What Is a Hydrostatic Balance?

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  • Written By: Mal Baxter
  • Edited By: Daniel Lindley
  • Last Modified Date: 12 January 2020
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If a person wanted to know how much gold or other less valuable metals were in a crown, as Archimedes famously did, it would be necessary to understand the principle of hydrostatic equilibrium, or hydrostatic balance; this can be accomplished with the help of a piece of lab equipment that shares the name — a hydrostatic balance. Essentially, this device takes weight measurements of a substance or object, such as a gemstone or fluid. It does so by comparing the weight of the object suspended in the air and again in water, where, as swimmers know, objects seem to be much lighter; this is due to the upward thrust that water places on an object, which may also displace water from its container. The equipment that measures these characteristics often resembles a mechanical or electronic scale with a vessel attached for holding the substance in question.

To determine density characteristics using a hydrostatic balance, a substance is first weighed in the air, and then the same substance is dipped into a larger vessel containing water. These two measurements are compared and formulated to describe the substance's gravity. The gravity measure tells researchers vital information about the item's density, or how a substance or object will behave under varying pressures.


Hydrostatic equilibrium often refers to the relationship between gravity and gas pressure on a substance. Substances experience different amounts of pressure at different depths. The deeper the area, the more pressure it endures from gravity. Essentially, as gravity presses down — a relative increase — the substance's pressure gradient also increases. Put simply, to measure the gold in a crown, it may be helpful to know that the upward force of water on an object equals the weight of its displaced fluid.

These forces can interact with a substance in other aspects, such as temperature and surface area, generating pressures or energy. The hydrostatic balance can suspend elements in other mixtures as well as water, in order to measure the gravity of substances. Measures can also detect the purity of substances that might be mixed with other elements; for example, whether a crown contains impurities, or in Archimedes' case, silver, which has a measurably different hydrostatic equilibrium than gold. Today the technology finds uses in applications such as gemology, astrophysics, geology, and atmospherics; on larger scales, the principle is used to describe characteristics of planets and stars. In all these contexts, hydrostatic equilibrium refers to how a substance may behave or retain its shape or composition under varying pressures.

Traditionally, hydrostatic balance technology relied on mechanical equipment. This technology was typically shaped like a common balance: a T-shaped structure supporting two dishes on either end. Differences in weight would cause the rocker arm to bow toward the heavier element.

Contemporary hydrostatic balance equipment relies on exacting electronic sensing. Technology may resemble a scale with a single vessel suspended from an electronics platform. Instruments can be scaled to various sizes; some are designed for benchtop operation, while others are bigger, standalone equipment to handle larger vessels.

Modern devices offer digital outputs and numerous other precision readings. Computerized digital sensors can report not only the given hydrostatic equilibrium of an element, but can also output additional computed information. Such data might include running analyses and comparisons between measurement series, as well as unit conversions between Imperial or metric systems; for ready output in whatever measures called for.


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