Yokogawa 415 Steam and Gas - Manual (Page 37)

3.5.2 Analog Input Linearity Correction
For single analog flow inputs, an input table can be programmed to correct for
any non-linearities between the flow signal and the actual flowrate. This feature
is very useful when using the flow computer with some types of VA flowmeters
or laminar flow tubes which may exhibit slight non-linear characteristics.
Up to 20 points can be programmed, and linear interpolation is then used
between points in the curve. The table works by inputting a normalised flow
input signal A, into the table and produces a corrected output A
C
. The table
ranges between 0 and 1.0000 so that, at a 4mA input, both A and A
C
equal 0 and
at 20mA both values must also equal 1.0000.
The user programs the table starting with 1.0000 and programs corresponding
values of A and A
C
. As soon as A is programmed as 0.0000, no further input to
the table is possible and the program will correct over the number points that
were programmed. A maximum of 20 points can be programmed.
At the base temperature and pressure, the flow equation, with non-linearity
correction is defined as:
Q = Span . A
C
Note: The square root relationship for conventional differential pressure
flow devices is handled separately and not by the linearity
correction described in this section.
Operation 35