50
7.3 Correction for Test Pressure Variation
The following describes how to correct flow numbers when a test pressure other than the calibration
pressure is used. SuperFlow benches require this correction which amounts to about 1% error in flow when
the test pressure varies considerably from the calibration pressure.
In the figure below, the flowbench is drawn as a box with an upper test plate and a lower (internal) flow
measurement orifice plate. On the left, the bench is drawing volume Q
0
through the test plate. Because the
air pressure is lower inside the plenum, the volume Q
1
through the flow orifice is larger than Q
0
.
Figure 7.1: Two Different Test Pressures
On the right side, the test pressure was changed to TP
2
which causes a different flow (Q
2
) to flow through
the orifice plate and a pressure (Pf
2
) across the flow orifice. Note that the flow Q
0
is the same flow as on the
left side of the diagram which means the test orifice was adjusted so the same flow Q
0
goes through the
test plate at test pressure TP
2
.
First, we derive the measured flow for TP
1
and then for TP
2
. These two readings will differ, and they should
be the same because Q
0
is the same flow in both cases. The ratio between the readings is a correction
factor that can be used to correct flow readings at test pressures other than TP
1
. Most SuperFlow benches
are calibrated at a test pressure of 25 inches of water.
Flow Measurement at TP
1
First, note that Q
0
and Q
1
differ even though both flows are composed of the same airstream. Assuming
the temperature of both streams is the same (which it is as long as the airstream velocity is brought to near
zero before it passes through the flow orifice), then the two flows are related this way:
Equation 1:
=
Q
0
X
P
0
Q
1
X
P
1
=
P
1
P
0
-
TP
1
=
Q
1
Q
0
P
0
P
0
-
TP
1
Where
P
= the pressure of the respective stream.
The flow through an orifice is a function of the pressure drop across the orifice and the density of the air.
For our purposes (because temperature doesn't change), we can substitute the upstream pressure for
density. All other factors such as coefficient of discharge are wrapped up in one constant—K
f
.
7.0 Flowbench Theory
