# Gravimeter

With gravimeters which is gravitational acceleration determined on a geographical location. It is made up of gravitational acceleration , centrifugal acceleration , the distance to the earth, the tidal effect, the influence of the surrounding terrain and other factors. See also gravimetry .

The term gravimeter used to refer to a special form of the hydrometer (liquid density meter), the weight hydrometer.

## Absolute gravimeter

Absolute gravimeters measure the absolute value of the gravitational acceleration. They can therefore be used anywhere, even outside of the earth, without further calibration .

One type of absolute gravimeter are those that measure the fall of bodies. The speed of fall from an object (usually a reflector) can be measured. It is also possible to shoot an object (reflector) into the air and to determine the gravitation on the basis of the transit times measured at two points (= four points in time). Such absolute gravimeters are usually large and unwieldy. They are often only used on firm ground. But there are also absolute gravimeters for use in aircraft (Absolute Airborne Gravimetry) .

Another type of absolute gravimeter uses the principle of pendulum oscillations, see pendulum gravimeter .

## Relative gravimeter

Schematic structure of the LaCoste-Romberg gravimeter

Relative gravimeters measure the change in gravity acceleration compared to a zero point.

Relative gravimeters are based on the principle of Newtonian spring scales. However, the change in the spring length is not measured, but the change is compensated and it is measured how much one has to compensate.

The simple form of a vertically suspended spring is too imprecise to achieve satisfactory measurement accuracy. For example, in order to be able to resolve gravitational accelerations in the mGal range, changes in the deflection in the µm range would have to be measured.

One way out is the astasis of gravimeters. The spring is attached in such a way that a small change in the gravitational acceleration results in a large deflection of the spring. In the LaCoste-Romberg gravimeter this is achieved by the inclined suspension of the spring. With the help of the adjusting screw, the gravimeter is brought to the zero position to read the value of the gravitational acceleration. With the help of a reflected light beam, the acceleration due to gravity can be read off very precisely on a relative scale.

## Superconducting gravimeter

By capacitive measurement of the floating height of a niobium ball with a diameter of about 25 mm above a superconducting ring in which a current flows without resistance at the temperature of liquid helium , the force of gravity is measured very precisely. The measurement resolution is 0.1 nm / s².

## Individual evidence

1. https://www.bkg.bund.de/DE/Observatorium-Wettzell/Messsysteme-Wettzell/Lokale-Messsysteme/Gravimeter/gravimeter_cont.html The superconducting gravimeter, Geodetic Observatory Wettzell (BRD), (c) 2017, accessed on 23 May 2017
2. http://www.dgg2009.uni-kiel.de/DGG-Tagungsband.pdf DGG-Tagungsband, 2009, accessed June 17, 2013