Basic knowledge of vacuum gauge

　　Vacuum is a space state where no matter exists. It is a physical phenomenon. In a "vacuum", sound cannot be transmitted because there is no medium, but the transmission of electromagnetic waves is not affected by vacuum. In fact, in vacuum technology, vacuum refers to the atmosphere. When part of the matter inside a specific space is discharged, making its pressure less than a standard atmospheric pressure, we generally call this space a vacuum or vacuum state. 1 Vacuum often uses Pascal or Torr as the unit of pressure. At present, in the natural environment, only outer space can be called the space closest to vacuum. Vacuum Gauge, also known as gauge, is an instrument for measuring vacuum or air pressure . Generally, the air pressure is measured by using the change of certain physical effects of gas under different air pressures. It is widely used in scientific research and industrial production.

　　Vacuum gauges are based on different physical mechanisms used in the measurement principle of vacuum gauges. The main vacuum gauges can be divided into three categories, namely vacuum gauges that use mechanical properties, vacuum gauges that use gas dynamic effects, and vacuum gauges that use charged particle effects. Typical vacuum gauges that use mechanical properties include Bourdon gauges and film capacitor gauges; typical vacuum gauges that use gas dynamic effects include Pirani resistance gauges and thermocouple gauges; typical vacuum gauges that use charged particle effects include hot cathode ionization gauges and cold cathode ionization gauges.

　　Classification by vacuum degree scale method

　　(1) Absolute vacuum gauge: directly reads gas pressure, and its pressure response (scale) can be calculated from its own geometric dimensions or determined by force measurement. Absolute vacuum gauges are accurate for all gases and are independent of the type of gas. Absolute vacuum gauges include U-type pound pressure gauges, compression vacuum gauges, and thermal radiation vacuum gauges.

　　(2) Relative vacuum gauge: The pressure is determined by some quantities that have a functional relationship with the gas pressure. It cannot be calibrated by simple calculation. It must be calibrated before it can be calibrated. The relative vacuum gauge is generally composed of a vacuum gauge tube (or gauge head) as a sensor and a measuring device for control and indication. The reading is related to the type of gas. There are many types of relative vacuum gauges, such as thermal conductivity vacuum gauges and ionization vacuum gauges.

　　Classification by vacuum gauge measurement principle

　　Direct measurement vacuum gauge

　　This vacuum gauge directly measures the force per unit area and is:

　　(1) Static liquid level vacuum gauge: uses the difference in liquid levels at both ends of a U-shaped tube to measure pressure.

　　(2) Elastic element vacuum gauge: It measures the pressure by utilizing the elastic deformation of the container surface connected to the vacuum under the action of pressure.

　　Indirect measurement vacuum gauge

　　When the pressure is 10-1Pa, the force acting on the surface of 1cm2 is only 10-5N. Obviously, it is difficult to measure such a small force. However, the pressure change can be indirectly measured based on the change of physical quantities related to gas pressure under low pressure. Vacuum gauges belonging to this category include:

　　(1) Compression vacuum gauge: Its principle is to apply Boyle's law on the basis of the U-tube, that is, a certain amount of gas to be measured is isothermally compressed to increase its pressure so that it can be measured with a U-tube vacuum gauge, and then the measured pressure is calculated using the relationship between volume and pressure.

　　(2) Thermal conductivity vacuum gauge: It is made based on the principle that heat conduction of gas under low pressure is related to pressure. Commonly used ones are resistance vacuum gauge and thermocouple vacuum gauge.

　　(3) Thermal radiation vacuum gauge: It uses the principle that thermal radiation of gas under low pressure is related to pressure.

　　(4) Ionization vacuum gauge: It uses the principle that the gas molecules under low pressure are ionized by the collision of energetic particles, and the generated ion flow changes with the electric force. Such as: hot cathode ionization vacuum gauge, cold cathode ionization vacuum gauge and radioactive ionization vacuum gauge.

　　(5) Discharge tube indicator: The vacuum degree is determined by using the gas discharge conditions and the properties of the discharge color related to pressure. It can generally only be used as a qualitative measurement.

　　(6) Viscous vacuum gauge: It uses the principle of external friction, i.e., the momentum exchange between gas and container wall under low pressure. Such as diaphragm vacuum gauge and magnetic levitation rotor vacuum gauge.

　　(7) Field microscopy: The pressure is calculated based on the relationship between adsorption and desorption time and pressure.

　　(8) Partial pressure vacuum gauge: Use mass spectrometry to measure the partial pressure of mixed gases. Commonly used ones include quadrupole mass spectrometer, cyclotron mass spectrometer and radio frequency mass spectrometer.