Trying to select the best turbo pump for your application can be a difficult and sometimes frustrating exercise. The questions range from pumping speed to bearing type and mistakes here can cost not only money but also the performance you need. This is a very basic guide to selecting a turbo pump but for more information it is wise to discuss your application with the pump manufacturer.

 If you are trying to replace a turbo pump on an existing piece of equipment you should contact the instrument manufacturer or the turbo pump manufacturer. This will ensure that you replace your old pump with one that is compatible with your instrument because in addition to the pump performance changes, it is common for the turbo pump to communicate with the instrument via analog or serial methods. If the wrong pump is used this could disrupt these communications and prevent the proper operation of your instrument.

With a locally built system you have a bit more flexibility on selecting a pump that will meet your needs. Here are some items to consider when selecting a turbo pump for your system:

1. What size inlet port should I get on my turbo?
   It is a common practice to match the inlet size of your turbo to the size of the port on your chamber. This will eliminate the need for adapters which can restrict conductance and increase the number of possible leak points. Having a turbo pump with a larger inlet than your chambers will typically result in a higher cost without realizing any significant benefit in effective pumping speed.

2. What type of flange, NW, ISO or ConFlat?
   There are two basic inlet seal types, Elastomer oring seals and Metal seals. If selecting a turbo for an existing vacuum chamber you should once again match the pump flange to the chamber flange to eliminate the need for adapters. If you are building a custom chamber, the type of inlet should be based on the vacuum level that you need to achieve in order to run your specific application. If your application requires vacuum levels between atmospheric pressure and 10-7mbar an elastomer seal is recommended for cost and ease of use. Metal seals are recommended for UHV applications better than 10-7mbar and when you are using bakeout. The metal seals have better outgassing characteristics and are leak-tight to less than 10-10mbarls-1 but the metal seals should not be re-used.

3. Required pumping speed.
   To determine you pumping speed requirements you will need to understand the outgassing characteristics of your chamber, amount of gas generated by your process and your required chamber pump-down time. With this information the pump manufacturer can help you determine the pumping speed you will require.

4. Compression ratios.
   Some turbo pumps are better than others at pumping light gases such as helium and hydrogen. If your application requires the use of these light gases you should select a turbo pump that has enough compression to move these light gases from the chamber to the turbo exhaust.

5. Backing pump requirements.
   The backing pump should have a high enough capacity to handle the gas throughput from your process while maintaining sufficient vacuum level to back your turbo pump. Turbo pumps without a drag stage typically need backing pressures better than 0.1mbar while compound turbos with molecular drag stages can usually operate at pressures above 2mbar.

6. Ultimate pressure requirements.
   Most turbo pumps will have their ultimate listed in their product literature but note that this specification is based on stringent test standards such as AVS or Pnuerop. These test standards call for specific size chambers, outgassing rates, backing pressures, gauging and baking time/temperature. Because of this fact the ultimate pressures in the manufacturer’s documentation should be used as a guide to assist in selecting the correct pump.

7. Should I use a ceramic bearing pump or a magnetically levitated turbo?
   Ceramic bearing or hybrid magnetic/ceramic bearing pumps may be used for the majority of applications. Fully magnetically levitated turbo pumps are an option in applications where minimal maintenance is required as there are no bearings to subject to wear.
   
   In corrosive environments ceramic bearing and hybrid magnetic/ceramics bearing pumps should be protected with a nitrogen purge to prevent decomposition of the bearing lubricant by the corrosives. Additionally the internal surfaces of the pump may be treated with an inert coating.
   
   There are some harsh environments which require the turbo pump to be heated and in these cases a fully magnetically levitated turbo is an excellent choice as there are no mechanical bearings. Heating mechanical bearings accelerates lubricant decomposition and increases wear bearing wear rates which can lead to premature failure.
   
   Fully magnetically levitated pumps are also better suited to applications, such as electron microscopy, where lower vibration levels are required. The magnetic bearing system will typically exhibit a lower vibration signature than mechanical bearing systems. Magnetic bearings also provide a constant vibration signature that does not change over time allowing system designers to effectively damp the frequencies of concern. Ceramic bearing or hybrid magnetic/ceramic bearing pumps have a vibration signature that changes over time as the mechanical bearing wears and this makes damping challenging and in some cases impossible.