GRAVIMETER

A 'gravimeter' is a device designed to measure the local gravitational field. Also sometimes called a gravitometer, a gravimeter is a type of accelerometer.
According to the equivalence principle of Albert Einstein, the effects of a gravitational field are rigorously equivalent to acceleration. Therefore, gravimeters are susceptible to vibration which causes small, oscillatory accelerations. This is counteracted by integral vibration isolation and signal processing.
Though the essential principle of design is the same as accelerometers, gravimeters are typically much more sensitive than accelerometers in order to measure very tiny changes within the earth's gravity of 1 ''g)''. In contrast, accelerometers are often designed to measure 1000 ''g'' or more, and many perform multi-axial measurements. The constraints on ''temporal resolution'' are usually less for gravimeters, so that resolution can be increased by processing the output with a longer "time constant".
In current use, there are two basic types of gravimeter: relative and absolute.
A relative gravimeter is basically a weight on a spring, and by measuring the amount by which the weight stretches the spring, local gravity can be measured. However, the strength of the spring must be calibrated by placing the instrument in a location with a known gravitational acceleration.
An absolute gravimeter is used to calibrate relative gravimeters, and operates by letting a mass free-fall in vacuum and measuring its rate of acceleration. The mass includes a retroreflector and terminates one arm of a Michelson interferometer. By counting and timing the interference fringes, the velocity of the mass can be measured.[1] A more recent development is a "rise and fall" version that tosses the mass upward and measures both upward and downward motion.[2] This allows cancellation of some measurement errors, and twice the measurement time for a given instrument height.
The field of gravimetry deals with measuring gravitational fields. A high-grade, calibrated gravimeter such as the portable LaCoste-Romberg gravimeter can measure the earth's gravitational field to within 1 μgal, or 1 ng (10^-9 ''g''). Measurements of the surface gravity of the earth are part of geophysical analysis.
There has also been some interest in the use of gravimeters to detect possible changes in the gravitational field in various anti-gravity experiments.