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Factors That Can Affect the Accuracy of Your Lab Balance Readings

By Aimee O'Driscoll, 24 August 2018

When using a lab balance, there are a variety of factors that can ultimately affect how accurate your readings will be, including:

  • The type of balance you’re using
  • Calibration of the equipment
  • The placement of the unit
  • Proximity to air draft sources
  • The ambient environment

In this post, we’ll explore these in more detail to help you get the accuracy you need.

1. The Type of Balance You’re Using

One of the most obvious things that will affect the accuracy of your reading is the type of balance you’re using. For example, an analytical balance can typically measure masses of up to about 200 g with a resolution of 0.1 mg. A precision balance is less accurate with a typical resolution of 1 mg, but can accommodate larger masses.

 

A precision balance and an analytical balance.

An AG Pro Precision Balance and an AGN Pro Analytical Balance.

For greater accuracy, there are different types of analytical balances, some that can even provide readings as low as 0.01 µg.

2. Calibration of the Equipment

This is one of the most obvious causes of inaccuracy in balance readings, but nonetheless can be often overlooked. Some balances come with an internal calibration feature which should negate the need for manual calibration.

 

Accuris specs.

Certain models of the Accuris Analytical Balances come with an internal calibration feature.

However, because lab environments differ, it’s a good idea to calibrate equipment with certified calibration weights.

 

 Calibration weights.

Accuris class F1 individual calibration weights and an OIML precision weight set.

It’s a good idea to check the calibration regularly, perhaps once every three months, or have a calibration service company come in to do the calibration for you.

3. The Placement of the Unit

To ensure the most accurate readings, you need to consider external factors that may have an affect on the unit. The balance should be placed on a flat, stable surface that doesn’t allow for any movement. For example, placing it on a trolley wouldn’t be a good idea.

Most balances come with a built-in level to help you ensure it is indeed level. It’s also common for a balance to have two adjustable legs as part of a three-point system which is designed to prevent rocking. Once the scale has been leveled, it shouldn’t be moved, otherwise the leveling will need to be checked again.

 

Balance examples.

Most Torbal balances come with a bubble (spirit) level and adjustable legs.

Similarly, it’s not good to have a balance sharing a surface with equipment that vibrates, such as a refrigerator or overhead stirrer run at high speeds. Not only could the vibrations affect the accuracy of readings, you could spill the sample or even damage the internal components of the balance itself.

4. Proximity to Draft Sources

Sources of airflow close to the unit can affect the mechanism of the balance or put pressure on the balance plate, distorting the readings. Examples of such sources are air conditioners, heaters, ceiling fans, windows, and doorways.

 

Analytical balances.

A Cubis® Analytical Polyrange Balance and a Series Dx Analytical Balances.

Many analytical balances come with a draft shield for this reason, so be sure to close the shield before taring or measuring. Although most precision balances don’t come with draft shield, some precision balance manufacturers, such as Ohaus and Torbal, sell compatible draft shields separately.

5. The Ambient Environment

When dealing very small samples, you need to bear in mind the ambient temperature and humidity. A too humid environment could cause a sample to absorb water and gain mass. Too high a temperature could cause evaporation of water or other components within the sample. Some samples may even undergo reactions at certain temperatures, changing their composition and mass.

Wrapping Up

Many of these factors come down to common sense, but it’s easy to forget about them in a busy lab environment. Given that the accuracy of weighing samples is crucial to many applications, it’s worth double-checking the above list before you start measuring.