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A platform balance measures the weight of an object using a platform upon which the object is placed. Typically, the term is applied to devices designed to weigh objects that are very heavy or difficult to weigh in other circumstances. Technically, scales and balances differ, although the terms are used interchangeably. The mechanism of the measuring device depends on the application and weight range required. A platform balance can refer to either a balance or a scale.
The mechanism behind the balance is based on the comparison of the object to a set of standard weights on opposite arms of a central fulcrum. A two-pan balance, as seen in grocery produce aisles, is an example of this type. Likewise, a gym or health clinic will often have a platform that weighs a person by moving weights across an arm. The arms in such a balance are not as obvious as the vegetable balance and are uneven in length. This creates a multiplier, allowing larger weights to be accommodated in a smaller space.
In a scale, a movement of a pointer across a dial face is caused by deflection of a spring or deflection of pendulum weight. Strain-gauge load cells and pneumatic pressure cells also sense force and so can measure weight. These devices have the advantage of no weight-bearing moving parts.
Strain gauges use the four circuit legs of a Wheatstone bridge, an electric device, to measure weight in a load cell. Stress is applied to a calibrated metal foil due to the pressure of the item being weighed on the platform. This strain causes the resistance of the metal to electrical flow to change, which causes the Wheatstone bridge to be out of balance. The comparative lengths of the bridge legs are adjusted, and the circuit flow comes back in balance.
The amount of adjustment corresponds to the weight of the object. Output of strain gauges can be added to enable multiple sensors to be employed in weighing very heavy loads like trucks and tankers. The small physical package of a strain gauge, coupled with its relative low cost and maintenance, means that the platform balance may be just a few inches or centimeters tall and not require a pit.
Pneumatic pressure cells are devices that measure pressure by force against a liquid or gas. They may be paired with a strain gauge to change the range of weights that can be measured. These devices may also use electronics such as the Wheatstone bridge to measure the weight directly, although they are not as sensitive.
Trucks and tankers are weighed on large drive-on platforms that are almost level to the road. Train cars are also weighed on platforms that compensate for the fact the cars are coupled to each other. These scales accommodate loads in excess of 50 tons (45 megagrams). Ships are usually not weighed by a platform balance but rather by use of a sling or by measuring water displacement.