Throughput and Pumping Speed

A different measurement of gas transport is the volumetric flow rate (S), which is also called the pumping speed. The units of volumetric flow rate are simply vol-ume/time, such as liters per second. The difference between Q and S is that S is independent of the quality of the vacuum. Throughput and volumetric flow are related to each other by the pressure P and are expressed in Eq. (7.7). The relationship of mass flow, volumetric flow, and pressure is demonstrated in Fig. 7.9. 

In a typical vacuum system, tubes are connected to tubes, which are connected to traps, which are connected to other tubes, and so forth. The transport of gas through a system of these tubes is called conductance (C). Every tube within the system offers a different conductance depending on its length and diameter. 

Conductance is the throughput (Q) divided by the difference between the pressure going into the tube (Pj) and the pressure going out of the tube (P2) [see Eq. (7.8)]. Although conductance and volumetric flow rates have the same units, conductance is used to describe the ability, or efficiency, of a gas flowing through a 

Vacuum systems are generally not made with one tube and one easy way to calculate conductance. Rather, vacuum systems are constructed with many tubes of different sizes. The total conductance of an entire system is calculated by adding all the reciprocals of the conductance of the various parts [see Fig. 7.10)] and is shown in Eq. (7.9)  

What can be seen from Eq. (7.9) and Fig. 7.10 is that by placing a stopcock mid-length on a tube will minimally effect conductance, especially if the length of the stopcock section is as short as possible and the size of the stopcock is as large as possible. Common sense, economy, and experience are good guides for this. 

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