An analytical balance for lyophilisation systems

The description of industrial lyophilisation 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 an 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 lyophilisation processes are shown