The Mass-Change Sensor

A change in mass of the crucible contents initially results in a movement of the balance beam. This movement must be sensed and the signal from the sensor (which is made to be proportional to the movement) used to activate the current through the coil in the magnetic field to bring the beam back to its original position. The movement sensor is usually optically based and consists of a light source focused on a photoelectric cell that passes a baffle plate situated on the end of the balance beam. On loss of mass the beam moves up letting more (or less) light to the photocell and the resulting change in output made to increase the coil current and bring the beam and baffle plate back to their original position. This is basically a null-recording system. The current is recorded by the computer and defines the new weight of the sample. Loss of weight produces the reverse effect. As the basic signal results only from light ray deflection, the system is weightless and, thus, the registered weight can be exceeding precise, sensitive and accurate.