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Answer it Forward Challenge
Asked: July 26, 2020In:
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atmospheric pressuremaximum levels
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    This answer was edited.

    Atmospheric pressure directly limits the maximum vacuum level a pump can achieve because a pump cannot create a perfect vacuum; it can only reduce the pressure relative to the surrounding atmospheric pressure. Here’s how it affects the maximum achievable vacuum: 1. Maximum Theoretical Vacuum Level TRead more

    Atmospheric pressure directly limits the maximum vacuum level a pump can achieve because a pump cannot create a perfect vacuum; it can only reduce the pressure relative to the surrounding atmospheric pressure. Here’s how it affects the maximum achievable vacuum:

    1. Maximum Theoretical Vacuum Level

    • Theoretically, a perfect vacuum is 0 Torr (or 0 Pa absolute pressure), but achieving this is impossible in practical conditions.
    • Most vacuum pumps operate by creating a pressure differential with the surrounding atmosphere, so the best practical vacuum level is determined by how low the pump can reduce the chamber pressure relative to atmospheric pressure.

    2. Local Atmospheric Pressure Variation

    • Atmospheric pressure changes with altitude and weather conditions:
      • Higher Altitudes: At high altitudes, atmospheric pressure is lower, which slightly reduces the maximum achievable vacuum level because the starting reference pressure is lower. For example, at sea level, atmospheric pressure is around 101,325 Pa (or 760 Torr), but at 3,000 meters, it drops to approximately 70,000 Pa.
      • Weather Conditions: Barometric pressure varies with weather, influencing the initial atmospheric pressure slightly, which can also impact the maximum vacuum level achievable.

    3. Pump Specifications Relative to Atmospheric Pressure

    • A pump’s “ultimate vacuum” or maximum achievable vacuum level is specified in absolute pressure terms. For example, if a pump’s ultimate vacuum is rated at 10 Pa, this means it can reduce the chamber pressure to 10 Pa above a perfect vacuum, regardless of atmospheric pressure.
    • When atmospheric pressure decreases (e.g., at high altitude), the relative pressure difference that the pump can achieve decreases, which slightly affects the actual vacuum level relative to the ambient pressure.

    4. Impact on Process Requirements

    • For processes requiring a specific absolute vacuum level, such as 10 Pa, changes in atmospheric pressure have little effect if the pump is rated for that pressure.
    • However, for processes defined by relative pressure (gauge pressure), variations in atmospheric pressure will directly affect the achievable vacuum level, as gauge readings are dependent on the surrounding atmospheric conditions.

    In summary, while a vacuum pump’s absolute maximum vacuum is an inherent characteristic, local atmospheric pressure sets the practical baseline for this limit. Lower atmospheric pressures (such as at higher altitudes) reduce the maximum achievable vacuum level relative to the surroundings, which can be relevant in high-precision applications or when working close to the pump’s ultimate vacuum limit.

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Answer it Forward Challenge
Asked: July 26, 2020In:
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ion vacuum gauge
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    An ion vacuum gauge measures extremely low pressures (high vacuum) by ionizing gas molecules within the gauge and detecting the resulting ions. This type of gauge is commonly used in high and ultra-high vacuum systems, capable of measuring pressures down to 10⁻¹⁰ Torr or lower. Working Principle 1.Read more

    An ion vacuum gauge measures extremely low pressures (high vacuum) by ionizing gas molecules within the gauge and detecting the resulting ions. This type of gauge is commonly used in high and ultra-high vacuum systems, capable of measuring pressures down to 10⁻¹⁰ Torr or lower.

    Working Principle

    1. Electron Emission: The ion gauge contains a hot filament, typically made of tungsten or another refractory metal, which emits electrons when heated. These electrons are accelerated into the gauge by an applied voltage.

    2. Ionization of Gas Molecules: As the emitted electrons move through the gauge, they collide with gas molecules present in the vacuum chamber, ionizing them. This process creates positively charged ions from neutral gas molecules.

    3. Ion Collection: The positive ions are attracted to a collector electrode (a wire or a plate) maintained at a negative potential. When these ions strike the collector, they generate a small current.

    4. Current Measurement: The ion current is directly proportional to the density of gas molecules in the vacuum, and hence to the pressure. By measuring the ion current, the gauge provides a reading of the pressure in the chamber.

