Gas flow in a vacuum system occurs differently than in higher-pressure environments. In vacuum systems, the behavior of gas molecules changes based on the pressure range, affecting how gas flows through the system. There are three primary flow regimes based on pressure:

1. Viscous Flow (Continuum Flow)

• Pressure Range: This occurs at higher pressures, typically above 1 mbar (750 mTorr).
• Characteristics: In viscous flow, gas molecules are close enough to interact and collide frequently with each other, creating a continuous flow similar to how gases behave at atmospheric pressure.
• Flow Nature: The flow is dominated by molecular collisions, and it behaves in a predictable way according to fluid dynamics laws, such as Poiseuille’s law.
• Applications: This regime is relevant at the beginning stages of evacuation when a system is not yet in a high vacuum.

2. Transitional Flow (Knudsen Flow)

• Pressure Range: Transitional flow occurs between about 0.01 to 1 mbar (10 mTorr to 750 mTorr).
• Characteristics: Here, the mean free path of gas molecules (average distance between collisions) becomes comparable to the size of the chamber. Both molecular and viscous flow properties are present, but molecular interactions begin to dominate.
• Flow Nature: It is a mixed regime, where neither viscous nor molecular effects dominate completely, making flow behavior more complex.

3. Molecular Flow

• Pressure Range: Molecular flow is dominant in high and ultra-high vacuum, typically below 0.01 mbar (10 mTorr).
• Characteristics: In this regime, the mean free path of molecules is much larger than the dimensions of the vacuum chamber or the tubing.
• Flow Nature: Gas molecules rarely collide with each other and instead move independently, bouncing off surfaces of the chamber and components. Flow becomes random and directional based on the probability of molecules hitting surfaces rather than colliding with each other.
• Applications: Molecular flow is crucial in high-vacuum and ultra-high vacuum processes, like semiconductor manufacturing, surface science, and space simulation.

Factors Affecting Gas Flow in a Vacuum

• Mean Free Path: As pressure decreases, the mean free path increases, and molecules travel further between collisions. This transition marks the shift from viscous to molecular flow.
• Conductance: The ability of a vacuum system’s piping to allow gas flow decreases as molecular flow dominates, since molecules randomly strike surfaces rather than moving with the bulk flow.
• Pumping Speed: In molecular flow, the pumping speed of a vacuum pump becomes less efficient, as fewer molecules reach the pump inlet per unit time. This is why high-vacuum systems are often designed with optimized geometries to maintain effective gas flow.

Practical Implications in Vacuum Systems

• Transition Between Regimes: As a vacuum pump evacuates a chamber, the system transitions through these flow regimes. Understanding each is key for optimizing vacuum system performance.
• Leak Detection: Helium leak detection often relies on molecular flow principles, as helium atoms can travel directly to the detector without molecular collisions, allowing detection of very small leaks.

In summary, gas flow in a vacuum system changes from continuous, viscous flow at higher pressures to random, molecular flow at very low pressures.

A Pirani gauge is a thermal conductivity vacuum gauge used to measure low to medium vacuum levels, generally from about 0.5 Torr down to 10⁻⁴ Torr. Its operation is based on the principle that the thermal conductivity of gases changes with pressure.

Working Principle

1. Heating Element: The gauge consists of a thin filament, usually made of tungsten or platinum, which is heated by a constant electric current. When the filament is in a vacuum, the heat dissipates through conduction to the surrounding gas molecules.

2. Thermal Conductivity: As gas pressure decreases, fewer gas molecules collide with the filament, resulting in less heat being conducted away. At higher pressures, more gas molecules are present, leading to more efficient heat transfer.

3. Measuring Resistance Change: As the filament heats up, its electrical resistance changes. This resistance variation is measured and correlated to the gas pressure. When the vacuum level is high (fewer gas molecules), the filament stays hotter, leading to higher resistance. When the pressure is higher, the filament cools more due to increased molecular collisions, leading to lower resistance.

4. Calibration: The gauge is calibrated to translate this resistance into a pressure reading. Pirani gauges are generally calibrated for specific gases (usually air or nitrogen), so the accuracy can vary with different gases due to differences in thermal conductivity.

Applications and Limitations

• Applications: Pirani gauges are commonly used in vacuum systems that operate in the low to medium vacuum range, such as in vacuum furnaces, coating processes, and other industrial applications.
• Limitations: The accuracy of a Pirani gauge can be affected by changes in gas composition, as different gases have different thermal conductivities. Additionally, they are less effective at very high vacuums (e.g., below 10⁻⁴ Torr), where other types of gauges like ionization gauges are preferred.