An analytical balance for lyophilisation systems

The description of industrial lyophilization processes by means of theoretical models requires a balance that is suitable for the very severe environmental conditions that take place in a freeze-dryer. A capacitive analytical balance that is able to operate in the temperature range of -80 °C to +20 °C and in the pressure range of 0.5 Pa to 100 kPa is described in this paper. Such a balance is based on a parallel-electrode capacitor whose capacity is affected by the mass of the substance that is placed on the moving electrode of the capacitor. The capacitor acts as the reactive element of a oscillator circuit that converts the capacity into frequency. Such a frequency is measured and eventually converted in a mass measurement. The prototype the authors have made has a range of about 300 g, is 6 cm high and has a diameter of about 18 cm. The operating principle of the balance is described and the results obtained during different lyophilization processes are shown.