How does a Laboratory Balance or Scale work?

There are two basic types of electronic balance designs.

1. Electromagnetic balancing type
2. Electrical resistance wire type (load cell type)

These are based on different principles, but both have neither directly measures mass. They measure the force that acts downward on the pan. This force is converted to an electrical signal and displayed on a digital display.
 

As a means of measuring force, the electromagnetic balance method uses the electromagnetic force generated from a magnet and coil, whereas the electrical resistance wire method uses the change in resistance value of a strain gauge attached to a piece of metal that bends in response to a force.
 

 
So why do electronic balances display mass values when that is not what they measure? It's because the reference standards for mass are weights, which are placed on a pan to inform the electronic balance that a given force is equivalent to a given number of grams, which is used for conversion. Consequently, electronic balances that do not perform this conversion accurately cannot display accurate mass values.

Readability and accuracy are not the same thing?

The readability of a balance is the smallest quantity that the balance will display. Accuracy is the difference between the known weight of a sample and the displayed weight. The accuracy of a balance can be measured only when the balance is in its operating environment

Location of the Balance

The precision and reproducibility of weighing results is closely associated with the location of the balance. To ensure that your balance can work under the best conditions, please observe the following guidelines:

Weighing bench

• Stable (lab bench, lab table, stone bench).  Your weighing bench should not sag when work is carried out on it and should transfer as few vibrations as possible.
• Antimagnetic (no steel plate).
• Protected against electrostatic charges (no plastic or glass).
• Wall or floor installation.  The weighing bench should be fixed either to the floor or on the wall.  Mounting the bench on both places at once transfers vibrations from wall and floor.
• Reserved for the balance.

The place of installation and the weighing bench must be stable enough that the balance display does not change when someone leans on the table or steps up to the weighing station. Do not use soft pads underneath, such as writing mats.  It is better to position the balance directly over the legs of the bench, since the area is subject to the fewest vibrations.

 

The article above is a very brief outline of laboratory balances or scales.  many factors affect the weighing accuracy besides location - eg temperature, humidity etc..Visit the last link below to download a complete guide to weighing.

For more information visit:-
http://www.shimadzu.com/an/hplc/support/lib/lctalk/66/66lab.html
http://electronics.howstuffworks.com/everyday-tech/digital-scale.htm
http://www.prlabs.co.uk/news/article.php?Id=140