Yokogawa 414 Batch Controller - Manual (Page 21)

3.3.2 The Cutoff Point
Because many transducers do not always exactly transmit 4mA (0mA, 1V or 0V)
when they are at zero rate, it is often necessary to define a rate below which no
integration takes place. This is termed the cutoff point and is programmed as a
percentage of the Span, S.
For example, if S = 2200 kg/min with an offset of 100 kg/min in a square law
system, and the cutoff point is set at 20.0%, the actual cutoff rate Rc can be
determined as follows:
The cutoff rate is defined as:
Rc
=
2200 A
+
100
At 20% cutoff:
Rc
=
2200 x 0.2
+
100
The value of A which would produce this cutoff is:
A
=
0.04
(
since 0.04
=
0.2
)
and the input signal would be:
I
=
16mA x 0.04
+
4mA
Operation 19
=
540 kg/ min
=
4.64mA