During Balance Awareness Week, An Overview of Maintaining a Correct Balance

Let’s start with the basics. A balance measures the mass of an item by comparing the mass of a known object to an unknown object. In electrical balances, a load cell containing a strain gauge is calibrated with a known object and used to give the mass of an unknown object. Note that mass does not change when pressure or gravity change. So, why refer to the mass of an object and not the weight of an object? Weight is the force with which a body is attracted to Earth or another celestial body, equal to the products of the object’s mass and the acceleration of gravity. Mass, however, is a property of matter equal to the

measure of an object’s resistance to changes in either the speed or direction of its motion. The mass of

an object is not dependent on gravity and therefore is different from, but proportional to, its weight.

So how does an electrical balance use a load cell and strain gauge to determine the mass of an unknown object? Wires are embedded in the object and voltage is applied to the wires. As the wires are stretched or compressed, resistance changes. The resulting change in current flow is then converted to a measure of mass.

Basic troubleshooting

If you experience fluctuating readings when using your balance or scale, or measurements are going up and down, you may be susceptible to drafts caused by airflow. When weighing small items or when accurate

measurements are critical, use a draft shield.  Draft shields are often used with analytical balances and precision weighing instruments to reduce the effects of drafts on your sample. You may also purchase a separate draft shield to surround your entire weighing instrument.  Never weigh your samples near fans or vents or heavily-trafficked areas of the lab.

The size of your weighing pan or surface also plays a role in the performance of your scale.  The larger the weighing surface area, the more susceptible your balance or scale is to drafts.  Square weighing pans have more surface area than round pans. Always select the smallest pan size that fits the material being weighed.

Vibration forces may also affect your readings, so consider weighing on an anti-vibration table or platform to minimize the effects of vibration.  Again, it is wise to weigh samples in an area with as little traffic as possible.

Basic balance skills

In order to improve your weighing skills, here are some helpful hints that will improve your accuracy:

Before weighing, ensure that your balance is leveled correctly. When the balance level is changed, the metrology of the balance is affected. Checking the level of the balance shows if the balance has been disturbed or moved. Balances with Automatic Internal Adjustment should have this function switched ON. This will take care of any changes due to environmental factors (drift). However, this does not excuse a user from regularly calibrating the balance with a certified weight to prove traceability of measurement. These two actions, combined with regular preventative maintenance technical service, will give a sensible, three-pronged approach to accurate weighing.

Important balance and scale terms

Accuracy: The degree to which sample weight conforms to a standard (calibration mass). Accuracy is a function of reproducibility and linearity.

Capacity: Maximum load a balance can weigh.

Linearity: The maximum amount a weight reading may deviate from a straight line between zero and the maximum capacity of the balance.

Readability: Smallest increment of weight a balance will display.

Repeatability: The degree of agreement between repeated measurements of the same mass, on the same balance, under the same conditions.

See a full list of balance and scale terms