Capacitance manometers in freeze-drying

<p dir="ltr">A <b>capacitance manometer</b> is a precise pressure sensor commonly used in freeze-drying (lyophilization) to monitor chamber pressure. Its core principle relies on a <b>flexible diaphragm</b> that forms one plate of a capacitor. As the surrounding pressure changes, the diaphragm deflects, altering the capacitance between the plates. This capacitance change is converted into an <b>electrical signal proportional to pressure</b>, allowing accurate measurement over a wide pressure range, typically from 10⁻⁴ to 10³ mbar.<br><br><b>Calibration:</b> The device is <b>calibrated using reference pressures</b> from a primary standard, such as a spinning rotor gauge or a deadweight tester, to ensure the pressure reading is traceable and accurate. Calibration typically involves adjusting the sensor output to match known pressures across its operating range, often performed at regular intervals to maintain measurement fidelity.<br><br><b>Use in Pharmaceutical Freeze-Drying:</b> In a freeze-dryer, capacitance manometers provide <b>real-time monitoring of the chamber pressure</b>, enabling precise control of the sublimation process. By tracking pressure, in comparison with a Pirani gauge, which is sensitive to moisture vapour, operators can determine the <b>endpoint of primary drying</b>, detect leaks or valve issues, and ensure consistency between batches. Accurate pressure measurements are also crucial for <b>process development and scale-up</b>, where the sublimation rate and product temperature must be tightly controlled to prevent collapse or overdrying of sensitive pharmaceutical formulations.</p>