Yokogawa 415 Steam and Gas - Manual (Page 20)

D. Differential Pressure Flowmeters With 4-20mA Output And With A
Linear Flow Relationship.
eg. D.P. transmitters with a square root extractor or VA meters.
......(8) Q
VB
= S
VB
.
P
F
P
B
.
T
B
T
F
.
Z
B
Z
F
. A
Q
M
=
ρ
B
. Q
VB
Note that the pressure and temperature are still square rooted, even though
the flow signal A is not. This is because the output from the D. P.
transmitter is not truly volumetric, but will be affected by a change in
density of the gas being measured. Therefore, the equations relating to
differential pressure will apply.
E. Dual Differential Pressure Flowmeters With 4-20mA Output.
To increase the range over which flow can be measured, two D. P.
transmitters with different spans can be connected across a common
orifice or other differential pressure device.
Equations 5 & 6 or 7 & 8 above would be used depending upon whether
the D. P. transmitters have square root extractors. Separate scaling using
these equations is then programmed for each transmitter.
At lower flowrates, transmitter 2 will be used as a basis of measurement
and at higher flowrates, transmitter 1 will be used. The crossover point
will occur when the input on transmitter 2 exceeds 20mA.
18 Operation