25
Appendix A
Back Pressure
It is commonly known that an air capture hood may induce a back
pressure. In general, back pressures are caused by restrictions in the
flow path as well as frictional pressure losses. In order to improve
accuracy and sensitivity, all air capture hoods incorporate a contracted
flow section. The contracted section restricts the flow through the hood
which induces a back pressure. A hood having a more abrupt contraction
than the AccuBalance
®
capture hood may induce greater back pressure
due to the turbulence caused by the abrupt contraction. In addition any
apparatus within the hood, such as supporting poles and the flow
manifold, also induces some frictional pressure drop.
Back pressure may cause slight errors in the flow measurements. Figure
11 shows the pressure drop through the AccuBalance
®
capture hood
over its entire flow range. As shown in the Figure, the back pressure is
only 0.05 in. H
2
O at a flow rate as high as 1000 CFM.
Figure 11: Pressure Drop through the
AccuBalance
®
Air Capture Hood
