Q. What Is Readability,

To fully understand how balances and scales operate, there must be an understanding of the difference between mass and weight. Mass is a constant unit of the amount of matter an object possesses. It stays the same no matter where the measurement is taken. The most common units for mass are the kilogram and gram. Weight is the heaviness of an item. It is dependent on the gravity on the item multiplied by the mass, which is constant. The weight of an object on the top of a mountain will be less than the weight of the same object at the bottom due to gravity variations. A unit of measurement for weight is the newton. A newton takes into account the mass of an object and the relative gravity and gives the total force, which is weight. Although mass and weight are two different entities, the process of determining both weight and mass is called weighing. Accuracy The ability of a scale to provide a result that is as close as possible to the actual value. The best modern balances have an accuracy of better than one part in 100 million when one-kilogram masses are compared.

Calibration The comparison between the output of a scale or balance against a standard value. Usually done with a standard known weight and adjusted so the instrument gives a reading in agreement. Capacity The heaviest load that can be measured on the instrument. Precision Amount of agreement between repeated measurements of the same quantity; also known as repeatability. Note: A scale can be extremely precise but not necessarily be accurate. Readability This is the smallest division at which the scale can be read. It can vary as much as 0.1g to 0.0000001g. Readability designates the number of places after the decimal point that the scale can be read. Tare The act of removing a known weight of an object, usually the weighing container, to zero a scale. This means that the final reading will be of the material to be weighed and will not reflect the weight of the container. Most balances allow taring to 100% of capacity.

Analytical Balance These are most often found in a laboratory or places where extreme sensitivity is needed for the weighing of items. Analytical balances measure mass. Chemical analysis is always based upon mass so the results are not based on gravity at a specific location, which would affect the weight. Generally capacity for an analytical balance ranges from 1 g to a few kilograms with precision and accuracy often exceeding one part in 106 at full capacity. There are several important parts to an analytical balance. A beam arrest is a mechanical device that prevents damage to the delicate internal devices when objects are being placed or removed from the pan. The pan is the area on a balance where an object is placed to be weighed. Leveling feet are adjustable legs that allow the balance to be brought to the reference position. The reference position is determined by the spirit level, leveling bubble, or plumb bob that is an integral part of the balance. Analytical balances are so sensitive that even air currents can affect the measurement. To protect against this they must be covered by a draft shield.

This is a plastic or glass enclosure with doors that allows access to the pan. Equal Arm Balance/Trip Balance This is the modern version of the ancient Egyptian scales. This scale incorporates two pans on opposite sides of a lever. It can be used in two different ways. The object to be weighed can be placed on one side and standard weights are added to the other pan until the pans are balanced. The sum of the standard weights equals the mass of the object. Another application for the scale is to place two items on each scale and adjust one side until both pans are leveled. This is convenient in applications such as balancing tubes or centrifugation where two objects must be the exact same weight. Platform Scale This type of scale uses a system of multiplying levers. It allows a heavy object to be placed on a load bearing platform.