The working principle of the densimeter is floating, so the densimeter is in a state of two forces balance, that is, the buoyancy of the liquid when the densimeter is immersed in the liquid is equal to the gravity of the densimeter.

Put the same densitometer in salt water, clear water and oil respectively, and they are all in a floating state at rest, so the buoyancy of the liquid received by the densitometer is equal to the gravity of the densitometer, that is, F salt =G meter, F water =G meter and F oil =G meter. Because the gravity of the same densitometer is equal, the buoyancy of the densitometer is equal in these three liquids, that is, F salt =F water =F oil =G meter.

Note: the buoyancy here is not judged by Archimedes principle. If we use Archimedes' principle to judge the buoyancy, for example, the greater the density of liquid, the greater the buoyancy of an object subjected to liquid. There is a prerequisite here, and the volume of liquid displaced by an object remains unchanged. In this question, the densitometer is put into salt water with higher density, and the volume of salt water discharged by the densitometer is smaller; The densimeter is put into the oil with lower density, and the volume of the oil discharged by the densimeter is larger.