Yokogawa 415 Steam and Gas - Manual (Page 30)

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.
Mass Flow
......(18) Q
M
= S
M
.
ν
B
ν
F
. A
Energy Flow
Equations 13 & 14 are used.
Note that the Specific Weight (density) is 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 the
steam density. Therefore, the gas equations relating to differential
pressure must 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 flowmeter.
Equations 17 or 18 would be used depending on whether the D. P.
transmitters have square root extractors. Separate spans are then
programmed for each transmitter and at lower flowrates transmitter 2 will
be used. At higher flowrates, where the output of transmitter 2 exceeds
20mA, transmitter 1 will be used.
28 Operation