Revision 14
29 August 02, 2019
2.8.4 Eductor
The eductor produces a positive pressure at the exhaust while allowing the analyzer vent to
remain at ambient pressure. An eductor is shown in Figure 2-13.
Figure 2-13: Eductor
The eductor has two purposes:
• Changes in ambient pressure can affect the results produced by the system. If the system
is inside a building where there is a fluctuating static pressure in comparison to the outside
ambient pressure, the eductor will eliminate any influence on the tape staining. Static
pressure changes inside the building are normally caused by building vent fans coming on
and off which will change the differential pressure across the sample chamber. Pressure
can also be affected by a strong wind blowing across the vent. When this happens, a slight
change in the pressure of the sample between the tape and the sample chamber occurs.
The slight change in differential pressure across the tape can cause a reduction in the seal
between the tape and the sample chamber, resulting in poorly shaped stains that give poor
results. It can also cause more air to be introduced into the sample chamber, diluting the
sample gas and producing lower-than-expected results.
• If the system is located in a cold climate, the humidified sample gas passing out of the vent
can freeze, blocking off the flow through the sample system. By using a eductor, the
residence time of the sample gas within the system is reduced, making it less likely that the
moist air will freeze inside the analyzer.
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