TSI Instruments AccuBalance 8370, Operation And Service Manual

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Appendix B
Back Pressure
It is commonly known that a flow measuring 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 flow measuring 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 A
CCU
B
ALANCE
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 15 shows the pressure drop through the A
CCU
B
ALANCE
over its
entire flow range. As shown in the figure, the back pressure is only
0.05 in. H2O at a flow rate as high as 1000 CFM.