Design Factors

The basic instrumental requirements for TG are a precision balance, a furnace capable of being programmed through a required regime of temperature change, and a computer workstation capable of programming the furnace, recording the weight change, and processing the data (see Fig. 9).

The essential requirements of an automatic and continuously recording balance are similar to those of an analytical balance and include accuracy, sensitivity, reproducibil-ity, and capacity. In addition, a recording balance should have an adequate range of automatic weight adjustment, a high degree of mechanical and electronic stability, and a rapid response to weight changes and be unaffected by vibration.

FIGURE 9 Schematic diagram of a thermobalance and control unit.

Two weighing systems need to be noted, namely, deflection and the null-point balances. There is a variety of deflection balances that can be constructed—beam type, helical spring, cantilevered beam, torsion wire, etc.—but they suffer in that the sample under observation will not remain in a fixed position in the furnace. For this reason most units employ a null-point balance.

In the use of null-point balances, a sensor must be used to detect the deviation of the balance beam from its null position, and a variety of methods may be used to detect deviations from the horizontal or vertical norm. One common system would is make use of the varying intensities of a light source impinging upon a photoelectric cell. The usual arrangement incorporates a light source, a shutter or mirror, and either single or double phototubes. The displacement of the shutter attached to the balance beam (or spring) intercepts the light beam, thus either increasing or decreasing the light intensity acting on the phototube. The resulting change in current magnitude from the phototube is then used to restore the balance to its null point.

Furnace design features and temperature measurement have already been discussed.

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