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What is the difference between a Balance and a Scale?

Written by Janice M on . Posted in Laboratory Equipment

Early Measurements

To confuse us, the "scale" started life as a balance. If you think of an old fashioned kitchen scale, it had a horizontal lever and two pans – you placed small brass weights on one side and flour on the other side until the lever was in equilibrium. You may also remember another kind of balance from a visit to the doctor. You stood on the "scale" and a slider weight was then moved along a graduated scale. In today’s terms, these were both technically balances, but were commonly referred to as scales.

The science of weighing objects has become more accurate today. A greater selection of weighing instruments is available and scales and balances now have different definitions, but a similar purpose – to weigh an object.

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SCALES:

Scale: A scale measures weight relative to the force of gravity.

Weight: Refers to the heaviness of an item. It is determined by the gravity exerted on the item, multiplied by the mass which is constant.

Example: A spring scale measures flour by measuring the difference between the unflexed state versus the flexed state of a spring. Force gauges inside the scale operate on the assumption that the amount of force on a spring is proportional to the amount of stretch in a spring (this is known as Hook’s Law).

Scales generally:

  • Have slightly different results at different gravity levels – an object measured on the top of a mountain will be slightly less than at sea level, because there is less gravity at the mountaintop. Results can vary by up to 0.5%
  • Have a higher measurement range than balances
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A modern digital A&D scale

BALANCES:

Balance: A balance compares the mass of two different objects.

Mass: Mass is the amount of matter in an object. It stays the same, no matter where the measurement is taken.

Examples: The old-fashioned "scales of justice" balances with rice in one pan and brass weights in the other pan. A modern analytical balance contains a "load cell" that is displaced when weight is placed on top of the pan. The amount of displacement is measured.

Balances generally:

  • Are more accurate and precise than scales because their accuracy is not affected by differences in gravity, which can vary according to your location on earth
  • Have a more limited range and lower resolution than scales, eg a balance may weigh up to 100 g x 0.001 g and a scale may weigh up to 5 kg x 0.01 kg

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An A&D Analytical Balance

6 Words you should Understand when using a Balance / Scale:

  1. Capacity: The heaviest amount that can be measured on the instrument.

  2. Resolution / Readability: Smallest division or fineness the instrument will register, eg. 0.1 g or 0.0001 g. In other words, the number of places after the decimal point that can be read.

  3. Precision / Repeatability: The amount of agreement between repeated measurements of the same quantity.

  4. Accuracy: The ability of an instrument to provide a result that is as close as possible to the actual value, in other words the "correctness" of the result.
    Example: A balance has a resolution of 0.1g. A 100 g standard weight gives a result of 108.0 g. When you repeat this test 10 times, the result is always 108.0 g. This means that the reading is precise but the accuracy is 8 grams and not so good.

  5. Calibration: A comparison between the results of an instrument and a standard known weight, eg a 100 g brass weight. After calibration, the instrument may be adjusted in order to give a reading in agreement. Calibrations are usually done once a year by using calibration weight sets or a professional, such as Selectech – contact us.

  6. Tare: To zero the display and remove the weight of a container before filling the container, so that one can measure only the contents added to the container. This action can be repeated to add different measured ingredients to the same container.

 

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