How does a Residual Gas Analyzer (RGA) work?
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 coRead more
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.
A residual gas analyzer (RGA) is a type of mass spectrometer used primarily to identify and quantify the gases present in a vacuum system. It allows for the analysis of gas species in the low-pressure environments often required in high-tech industries such as semiconductor manufacturing and surfaceRead more
A residual gas analyzer (RGA) is a type of mass spectrometer used primarily to identify and quantify the gases present in a vacuum system. It allows for the analysis of gas species in the low-pressure environments often required in high-tech industries such as semiconductor manufacturing and surface science. Here is a simplified description of how an RGA works:
Ionization of Gas Molecules: Gas molecules within the RGA’s sampling volume are ionized, usually by electron impact. This means that electrons are fired at the gas molecules by an electron gun, knocking off an electron and creating positively charged ions.
Mass Analysis: The ions are then directed into a mass filter, which is often a quadrupole mass filter. A quadrupole consists of four parallel metal rods, with each opposing rod pair connected to a radio frequency (RF) voltage with a direct current (DC) offset. The RF and DC fields are applied to the rods in such a way that only ions of a specific mass-to-charge ratio (m/z) can pass through the filter at any one time, with other ions being deflected and lost. By scanning through a range of RF and DC settings, ions of different m/z can be selectively filtered through the quadrupole, allowing the RGA to scan across a range of masses and thus detect multiple gas species.
Detection of Ions: The ions that pass through the mass filter reach a detector, which is often a Faraday cup or a secondary electron multiplier (SEM). These devices generate a signal proportional to the number of ions hitting them. The detector’s output is then processed and read out as a mass spectrum. Peaks on the spectrum correspond to the m/z of the ions, which can be used to identify the gas species. The height or area of the peak is proportional to the concentration of that species in the gas mixture.
Data Interpretation: The resulting data are interpreted to determine the types and quantities of gases present in the vacuum system. RGAs are crucial for quality control and system maintenance in vacuum systems, as they help identify contaminants, leaks, and outgassing sources which may compromise the integrity of the vacuum environment or the processes occurring within it.
For a more detailed explanation visit: Residual Gas Analysers – VAC AERO International
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