Everyone working with a vacuum system will at some time in the career ask themselves the question: “Why is my vacuum system pressure not as low as I think it should be?” Let’s start with the obvious:-
Pump down to the 10-6 mbar range (or as low as you can) and close the main valve. Watch the pressure rise. If the pressure rise is linear you have an air leak. If you have an air leak use a leak detector to find out where the leak is situated. Usually these are located on flanges (‘O’ rings), lid seals, or moving parts, seals, and feedthroughs. If your pressure rise is not linear but the pressure rise becomes slower after a while you have a degassing problem. If this is the case try to find the degassing material. Very often these are polymer materials (e.g. PTFE) which do not belong in a vacuum system. You could also have a virtual leak. Then you have air pockets in narrow parts of the system (e.g. if you use screws in the system which do not have a hole in the middle). Also clamped or screwed surfaces can hold air pockets which are difficult to remove.
2. Pumping speed
If you haven't found leaks then there is the possibility your pumping speed has become lower over time. If you are just using a rotary pump then this might need maintenance since they are mechanical and wear out. Also an oil change can help if the oil is old and contaminated. If you use a cryo pump check when the last time regeneration was done. Also cryo's need maintenance since the 10K shield needs new absorption materials (charcoal) every 1-2 years. If you use a turbo pump there is very little chance something will change in the pumping speed, unless you use an older pump with oil lubrication of the bearing. Change the oil and make sure the oil level is OK.
A simple, but often overlooked problem. If you block the pumping opening of the chamber with a table or shielding, pumping speed will drastically go down because you influence the conductance. If you come down in the molecular regime of pumping (around 10E-5 to 10E-6) molecules will have difficulty finding the pumping hole due to the shielding. Make sure you always have direct line of sight of the pumping hole to the vacuum, this also applies for cryo's.
4. Leaking gas valves
Occasionally gas valves (usually pneumatically driven) might be leaking. In this case your vacuum system is OK but you have an internal leak. To check for this empty the gas lines and see if this has impact on the pressure. A Residual Gas Analyser (RGA) is a great diagnostic tool for identifying gas species and troubleshooting the source of poor vacuum.
Outgassing of materials within your vacuum system is always present. You can minimise these problems by careful choice of what goes in the vacuum system, cleanliness, and use of heating. Use isopropanol for cleaning, not acetone, and when you have performed all of the above checks to ensure that there are no leaks then you can heat your system to speed up the outgassing. Water is the principle source of slow pump-down times as it is adsorbed into the walls of the chamber etc. You can speed up pumping by heating the chamber lids and wall with heating pads before you close the system. It can be a good idea to heat up the system before you close it, water levels become low and pump-down time will be faster. Keep the heating pads on while pumping down the system. A vacuum bakeout is recommended for UHV conditions – say a minimum of 150°C for 12 hours. To keep your vacuum system clean and ready for quick pump-down always back-fill the chamber with dry nitrogen when letting up to atmospheric pressure.