Floats are free drifting underwater buoys. They measure ocean currents, temperature, salinity and an increasing number of parameters according to availability of appropriate sensors. Their compressibility is less than or equal to seawater and, hence, they gain positive buoyancy as they sink. For this reason, float pressure cases are often made of aluminum or glass. At a certain depth, the buoyancy force acting on the pressure case equilibrates with the weight of the float and it stops sinking. This physical concept, called neutral buoyancy, is the basis of floats.
There are floats which remain in constant depth. They use acoustic methods to be located. Either, they transmit sound signals which are received by moored instruments or ships (then called SOFAR) or they receive sound signals transmitted by moored sound sources, then it is called RAFOS. From the time of arrival of the sound signal the distance from source to receiver can be calculated. From the known distances, the location of the float can be derived. Argos floats return at fixed time intervals to the sea surface by help of a bladder at the instrument which can change the buoyancy of the instrument. By this method the measured data can be transmitted by satellite links and their position can be determined by satellite navigation.