Pressure is defined as force per unit of area: p=F/A (Formula 1-3) where F is the force and A the area to which the force is applied. The SI unit of pressure is 1 N / m² = 1 Pa. Other frequently-used units of pressure are: 1 mbar = 1 hPa = 100 Pa and 1 Torr = 133.322 Pa. If pressure is measured via the force that is exerted on an area, the pressure measurement is independent of the type of gas.
Pressure measurement on the basis of force reaches its limits at pressures of less than 1 hPa, because the exerted forces become too small. Consequently other processes must be used. The thermal conductivity of the enclosed gas can be used, for example, or the gas molecules can be ionized and the ion current flowing between electrodes measured. These indirect measurements which determine the pressure from a gas property consequently deliver a measurement result that is dependent on the type of gas.
In vacuum technology, no single measurement method covers the entire pressure range. It is therefore necessary to use different sensors. The criteria for selecting a pressure sensor are based upon various conditions:
The pressure range to be detected
Gas composition: Inert or corrosive
Required accuracy and repeatability
Environmental conditions, such as radioactivity
In the case of a diaphragm vacuum gauge, pressure is measured in accordance with the definition. A pressure p is exerted on a diaphragm having a defined area A and deflects the diaphragm proportionally to the pressure. A sensor measures the deflection; in the most straightforward case, the deflection is transmitted mechanically to the needle moving over a pressure dial. Piezo-resistive or capacitive sensors receive the pressure signal and convert it into an electrical signal.
Piezo-diaphragm vacuum gauges
A simple and extremely robust method involves the use of a piezo-resistive pick-up. The design is shown in Figure 5.1. A diaphragm into which strain resistances have been diffused is arranged over an evacuated volume having a reference pressure p0. The measured change in resistance as a result of diaphragm deflection serves as a parameter for the pressure. This sensor is characterized by its insensitivity to gas inrush and its high accuracy.
:grayscale(false):format(webp))
Figure 5.1: Design of a diaphragm vacuum gauge
Capacitive diaphragm vacuum gauges
In a capacitive vacuum gauge (Figure 5.2), deflection of the diaphragm is measured as the change in capacity of a plate capacitor that is formed by the diaphragm and a fixed counter-electrode in a well-evacuated space having a pressure p0. The diaphragm is comprised either of ceramics with a vacuum-metalized coating or of stainless steel. This method and diaphragms of varying sensitivity can be used to perform measurements of four decades each. The lower measurement limit is 10-5 hPa.
The limiting effects are:
Change in clearance between the capacitor plates due to the influence of temperature
Decreasing forces acting on the diaphragm at low pressures
The influence of temperature can be minimized through electronic compensation of a known temperature drift or by means of an integrated heater that maintains the sensor at a constant temperature. The influence of temperature can be further reduced through the use of ceramic diaphragm material; in addition ceramic diaphragms give capacitive vacuum gauges excellent resistance to corrosive gases.
:grayscale(false):format(webp))
Figure 5.2: Design of a capacitative diaphragm vacuum gauge
Spinning rotor gauge
A spinning rotor gauge (SRG), a so-called gas friction gauge, is used for calibration purposes. A sphere is magnetically suspended in the vacuum and caused to rotate rapidly, at which point the drive is then de-energized. The pressure of the type of gas that is present can be calculated from the decrease in rotational frequency due to gas friction. In the molecular flow range, these devices measure up to pressures p > 10 -7 hPa. The calibration of the device is dependent only on the sphere, which means that calibrated spheres can be used as a transfer standard. These vacuum gauges are not as suitable for vacuum processes since the time taken for the measuring process increases as the pressure decreases.
