Gravimeters measure the local gravitational field of the Earth and are used in oil and mineral extraction and geophysical research. They come in two types: absolute and relative. Relative gravimeters are commonly used and measure how much a weight attached to a spring stretches due to gravity. The most accurate relative gravimeters are superconducting gravimeters, which use a small sphere of superconducting niobium suspended in a stable magnetic field to measure gravity down to a nanogal.
A gravimeter is a specialized type of accelerometer designed to measure the local gravitational field of the Earth. In a more basic conception, a gravimeter is simply a highly developed derivation of a scale used to weigh an object. In this way, gravimeters work on the same principle as any other accelerometer, measuring acceleration relative to a static basis, but are designed to be much more sensitive than a typical accelerometer to measure minute changes within the Earth’s gravity that they may occur due to the shape of the planet or local geological features.
Unlike other accelerometers which display measurements in common units of acceleration, a gravimeter reads in units known as gals. A girl is defined as 1 centimeter per second squared. The measurement of Earth’s gravity on its surface varies between 976 and 983 gallons. Fluctuations due to mountains and other ground features of varying density typically range from tens to hundreds of milligals.
Commonly, gravimeters are used in oil and mineral extraction, 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 gallons as an absolute unit, relative gravimeters compare gravity at one location to another and must be calibrated at a known location to give the relationship between the two locations. An absolute gravimeter is generally used to calibrate a relative gravimeter and works by measuring the acceleration of a mass in a free fall housed in a vacuum. There are both absolute and relative gravimeters available in dimensions therefore making them feasible for field work.
A common relative gravimeter typically consists of a weight attached to a spring. Measures how much the weight stretches the spring as gravity. The best relative gravimeters constructed in this way are accurate to about 1 microgal.
Currently the most accurate relative gravimeters, however, work by suspending a small sphere in a stable magnetic field, with the energy needed to create the magnetic field suspending the sphere being proportional to the Earth’s gravity. These devices are known as superconducting gravimeters. In such a gravimeter, the sphere itself consists of a sphere of superconducting niobium which is cooled with liquid helium, making it magnetically repulsive. These devices are capable of measuring gravity down to a nanogal, which is one-thousandth of a billionth the force of Earth’s gravity.
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