Fermion's 100 Questions, Issue 5 | Vacuum related

13. What are the differences and selection of UHV-24, FRG, and WRG gauges?

This question covers a wide range. In fact, we generally divide ultra-high vacuum gauges into two categories: hot cathode ion gauges and cold cathode ion gauges:
A hot gauge is a vacuum gauge that measures the vacuum degree by emitting electrons from a hot filament. Its biggest feature is that it has a replaceable filament for generating an electron beam for ionization; while a cold gauge does not have a hot emitting filament, and gas ionization relies on other means, such as a strong magnetic field. Among them, the hot cathode gauge has a very small amount of X-ray radiation during use. This amount is very small and will not cause safety problems, but it needs to be noted for some sensitivity tests.
The vacuum gauges mentioned in the question, UHV-24, UHV-24P, FRG 730 belong to the category of hot gauges, while FRG-700 and WRG belong to the category of cold gauges.
Different vacuum gauges have different conversion and signal transmission methods. UHV24 and UHV24P require a special transmission cable (the price of this cable may be higher than the ion gauge itself depending on the degree of bakeability) to transmit the weak ion current to the controller; while FRG730 has an ADDA module installed at the tail of the ion gauge to directly convert the current into vacuum degree and digital signals. Both have their advantages and disadvantages.

For UHV 24P type separate vacuum gauge
1. A dedicated XGS600 controller is required to read the vacuum value, including cables and connection joints.
2. The conversion efficiency of the dedicated vacuum controller is relatively more accurate. The ultimate vacuum is higher.
3. The structure of the vacuum gauge is simple, and maintenance, baking, and overhaul are easier.
4. Faults are easier to judge.

For FRG 730 type integrated vacuum gauge
1. Because the vacuum gauge tail is already equipped with digital circuits, in addition to the original controller, other communication methods can also be used to directly communicate with FRG730 to read the vacuum degree, which is relatively more integrated.
2. In addition to the vacuum gauge itself, the cables and connectors are all standard interfaces and standard communication protocols, which are more suitable for use as standard modules of standardized complete machines.
3. The integration is too high, and baking, repair, and maintenance are relatively complicated. Replacing the filament of the ion gauge is relatively difficult.
4. Due to the size and power consumption of the onboard module, the reading accuracy and limit test value are relatively poor.  

The main purpose of the hot cathode vacuum gauge is to be used in systems with a higher vacuum degree, generally at the level of 10E-10 mbar. However, the vacuum degree of many systems does not need to be so high. For such systems, cold gauges are generally recommended, such as vacuum gauges such as WRG and FRG700.

One of the biggest advantages of cold gauges over hot gauges is their long life. Because there is no hot filament, they can withstand vacuum shock during use and are more convenient to use. Because of this feature, cold cathode gauges are generally used in conjunction with Pirani gauges to form full-range vacuum gauges. For example, the FRG700 and WRG gauges are actually composed of two modules, the Pirani and the cold gauge. The Pirani gauge is in a long-term working state. Once the reading of the Pirani gauge exceeds the range (less than 10E-4 mbar), the cold gauge will start to complete the full-scale vacuum test. Due to the relationship between the control accuracy of the magnetic field and the control accuracy of the voltage, the measurement accuracy of the cold cathode gauge will be much worse. The nominal test accuracy of the manufacturer is generally (+/- 50%), so this type of vacuum gauge is generally used in environments with low vacuum requirements and even frequent exposure to the atmosphere, such as Load Lock.

14. When the temperature is determined and the environment in the vacuum chamber is the same, can the magnetic field strength in the chamber affect the vacuum index in the chamber? What about noise (ultrasound and infrasound)?

For vacuum gauges that use the principle of gas molecule ionization, because the movement of ions will be deflected by the magnetic field, the magnetic field will affect the trajectory of ion movement, and theoretically will have an impact on vacuum measurement. The magnitude of the impact depends on the magnitude of the magnetic field. For example, the magnetic field of the cold cathode ion gauge itself is generally as high as thousands of gauss, and the impact of the earth's magnetic field (0.25-0.65 gauss) can be almost ignored; if there is an external strong magnetic field within the movement range of the ions, it will seriously affect the function of the ionization vacuum gauge.
Noise (ultrasound and infrasound) is essentially air vibration. Our vacuum chamber is generally very heavy, and the energy of the noise is not enough to be transmitted to the chamber or generate vibration in the chamber, so it cannot affect the vacuum measurement in the chamber.

15. The purpose and function of the system Vent?

The system's Vent command generally refers to restoring the vacuum system from vacuum inflation to atmospheric state. Low-pressure dry nitrogen is generally used to prevent water vapor from entering the vacuum system, ensuring that the next vacuuming speed is faster and the system can reach the working state more conveniently.
Compared with directly removing the flange, the role of Vent can be simply described as follows:
1. When simply removing the flange, due to the effect of atmospheric pressure, the flange is sucked onto the cavity most of the time, and it takes a lot of force to break it off. This is not a standard operation and can easily cause injury to personnel or equipment.
2. If the flange is not removed through Vent, the air inhaled is often at normal pressure. Dust and water vapor in the air are additional sources of pollution. Even due to the huge suction force at the moment of flange removal, many solid fragments will be sucked into the vacuum cavity, becoming a potential source of pollution.