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Combining Conductances

Since a component's conductance in molecular flow is independent of pressure and is quoted as a volumetric flow, conductances for various components can be combined in series or parallel. If two chambers are connected together by:

  • a narrow tube on chamber 1
  • a right angle valve
  • a large port on chamber 2
their separate conductances can be combined as reciprocals to give a total conductance between the two chambers.

Notice that the total conductance is much less than any individual conductance. In addition, look at table below. Here, just two conductances, one variable and the other fixed at 10 L/s, are added together. The Total Conductance column demonstrates a critical rule in series conductances – the smallest conductance rules.

Alternatively, if two chambers are connected by two tubes of different diameters, each tube has its own conductance. To determine the total conductance between chambers, simply add the conductances together.

Conductance
C1
Conductance
C2
Total Conductance
1/(1/C1 + 1/C2)
10 10 5 L/sec
10 100 9.1 L/sec
10 1000 9.9 L/sec
10 10,000 9.99 L/sec
10 100,000 9.999 L/sec
10 1,000,000 9.9999 L/sec

Series Conductances

Series conductances are added as reciprocals:

1/C total = 1/C1 + 1/C2 + 1/C3

Given:
Narrow Tube — 120 L/s (C1)
Angle Valve — 230 L/s (C2)
Large Port — 1,400 L/s (C3)

The total conductance is:
1/C total = 1/120 + 1/230 + 1/1,400
1/C total = 0.0083 + 0.0043 + 0.0007
1/C total = 0.01339
1/C total = 1/0.01339

Total Series Conductance = 74.6 L/s

Parallel Conductances

Using two conductances simultaneously between two chambers or between a chamber and pump is not common but such arrangements do occur and are easily calculated. Suppose the two tubes have conductances of 1,800 L/s and 2,300 L/s.

The total conductance is:
C total = C1 + C2
C total = 1,800 + 2,300

Total Series Conductance = 4,100 L/s

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