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7.6 Air Density Correction and SuperFlow Flowbenches
There is no air density correction required to adjust for different atmospheric conditions when testing.
This is the concept that causes concern for many people. Doesn’t the flow through an orifice change with
changing air density? Yes, it does. Well, why doesn’t the flow bench perform a calculation to compensate
for the changing air density? Read on to understand why there is no correction, and why this is appropriate
to do.
The simple answer to this is that our flow benches are ratiometric measuring devices. In other words, the
density of the air in the room when you perform a test does affect the flow, but it affects both the test article
as well as the orifice plate internal to the flow bench by the same amount and this effect is automatically
accounted for in the way that the flow measurement is calculated. If the air density causes the mass flow to
go up by 10%, then the flow bench simply flows more, but it still measures the flow as if the air density was
at 60°F and 29.92 inches of Hg. This correction is not done in software, instead it is a natural consequence
of the mechanics of the flowbench. The flowbench only looks at pressures across the test article and
across the orifice internal to the bench. If you apply 10% more differential pressure across the test article,
then there will be the same 10% increase in differential pressure across the orifice plate. If the density of
the air changes and therefore the differential pressure across the test article changes, then the differential
pressure across the internal orifice changes a similar amount. If the differential pressure at the test article
goes up a certain amount, then the FlowCom will adjust the blower speed to bring the test article differential
pressure back to the original set point (usually 25” H20), which will then bring the orifice plate differential
pressure back to its earlier reading as well.
Technically speaking, the flow bench does not measure flow, it measures differential pressure across the
internal orifice and then calculates the flow as if the flow bench was in a room full of 60°F and 29.92” Hg air
with the same differential pressure.
You would be correct in your thinking that the flow would change as a result of the density change, but
the key point here is that the flow bench does not measure flow, it measures the pressure across the
orifice internal to the bench, and the pressure across the orifice changes in proportion to the change in
air density such that the calculated flow number is constant. If you really want to investigate this using
flow calculations, then a good reference is the ASME (American Society for Mechanical Engineer) MFC-
3M-2004 publication. According to equation 2-1 in this publication, the flow through a sharp edged orifice is:
Qm = 0.09970190
X
C
X
Y
X
d
2
X
h
w
x
p
1
-
B
4
Where:
Qm = mass flow in lbm/sec
C = Discharge Coefficient
Y = Expansibility Factor
d = throat diameter of orifice in inches
h
w
= Differential Pressure across orifice in inches of water
p = upstream air density in lbm/ft^3
B = Beta (throat diameter ratio of orifice to tube).
7.0 Flowbench Theory
