Balances (Precision)

Scales, balances, serve as a measuring instrument for determining mass. This may be done either by directly measuring the weight force or by the comparison of the weight force with a known mass. Because of different gravity acceleration, scales which are based on measuring the weight force must be readjusted when changing location.

History of balances

The history of the scales goes back to 2000 B.C. Illustrations of simple balances derive from this millenium. The accuracy of the balance was improved by the Etruscans in about 500 B.C. The Romans were also familiar with off-centre beams. The longer beam was here provided with a weight and with a linear scale marking. In the Renaissance Age, highly sensitive analysis scales were used in alchimist laboratories. In 1669, the Robeval balance was invented by Joachim Rosenthal de Romée. The advantage of this balance was the independence of the position of the weighed object from the result. In 1763, inclination scales with direct weight display were built. In the first half of the 19th century decimal balances and kitchen scales were developed. In 1939, the age of electronic scales was initiated with the use of changing electric resistance. Typical scales used in chemical analysis are analytical and laboratory balances, precision scales, platform scales and moisture balances.

Functionality of balances

Mechanical scales are based either on direct measurement of the weight force or on comparing masses. Scales based on directly measuring the weight force have to be calibrated to local conditions of earth/gravity acceleration. This is not required for scales which are based on comparing the weight force. The simplest kind of spring balance directly measures the weight force by attaching the weighed object to a coil spring and then measuring the extension. With the help of the spring constant, the weight force can be determined and illustrated on a suitable scale. Basically, spring balances and dynamometers exist for different ranges. With mass comparison, the mass of an object is determined by comparing against standard weights. Though this balance also uses gravity, it does not need to be calibrated to different local conditions. Today, mechanical scales are more and more replaced by electronic scales. Electronic scales are more exact and may be read faster. Electronic scales convert the weight force, which cannot be measured directly, into a distortion or a path. The measurement is ultimately carried out with a spring balance or a bending beam. The direct measurement determines the distortion with a stretch measuring strip, the indirect path measurement, e. g., via the capacity shift of a capacitor, while the distance between the plates is changing. Table scales, precision scales and laboratory scales may be mentioned as typical examples of electronic scales.

Literature

• http://de.wikipedia.org/w/index.php?title=Waage&oldid=86530885 (called: 28.03.11).

• http://de.wikipedia.org/w/index.php?title=Analysenwaage&oldid=85204189 (called: 28.03.11).