Since we can’t actually weigh astronauts in weightlessness, body mass measurements are used to determine any change in weight.
Weight, in this case, is what the astronaut would weigh under the influence of earth gravity.
Such measurements are made with a spring-like device and a highly accurate timer.
By measuring the time, or periodicity, between the oscillations, the back-and-forth or up-and-down movement, of the spring, a correlation can be drawn between body mass and actual weight.
The periodicity of the motion varies based on the mass of the object or person on the springs.
On the shuttle, a chair-like device is used.
The astronaut climbs in the chair, which then moves back and forth.
The speed of the motion is determined by the mass in the chair.
On the Russian space station Mir, cosmonauts crouched over a device that moves up and down, but the result is the same.