There are many different techniques for body fat measurements. The most common measurement method in Israel (at least from my experience) is bioelectrical impedance analysis. These devices are frequently referred as Tanita, after a Japanese company that manufactures such devices. It’s usually scales on top of which you stand barefooted or a small device that you hold with both hands in front of your body. A small electrical charge is sent through the body (two conductors you stand on or hold in your hands) and according resistance between conductors a measure of body fat will be provided (since the resistance to electricity varies between fat, muscular and skeletal tissue). Main criticism on this methodology is: the place of the conductors. For example: if you’re holding the device in your hands (meaning that upper body fat percentage will be checked), but your problematic area is your lower body. But maybe bioelectrical impedance analysis can be used for measuring deltas? Manufacturers provided experimental results in which their devices showed high consistency. But, in order to get such results several preconditions should be performed, which unfortunately almost never executed. An example of a precondition: 24 hour prior each test should be the same in all food related matters. Meaning: eating and drinking (even water, since it’s a conductor) the same amounts at the same time.