Weighing scale

A scale is a measuring instrument for determining a mass. Usually this is done on the weight. The gravitational force is either measured directly, such as with a spring balance. Or it is compared with the weight of a known mass. An example of this is the beam balance.

On scales of the first type, the measurement of the local factor depends. In both species the weight value is influenced by the buoyancy in the air. Depending on the application and the required accuracy of these influences are neglected or taken into account by appropriate measures.

A special form is the counting scale that determined from the mass of one or more individual pieces, the number of many identical objects.

• 5.1 Operation of electronic scales
• 5.2 Electromechanical scales for industry and transport
• 5.3 Bench Scales
• 5.4 Precision and Laboratory Scales

• 7.1 Following application
• 7.2 After construction
• 7.3 After accuracy

Relationship between mass and acceleration due to gravity

At high demands on precision balances of the first kind ( weight measurement) must be adjusted at each location because the gravitational acceleration is different. It is the relationship

The deviation of the acceleration due to gravity at different points of the earth's surface is, however, less than 1 percent, so that when household scales, this effect is neglected.

Properties

• Weighing range
• Readability
• Precision and accuracy (see also: accuracy class of a weighing system )
• Reproducibility
• Linearity

History

A balance beam from the 5th millennium BC was discovered in a prehistoric grave in Egypt. The earliest Egyptian pictures simple beam scales come around 2000 BC The suspension of the balance beam consisted of a rope, at its outer ends hung the scales. Around 500 BC the Etruscans improved the accuracy of the beam balance. The Romans also knew ungleicharmige scales, the longer arm carrying a sliding Suspend weight as well as a bar code. During the Renaissance, highly sensitive analytical balances came in alchemical laboratories used.

1669 invented the Frenchman Joachim Rosentahl de Romée the board level. It had the advantage that the position of the load to be weighed to the weigh hoppers do not influence the result. 1763 built the Swabian pastor Philipp Matthäus Hahn tilt scale with direct weight display. Decimal and kitchen scales emerged in the first half of the 19th century. In 1850, attempts have been made, the weighing result to print automatically. 1895 arrived in the United States scales with simultaneous display on price ( price computing scales). 1939 initiated two American engineers with the use of electrical resistance changes in the age of electronic scales.

In 1927, the case for better distinction to cars of Wage was changed to balance.

Mechanical Balances

In mechanical scales, it distinguishes the above two principles, weight measurement and mass comparison.

Weighing: weight

In the simplest form of the spring balance Wägeobjekt is hung from a coil spring and the measured elongation. The weight force is determined and mapped to the corresponding scale on the mass basis of the spring constant. Related measuring principles are also based on deformation measurements, such as membranes, force sensors, load cells, piezoelectric effect or Ringtorsion.

There are spring scales and dynamometers for various measuring ranges. Examples:

• Budget balance, a hand-held device to about 10 kg; the simple spring scale is ( popular ) also called " rags " balance.
• Kitchen scales and baby scales, up to 5 or 10 kg
• Bathroom scales for body weight (usually to 130 or 150 kg, accuracy 0.1 to 3 kg).
• Stands for industrial purposes or commercially ( Weighing / Scale usually 25 kg / 50 g, 50 kg / 100 g or 100 kg / 200 g, accuracy 0,1 - 0,2 %).
• Membrane scales and barometer
• Micro balances and precision spring balances for laboratories, teaching physics, etc., accuracies of 0.5% to 0.1 %.

Weighing: Mass Comparison

The mass of a body is determined by comparison with standard weights. These scales work with gravity, but need not be adjusted to the different local conditions and could therefore also on other celestial bodies, such as used, for example the moon. The reference weights start at 0.1 mg for pharmacists scales and end at 10 kg for market scales.

• A beam balance measures the balance of two weights. In the simplest case, there is a bearing in the pivot point balance beam, which carries at the ends of two shells. A previously encountered in many households and kitchens variant of the beam balance is the Roberval balance, in which the scales are attached to a lever parallelogram. A mechanically -induced systematic error can be compensated, that the two are compared to comparative weights alternately on the same scale to finely dosed ballast on the other pan with beam balances.
• A link level or inclination scale measures the deflection of a fixed weight on a lever arm, this principle is used in many postal scales.
• A steelyard ( Steelyard, Roman scale, Besemer ) consists of a rod with scale and two unequal lever arms, said at a counterweight at the other is a hook for hanging of the sample. The ratio of the lever arms can be moved with a tongue and handle, entry to an attached object in equilibrium.
• A decimal scale works like a steelyard with the difference that the lever arm for the weight ten times as long as that for the sample weighed. With a 10 -kg reference weight body can be measured of 100 kg (see also Fig decimal scale ). Special forms are the road weighbridge for truck and tanker transportation, rail weighbridge for railway carriages, crane scales and pallet scales.

