Measuring vacuum pressure is a fundamental element of all vacuum applications, but there’s no such thing as a universal vacuum gauge.
When it comes to measuring vacuum pressure, no single gauge will respond accurately throughout the entire vacuum range (from atmospheric pressure to 10-12 mbar). That’s why a clear understanding of the differences between the types of gauges is essential to match them to the right operational contexts.
In this blog post, we assess direct vs indirect gauges and unpack the core characteristics of each kind for you to more easily compare them.
Direct vacuum gauges: an overview
A direct gauge measures vacuum pressure independently of gas species. There are four kinds of direct gauges worth considering for measuring vacuum pressure. Here’s an overview of how each one functions:
Elastic element vacuum gauges
In elastic element vacuum gauges, a sealed and evacuated vacuum chamber is separated by a diaphragm from the chamber in which the vacuum pressure is measured. The former serves as the reference chamber. With increasing evacuation, the difference between the pressure to be measured and the pressure within the reference chamber decreases. This causes the diaphragm to flex — the ‘elastic element’. This movement is then transferred to a dial, electrically or mechanically, and converted into an electrical measurement signal.
Bourdon vacuum gauges
This dial gauge helps roughly determine pressures between 10 mbar and atmospheric pressure. It’s considered the simplest, and most common of direct gauges. It’s derived from the inside of a tube that’s bent into a circular arc and connected to the vacuum system. The tube bends (more or less) during the evacuation process because of atmospheric pressure. This bending actuates the pointer arrangement attached to the tube, and the corresponding pressure is read off on a linear scale.
Capsule vacuum gauges
This gauge gives readings independent of atmospheric pressure changes and gets its name from the hermetically sealed, evacuated, thin-walled diaphragm capsule inside of it. As the vacuum pressure decreases, the capsule bulges. This movement is transferred to a dial via a system of levers and can then be read as the pressure on a linear scale.
Capacitance Diaphragm Gauge (CDG)
Capacitive measurement is taken when a plate capacitor is created by a diaphragm with a fixed electrode behind it. When the distance between the two plates of this capacitor changes, we record a “change in capacitance”. This change is proportional to the change in pressure and converted into a corresponding electrical measurement signal. An evacuated reference chamber is used as a reference for measurements.
CDGs can measure pressures from 10-5 mbar to well above atmospheric pressure. Depending on what pressure range needs to be measured, capacitance gauges have diaphragms of different thicknesses (and therefore, sensitivity).
Related: The quality of a vacuum is indicated by the amount of gas molecules remaining in the system, and a high-quality vacuum is one with relatively few molecules inside of it. Measurement gauges are used to determine their absolute pressure (and thus, the quality of the vacuum), — which makes knowing more about direct vs indirect gauges essential — but while considering what type of measurement gauge to employ, you should also think about the vacuum system’s maintenance and safety
Indirect vacuum gauges: on overview
Indirect gauges measure a physical effect which is proportional to vacuum pressure; there are 3 major indirect gauges to know. These are Pirani gauges, hot cathode ionisation vacuum gauges and cold cathode ionisation gauges.
This gauge uses the thermal conductivity of gases to measure pressures that range from 10-4 mbar to atmospheric pressure. In the diagram below, you’ll see that the filament within the gauge head forms one arm of a Wheatstone bridge. In one operating mode, the voltage applied to the bridge is controlled in a way that the filament’s resistance (and therefore, the temperature) remains constant regardless of the amount of heat it’s giving off. Since heat transfer from the filament to the gas increases with higher pressures, the voltage across the bridge is a measure of the pressure.
Hot cathode ionisation vacuum gauges
This gauge emits electrons from a cathode which are attracted to a positively charged anode, ionising gas molecules during their transfer. The ion current produced is proportional to the gas pressure being measured. The hot cathode sensors mostly used today are based on the Bayard-Alpert (BA) principle. These BA gauges follow the same principles, and were invented in 1950 to overcome a limitation in vacuum pressure measurement by the triode gauge of ~10-8 mbar.
Cold cathode ionisation gauges
In these gauges, the cathodes commonly operate on the inverted magnetron principle. A gas discharge is ignited by applying a high voltage, and the resulting current is output as a signal proportional to the prevailing pressure. Meanwhile, the gas discharge is maintained at low pressures with the help of an applied magnetic field.
This system requires a high ignition voltage of up to 3.5 kV DC. This high acceleration voltage causes electrons to take the shortest path to the positive anode ring or pin. The permanent magnetic field forces them into tight spiral paths, which increases their ‘residence’ time and the probability of the ionisation of gas atoms. This creates a permanent ion current proportional to the vacuum pressure.