Technical Manual
Page 3
CODEL
OPS.109
Issue : B
Rev. : 1
Date : 23/10/13
Doc. i/d : 0109/6
Ref. : 090028
2. Measurement Principle
Gas flow is rarely laminar. Turbulence in the flow produces a series of swirling eddies and vortices that are
transported with the bulk flow. Infrared radiation, emitted by a hot gas system, is characterised by a flickering
signal resulting from the swirling effect of these vortices. Two infrared detectors, placed a small distance apart,
will produce very similar flickering signals, but with a displacement in time equivalent to the time taken for the
bulk gas flow to carry the vortices from the first detector to the second.
The V-CEM5100 uses a cross-correlation technique to measure this time displacement and hence the flow. The
two signals from the infrared transducer units are defined as A(t) and B(t) as shown below.
The time-of-flight (and hence the flow velocity) of the naturally occurring turbulent eddies within the flow stream
can be determined by cross-correlating the two signals as shown in the following equation:
R
BA
LIM
.
T
1
T
B t
0
T
A t
dt
where
is a variable time delay imposed on the signal A(t). Using this function a correlogram can be computed
which has a maximum when the time-of-
flight and ‘t’ are equal. This can best be explained by considering the
two signals A(t) and B(t) as shown below.
Rx1
(Ch A)
Rx2
(Ch B)