    Types of Ion Gauges

    The most common types of ion vacuum gauges are:

    • Hot Cathode Ionization Gauge: Uses a heated filament to emit electrons, which are then accelerated to ionize gas molecules. This is one of the most widely used types for ultra-high vacuum (UHV) applications.
    • Cold Cathode Ionization Gauge: (Penning Gauge): Instead of a heated filament, this gauge uses a high-voltage field to create electrons. Cold cathodes are more durable in some cases because they don’t rely on a fragile hot filament, but they may require higher initial pressure to initiate discharge.

    Key Components

    • Filament (for Hot Cathode): Emits electrons when heated, essential for the ionization process.
    • Collector Electrode: Captures the positive ions created in the gauge.
    • Control Circuitry: Converts the ion current into a pressure reading.

    Applications and Limitations

    • Applications: Ion gauges are commonly used in scientific research, semiconductor manufacturing, surface science, and other applications requiring ultra-high vacuum.
    • Limitations:
      • These gauges cannot function well at pressures above approximately 10⁻³ Torr, as ionization becomes unreliable at higher pressures.
      • Ion gauges are sensitive to contamination, especially by hydrocarbons, which can coat the filament or other components, reducing accuracy.
      • Exposure to atmospheric pressure can damage the filament in hot cathode gauges, so they need to be carefully isolated or turned off before venting.

    Advantages

    • Ion vacuum gauges are capable of measuring extremely low pressures, making them ideal for ultra-high vacuum (UHV) environments where other gauges would not work effectively.
    • They provide a continuous and precise measurement of pressure in high vacuum regions, essential for many critical processes.

    In summary, ion vacuum gauges operate by ionizing gas molecules in the vacuum and measuring the resulting ion current, providing precise pressure readings in high and ultra-high vacuum ranges.

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Asked: July 26, 2020In:
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vapor
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    In a vacuum, vapor behaves quite differently from how it would in atmospheric conditions, primarily due to the lack of surrounding pressure. Here’s how vapor typically behaves in a vacuum: 1. Rapid Expansion: In a vacuum, there is little to no atmospheric pressure to contain the vapor molecules. AsRead more

    In a vacuum, vapor behaves quite differently from how it would in atmospheric conditions, primarily due to the lack of surrounding pressure. Here’s how vapor typically behaves in a vacuum:

    1. Rapid Expansion: In a vacuum, there is little to no atmospheric pressure to contain the vapor molecules. As a result, any vapor introduced into a vacuum will expand rapidly to fill the available space. This expansion lowers the vapor density and pressure.

    2. Increased Evaporation Rates: With reduced pressure, liquids vaporize more readily. Lower pressure decreases the boiling point of liquids, causing them to evaporate at lower temperatures. In vacuum heat treatment, for example, this can cause issues with volatile elements or alloy constituents potentially evaporating from the metal’s surface if temperatures are high enough.

    3. Reduced Collisions and Molecular Interactions: In a vacuum, the lack of pressure and low density result in fewer molecular collisions. This means that reactions between vapor molecules or between vapor and other materials are limited, slowing down the rate of some chemical reactions that would typically occur in higher-pressure environments.

    4. Sublimation of Solids: Some materials, like certain metals and organic compounds, may transition directly from solid to vapor in a vacuum through sublimation if the temperature is sufficiently high. This is particularly relevant in vacuum furnaces, where materials like zinc or lead can vaporize from solid alloy surfaces.

    5. Outgassing: Any surface in a vacuum environment may release trapped gases or vaporize certain compounds in a process called outgassing. This effect can introduce contaminants into the vacuum chamber, complicating processes like thin-film deposition, heat treatment, or semiconductor fabrication, where purity is critical.

    6. Low-Pressure Reactions: While reactions in a vacuum are generally minimized, certain reactions (such as the formation of oxides, carbides, or nitrides) can still occur if trace amounts of reactive gases or impurities are present. However, these reactions are usually slower or limited compared to those at higher pressures.

    In vacuum systems like vacuum furnaces, managing vapor behavior is essential to prevent unwanted contamination, oxidation, or alloy depletion, especially at high temperatures. Careful control of the vacuum level and temperature, along with appropriate materials, can help mitigate these effects.

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