Thermo-balances

There are several types of thermo-balances in use, most based on a null point measuring system. A diagram of a microbalance used in thermogravimetric analysis is shown in Figure 3. The balance is deflected as the sample weight changes as a result of temperature change and the consequent movement of the pan is sensed. The output from the movement sensor applies a correction (often magnetic in nature) that brings the balance pan back into its original position. This procedure ensures that, as the weight of the sample changes, the pan is kept in the same position in the furnace and, thus, in the same thermal environment. As a result the pan experiences consistent heating and exhibits greater stability. The actual weight of the pan is determined from the magnitude of the correction signal from the movement sensor. The beam suspension often consists of a ribbon and the center of the beam is located in the field of a permanent magnet. A coil carries a current that exerts a restoring force on the beam that is made to be proportional to the beam movement by appropriately modifying the magnitude of the signal from the movement sensor. Quartz crystal balances have also been designed. The thermo-microbalance can be extremely sensitive mass sensitivities of 0.2 nanogram have been reported.

Figure 3. Diagram of a Microbalance used in Thermogravimetric Analysis