In one of our prior posts, we talked about the key differences between direct and indirect vacuum gauges. One of the most common direct gauge technologies is the capacitive gauge. This post will review how these gauges are constructed and how they work.
At their core, capacitive gauges consist of a thin diaphragm and two capacitive plates that together form part of an electrical circuit. One side of the diaphragm is exposed to the process pressure of interest, while the other side of the diaphragm is exposed to a reference pressure. The reference pressure can be an internal reference – such as a sealed chamber that is evacuated to create a low pressure ‘0’ reference – or an external reference. In either case, the pressure differential across the diaphragm causes the diaphragm to flex.
The deflection of the diaphragm causes a change in the capacity of the circuit. The balance of the components in the electrical circuit convert this change in capacity to a proportional signal (e.g., a DC voltage) that can then be sent to a digital display, or simply read as an analog signal.
Typical capacitive gauges are able to measure pressures over the span of four order of magnitude. For example, VACUUBRAND’s VACUU·VIEW™ gauge measure from 103 mbar down to 10-1 mbar. The exact measuring range is dictated by the thickness of the diaphragm; thinner diaphragms will enable measure deeper vacuum levels. There are commercially available capacitive gauges that can measure as low as 10-5 mbar. Capacitive gauges can also be designed to measure positive pressure (i.e., pressures above atmosphere) by employing a thicker diaphragm capable of handling the greater force across the diaphragm.