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A platform balance is a device that measures the weight of an object using a platform or solid base in conjunction with a series of levers and balances that together calculate the overall mass. These tools are most commonly used to weigh objects that are very heavy, bulky, or awkward. People often step onto these sorts of devices in medical clinics and fitness centers in order to assess their own weight, for instance, and they’re often used to measure machinery and things like cars and trucks, too. Smaller models are common in science labs. From a technical standpoint this sort of device is different from a scale, but the two are often grouped together and as a result a balance is often thought of as a type of scale. A true scale usually works with a series of internal mechanisms and shows results on a dial or digital display, whereas a balance almost always operated with a series of external moving arms that must be manipulated and moved to counterbalance the weight on the platform and ultimately line up with the reading that is the final weight. The overall goal of each is the same, though; namely, to accurately and clearly display a particular weight.
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 a metal bar or arm. The arms in such a balance are not as obvious as the vegetable balance and are usually uneven in length. This creates a multiplier, allowing larger weights to be accommodated in a smaller space.
In a standard scale designed to measure smaller objects, the movement of a pointer across a dial face is caused by deflection of a spring or deflection of pendulum weight. The basic mechanics work similarly in a platform setting. Strain-gauge load cells and pneumatic pressure cells also sense force and so can measure weight in these contexts, and the devices have the advantage of no weight-bearing moving parts.
Strain gauges typically 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 typically corresponds to the weight of the object.
Pneumatic pressure cells are also commonly part of large-scale measuring operations. These 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 in most cases as sensitive.
Output of strain gauges can be added to enable multiple sensors to be employed in weighing very heavy loads like trucks and tankers, and in these cases there is often a pit below the platform where a lot of the heavy measuring is done. This isn’t really necessary in all circumstances, though. 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 depending on the intent and how it’s used.
Trucks and tankers are usually weighed on large drive-on platforms that are almost level to the road, which makes them fairly easy to physically get on top of. Train cars are also weighed on platforms that are specially calibrated to compensate for the fact the cars are coupled to each other. These scales can typically 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, unless they are out of water in which case they’re often treated in much the same way as a car or truck, particularly if they can be rigged to a trailer or wheeled platform that can be weighed separately.