Operating Range of a Lab Balance

The operating range of a balance is the weight range within which it will give you an accurate enough reading for your application.

In this post, we explain how to determine the operating range of a balance for your specific application. This method is as per USP Chapter 41.

The Operating Range (OR) comprises a minimum and maximum value:

• The maximum value (OR_max) is the maximum capacity of the balance.
• The minimum value (OR_min) is the lowest value at which the balance will give an accurate enough reading as determined by the outcome of the repeatability test.

Repeatability

A repeatability test comprises at least 10 measurements using one test weight. It’s recommended to use a half-load weight (one that is approximately one-half of the balance’s weighing capacity) for this test.

How to carry out a repeatability test:

1. Ensure your balance is calibrated, leveled, and warmed up.
2. Press the “Tare” button to set the display to zero.
3. Place the test weight in the center of the pan.
4. Record the stabilized weight.
5. Repeat the above steps a further nine times.
6. Use the 10 test results to calculate the SD.

Repeatability is satisfactory if the following is satisfied:

2SD/N ≤ 0.10%

Where:

• SD = standard deviation
• N = desired smallest net weight (the smallest weight you plan to use with the balance). This only refers to the sample (for example, it cannot include a weighing vessel).
• 0.10% is the minimum accuracy for important measurements

You can then use the SD to calculate OR_min using the following equation:

OR_min = 2*SD*1000

Note that there is a constraint to this equation:

The readability (d) is the lowest denominator by which you can read a weight on the balance. For example, many precision balances will allow you to read to 1mg, whereas many analytical balances will give readings to 0.1mg. If the standard deviation is lower than 0.41d, then it has to be replaced with 0.41d. 

In this case, OR_min = 2*0.41d*1000 = 820d

Essentially, the absolute minimum that OR_min can be is 820d. But if your SD is higher than 0.41d, OR_min will be higher.

Accuracy

As per Chapter 41, the accuracy of the balance must also be tested. For the accuracy test, the weight must have a mass that is between 5% and 100% of the balance’s capacity and the measurement uncertainty of the weight must be ≤ 1/3 of 0.10% (≤ 0.033%). The weight must be calibrated to prove the uncertainty.

To perform the accuracy test:

1. Place the test weight on the center of the balance pan.
2. Wait for the reading to stabilize and record.

The accuracy of the weight is acceptable if the deviation of the measured value is within 0.10% of the test weight.