readings are far from the nominal of the standard weights used.

This is why initial verification is performed then an adjustment follows.

Once we determined standard weights to be used, the next is to perform a balance verification. If we say verification, we will perform a simple test using a known mass, then we will compare the display of the balance with the nominal value of the standard weights. For the results to be acceptable, they should be within the *mpe* or tolerance limit of the balance.

The initial verification is performed by determining 80% capacity of the balance. After warm up time and exercising or preloading the pan, we then use a standard weight that is equal or near the 80 % capacity of the balance.

For example:

Balance Max Capacity = 5400 g

Tolerance Limit=0.1 g

Standard weights to use is 80% of 5400 = 0.8*6000 = 3000g, therefore, you can use the exact nominal value which is 3000 g standard weights.

As found value = 2999.86

Error = 2999.86-3000 = -0.14

Based on the results and tolerance limit , it is a** FAIL**, therefore we need to perform adjustment

We call this verification “Adjustment of Error Close to Max”. We can perform the adjustment either internal adjustment or external adjustment. (See the difference of internal and external adjustment in this link>> Balance adjustment)

But before performing the verification process, it is assumed that you already know the MPE or the tolerance limit of the balance.

To calculate the accuracy of a digital weighing scale, you can check my other posts here>> Accuracy

**Conclusion**

In order to perform verification of balance, it is important to determine the value of ‘e’, which is known as the ‘verification scale interval’

Verification scale interval ‘e’ is comparable to the resolution ‘d’, which is the smallest display you can read. But ‘e’ is used for determining the mpe or tolerance limit and the classification of the balance (accuracy class) for verification purposes. Calculations to determine e and the tolerance limit are discussed.

In this post, I have presented the following:

- Types of Balances and Its Accuracy Class
- What is the Verification Scale Interval ‘e’?
- How to Calculate the Value of the Verification Scale Interval ‘e”?
- How to Choose the Appropriate Standard Weights Used for Balance Calibration or Verification?
- How to Perform Verification of Weighing Instruments Before Starting Calibration?

Please understand that this is just a recommendation if ever you do not have a basis for your tolerance limit. You should verify its suitability before implementation. The calculation of the tolerance limit should be based on your own process and as per the actual performance of the balance.

**If you like this article, I appreciate if you could share, leave a comment and subscribe.**

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Best Regards,

Edwin

## 4 Responses

## Sriyani Chandrika

Thanks your post. It is very useful to me. I am working in NML Sri Lanka.

Can you post intermediate checking method for electricity meters / Energy meters

## edsponce

Hi Sriyani,

You are welcome. Thanks for visiting my site.

I will include in my list your concern in my future posts.

Happy to connect with a fellow Metrologist.

Best regards,

Edwin

## Joshua Keter

Elaborate!

I head a calibration Lab in Kenya and this soo useful to me

## edsponce

Hi Joshua,

I am glad that this post is useful to you.

Thanks for commenting and reading my post.

Best regards,

Edwin