Vacuum Techniques

In this experiment you will learn about vacuum technology and how to use it for measurements

Experimental setup

In this experiment we shell use the following devices:

Rotary Vane Pump

Rotary Vane Pump is a pump that draws gas from one chamber to an atmospheric pressure. It is made out of vanes mounted to a rotor that rotates inside of a cavity and the vanes are in contact with the cavity. The rotor is not aligned at the center of the cavity and the vanes are pressed against the cavity wall with the aid of springs and the centrifugal force. The vanes form compartments that traps gas. In the next figure a section view of a rotary pump is shown. For simplicity, only two vanes are shown but there may be more that two. Oil is used to lower the friction between the vanes and the cavity and, in addition, serves as sealing matter so gas will not flow between compartments.

Figure 1: A schematic illustration of cross section of a rotary vane pump (1) pump housing or a cavity (2) rotor (3) vanes (4) spring. Tanen from wikipedia.

When the rotor turns the vanes, three types of compartments are formed: (i) a compartment that grows and gas from the system that is pumped is drawn in; (ii) a compartment that shifts the gas toward an exit port; and (iii) a compartment that shrinks and gas from the previous compartment is pushed out to the atmosphere. As the rotor turns again and again, a suction is formed and gas will move from the system to the outside surrounding. Here is a video from youtube illustrating the way the pump works:

Resulting from the gas compression and friction, heat is generated and it is dangerous to touch the pump in its metal parts. In addition, one must remember that the pump contains oil. If a pump is turned off while being connected to a vacuum chamber, oil may flow to the vacuum chamber. To avoid that, a valve should be placed between the pump and the system. Before one powers off the vacuum pump, the valve should be closed, and only then, the user may turn off the pump and vent it cautiously.

Piezoelectric Pressure Transducer

The piezoelectric effect was discovered by Pierre and Jacques Curie in 1880. It remained a mere curiosity until the 1940s. The properties of certain crystals to exhibit electrical charges under mechanical loading was of no practical use until very high input impedance amplifiers enabled engineers to amplify their signals. In the 1950's, electrometer tubes of sufficient quality became available and the piezoelectric effect was commercialized. Piezoelectric Effect, appearance of an electric potential across certain faces of a crystal when it is subjected to mechanical pressure. Conversely, when an electric field is applied on certain faces of the crystal, the crystal undergoes mechanical distortion. Pierre Curie and his brother Jacques discovered the phenomenon in quartz and Rochelle salt in 1880 and named the effect piezoelectricity (from Greek piezein,"to press").

The piezoelectric effect occurs in several crystalline substances, such as barium titanate and tourmaline. The effect is explained by the displacement of ions in crystals that have a nonsymmetrical unit cell, the simplest polyhedron that makes up the crystal structure. When the crystal is compressed, the ions in each unit cell are displaced, causing the electric polarization of the unit cell. Because of the regularity of crystalline structure, these effects accumulate, causing the appearance of an electric potential difference between certain faces of the crystal. When an external electric field is applied to the crystal, the ions in each unit cell are displaced by electrostatic forces, resulting in the mechanical deformation of the whole crystal. Because of their capacity to convert mechanical deformation into electric voltages, and electric voltages into mechanical motion, piezoelectric crystals are used in such devices as the transducer, record-playing pickup elements, and the microphone. Piezoelectric crystals are also used as resonators in electronic oscillators and high-frequency amplifiers, because the mechanical resonance frequency of adequately cut crystals is stable and well defined.

In piezoelectric gauge the pressure exerts mechanical force on the crystal and a voltage which can be measured appears as result. Because the crystal can be deformed by fluctuations of the room temperature as well, the lower limit of this gauge is ~ 1 torr.

In the lab, you will use a APR250 model of Pieso from Pfeiffer company. Download its manual from and look at the "Measurement range max" and "Measurement range min" values (note that you will not be able to use all its range).

Pirani gauge

The Pirani gauge is a robust thermal conductivity gauge used for the measurement of the pressures in vacuum systems. It was invented in 1906 by Marcello Pirani. The gauge is composed of heated wire (The wire is electrically heated) suspended in a gas medium. The wire is often called a filament. The gas molecules collide with the heated filament and by heat convection cools the filament. As a result, the filament reaches an equilibrium temperature. As the pressure drops, fewer gas molecules are available and less heat is taken from the filament. This causes the filament to heat up and reach a higher temperature. The temperature of the filament is proportional to the resistance of the filament and by calibration curve, one can calculate the pressure of the surrounding of the filament by the resistance measurement.

In the lab, you will use a 945 model pirani from Kurt J. Lesker company. Download its manual from and look at the "Useful Set Point Range" under "Specifications" (note that you will not be able to use all its range).

Mass Flow Meter

Mass Flow Meter (MFM) measures directly mass flow, not volumetric flow. this is an important distinction in many applications since the mass flow rate (not volumetric flow rate) determines energy content and chemical reactivity. Volumetric flow measurement is less useful than mass flow measurement because a given mass of gas will change its volume with gas temperature and pressure. Mass flow measurements maximize process repeatability and the testing accuracy. Mass flow rates, when used for gas flow measurements, are conventionally made in volumetric units. Since volume varies with temperature and pressure it is necessary to specify a volume at standard conditions. The schematics of the MFM is shown on the following figure:

Figure 2: Mass flow rates are determined by measuring the heat required to maintain an elevated temperature profile along a laminar flow sensor tube. For a specific flow meter range and gas species, flow is proportional to the voltage necessary to maintain a constant temperature profile.