Vacuum Pump Knowledge - Vacuum Concept and Vacuum Calculation Formulas
1. Definition of Vacuum: A vacuum system refers to a rarefied gas state below atmospheric pressure in that region.
2. Vacuum Degree: The degree of rarefaction of a gas under vacuum conditions is usually expressed as "high vacuum degree" and "low vacuum degree." A high vacuum degree indicates a "good" vacuum, while a low vacuum degree indicates a "poor" vacuum.
3. Unit of Vacuum Degree: The Torr is usually used as the unit, but in recent years the Pascal (Pa) has been adopted internationally.
1 Torr = 1/760 atmospheres = 1 millimeter of mercury4. Conversion between Torr and Pascal:
1 Torr = 133.322 Pa or 1 Pa = 7.5 × 10⁻³ Torr
5. Mean Free Path: The average distance traveled by a gas particle undergoing random thermal motion in two successive collisions, denoted by the symbol "λ".
6. Flow Rate: The amount of gas flowing through any cross-section per unit time, denoted by the symbol "Q", with units of Pa·L/s or Torr·L/s.
7. Conductivity: The ability of a vacuum pipe to carry gas. Units are liters per second (L/s). In a steady state, the conductivity of a pipe is equal to the flow rate divided by the pressure difference across the pipe.
The symbol is "U".
U = Q/(P² - P₁)8. Pressure or Pressure Intensity: The force exerted by gas molecules on a unit area of the container wall, denoted by "P".
9. Standard Atmospheric Pressure: Atmospheric pressure equal to 101325 dynes per square centimeter, symbol: (Atm).
10. Ultimate Vacuum: The vacuum level at which a vacuum container stabilizes after being fully evacuated is called the ultimate vacuum. Typically, a vacuum container undergoes 12 hours of gas preparation followed by 12 hours of evacuation. Measurements are taken every 10 minutes during the last hour, and the average of these 10 measurements is the ultimate vacuum value.
11. Pumping Rate: Under a given pressure and temperature, the amount of gas removed from the pump inlet per unit time is called the pumping rate, or simply pumping speed. That is, Sp = Q/(P - P0).
12. Thermocouple Vacuum Gauge: A vacuum gauge that measures vacuum by utilizing the principle that the potential of a thermocouple is related to the temperature of its heating element, which in turn is related to the thermal conductivity of the gas.13. Ionization Vacuum Gauge (also known as Thermionic Cathode Ionization Gauge): Consists of a cylindrical collecting electrode, a grid, and a filament located at the center of the grid. The cylindrical collecting electrode is located outside the grid. The thermionic cathode emits electrons to ionize gas molecules, and the ions are collected by the collecting electrode. The gas pressure is measured based on the magnitude of the collected ion flow.
14. Compound Vacuum Gauge: Composed of a thermocouple vacuum gauge and a thermionic cathode ionization vacuum gauge. The measurement range is from atmosphere to 10⁻⁵ Pa.
15. Cold Cathode Ionization Gauge: Has a pair of cathode plates at both ends of an anode cylinder. Under the influence of an external magnetic field, a Penning discharge is generated inside the anode cylinder, producing ions. The gas pressure is measured based on the magnitude of the ion flow collected by the cathode plates.
16. Resistance Vacuum Gauge: Utilizes the principle that the resistance of a heating element is related to temperature, and the temperature of the element is related to gas conduction. A bridge circuit is used to measure the degree of vacuum.
17. McLaur vacuum gauge (compression vacuum gauge): An absolute vacuum gauge that compresses the gas to be measured to a very small volume using mercury (or oil), and then compares the difference in liquid column pressure between the open and closed tubes to directly calculate the gas pressure using Boyle's law.
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