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A gravimeter is a specialized type of accelerometer designed for measuring the local gravitational field of the Earth. In a most basic conception, a gravimeter is simply a highly developed derivation of a scale used for weighing an object. In this way, gravimeters operate on the same principle as any other accelerometer, by measuring acceleration relative to a static basis, but are designed to be far more sensitive than a typical accelerometer in order to measure the minute changes within the Earth's gravity that can happen to due the planet's shape or local geological features.
Unlike other accelerometers which display measurements in common units of acceleration, a gravimeter reads in units known as gals. A gal is defined as 1 centimeter per second squared. The measure of Earth's gravity at its surface varies between 976 and 983 gals. Fluctuations due to mountains and other terrain features of varying density typically range from tens to hundreds of milligals.
Commonly, gravimeters are used in petroleum and mineral mining, as well as for various types of geophysical research, including seismology. There are two distinct types of gravimeters: absolute and relative. While an absolute gravimeter measures local gravity in gals as an absolute unit, relative gravimeters compare gravity at one location with another, and must be calibrated at the known location to provide the ratio between the two places. An absolute gravimeter is generally used to calibrate a relative gravimeter, and works by measuring the acceleration of a mass in a vacuum-housed free-fall. Both absolute and relative gravimeters are available in sizes that make then feasible for field work.
A common relative gravimeter is typically composed of a weight attached to a spring. It measures the amount the weight stretches the spring as the strength of gravity. The best relative gravimeters constructed in this manner are accurate to roughly within 1 microgal.
Currently the most accurate relative gravimeters, however, function by suspending a small sphere in a stable magnetic field, with the energy required to create the magnetic field that suspends the sphere being proportional to the Earth's gravity. These devices are known as superconducting gravimeters. In such a gravimeter, the sphere itself is made of a superconducting niobium sphere that is cooled with liquid helium, rendering it magnetically repulsive. These devices are able to measure gravity down to a nanogal, which is a thousandth of a billionth the strength of the Earth's gravity.