Count: Number

Counting rotate the measuring principle of the weight comparison by a single one of these pieces is determined from the ratio of the weight of a piece quantity whose counting measure. For larger quantities compared with another piece set of known number, about 100 pieces.

Electromechanical and electronic scales

Mechanical devices are now largely replaced by electronic scales. They are more robust, more accurate, faster to read and often cheaper. Electronic analytical balances often have a casing to protect against air movements, and special programs for the calibration attenuation to prevent vibrations. They measure in kilograms area with sensitivity to about 0.1 mg, which corresponds to a dynamic range of 106.

Operation of electronic scales

Allen electric scales in common is the necessary transformation of the force of gravity, which can not be measured directly, in a deformation or a way. This is usually achieved by a spring or a beam deflection, wherein the spring in a crane scale may be a relatively massive metal block.

The more direct method of measuring the deformation of the spring / of the bending beam via a strain gauge, the indirect path measurement can be carried out, for example, on the change in capacitance of a capacitor with change in the distance plate.

Examples:

Electromechanical scales for industry and transport

• Platform and vehicle scales: weighing platforms ( weighbridge ) with multiple load cells, measuring terminal and programs ( for example, tare, tare, multiple measurements ... )
• Static / Dynamic weighing device
• Crane Scales with 10-20 ranges à ± 0.2 %, peak indicator, etc.
• Pallet scales (for example, 10 to 1500 kg ± 0.5 kg)
• Special Scales for long goods, heavy industrial use, etc.
• Weighing terminals with filling control for liquids, big bag, dosage, bagging, numbers, acceptance and loading ...
• Floor scale with function and numeric keypad, standard programs such as counting, checking, dosing ... ( for example, 10 kg ± 0.1 g).

In modern electromechanical scales mainly two sensor principles are used:

• Scales with load cells with strain gages (SG, Eng. Strain gauge, Strain Gage) work
• Scales according to the principle of electromagnetic force compensation

Rare One can also find scales with sensor elements which work with vibrating wire or in which capacitive or piezoresistive sensors are used.

Bench Scales

• Bench Scales with battery and mains operation, distance weighing platform display device to a few meters (verifiable in part ). Subgroups are: Compact or universal scales
• Electronic bench scales for industrial kitchen, bakery or laboratory
• Special Scales for grain samples, eggs, sorting, etc.

Precision and Laboratory Scales

• Precision scales with accuracies to 0.01 % (measuring ranges 100 g to 30 kg). Laboratory scales, micro and analytical balances ( for example, 200 g ± 0.0001 g)
• Scales to measure humidity, such as residual moisture at 40 ... 250 ° C

Special Scales for Engineering and Physics

• Six-component balance: By measuring the three forces and three torques in an orthogonal coordinate system can be with such a scale, for example, in a wind tunnel all the effects of air flow on a model to measure.
• Gravimeter of Geophysics and Geodesy: Precision " spring balance " ( oblique bars) or with superconductivity, measurement of the Earth's gravity 950-990 Gal ± 0.001 mGal (1 Gal = 1 cm / s ²).
• Cavendish balance for determining the gravitational constant (1798 )
• Watt balance for checking the original kilogram: it compares a time measurement, the mechanical with the electrical power.
• Comparator balance or mass comparator of the Physikalisch -Technische Bundesanstalt ( PTB ) for comparison of kilogram prototypes. The unit is in a housing, allows the measurements in a vacuum ( 0.005 mbar). This avoids the measurement errors caused by different buoyancy of prototype and test specimens in the air. The most accurate mass comparator of the PTB has a standard deviation of 1 × 10-10 kg. He serves, among other things, the research on the conversion of the old kilogram standard ( " Ur - kg " ) on a silicon sphere with a defined number of atoms.
• Cotton scale ( a scale magnetic ) for fast and accurate weight measurement of coins, invented by William Cotton ( banker ) during his time as director of the Bank of England from 1842 to 1845.
• Torque balance measures the torque current motors ( see figure).

Types of scales

After application

• Bathroom scales
• Briefwaage
• Crane Scale
• Multihead weigher
• Combination weigher
• Torsion balance
• Checkweigher
• Kitchen scale
• Wheel loader scale
• Wheel load scales
• Axle load scales
• Support load balance
• Stadtwaage
• Hive scale

According to type

• Beam balance
• Decimal scale, often as a blind scales with platform and wall to lean on the bag
• Spring balance
• Inclination balance
• Ring dynamometer
• Board scale
• Baby Scale
• Bathroom scales
• Apothekerwaage
• Body fat scales
• Hand Scale
• Roman steelyard with sliding weight and 2 measuring ranges by 2 hooks
• Moon Libra with elliptically curved leaf spring ellipse within which the protected pointer and crescent-shaped scale

By Accuracy

• Microbalance, eg with a increment of 1 ug
• Analytical balance, eg with a increment of 0.1 mg, 0.01 mg of rare
• Fine scale, with a scale interval 100-1 mg
• Trade balance
• Coarse scale