Section 4
Page 4 - 18
MODEL 346 INSTALLATION AND OPERATION MANUAL
HA243821
Issue 5 Mar 98
4.9 INSTRUMENT CONFIGURATION (Cont.)
4.9.11 CUSTOMER LINEARISATION TABLE CONFIGURATION
Custom Curve
The Custom Curve is an 11-point, user-entered linearisation. It is used as one of the linearisation functions
(described in Section 4.8) to define the relationship between the input signal and the recorded/displayed value.
When this option is present, ‘Custom Curve ?’ appears in the selection list for the channel Linearisation Function.
The ENTER key is used to access the Custom Curve for review or edit.
CUSTOM CURVE DESCRIPTION
The Custom Curve combines the flexibility of a line/segment curve with the curve matching power of a polynomial
equation. The curve is defined in five segments, each segment consisting of three point-pairs which are used to
calculate a second order polynomial equation for that segment. Segment one is defined by point pairs 1, 2 and
3, segment 2 by point pairs: 3, 4 and 5, segment 3 by point pairs: 5, 6 and 7, and so on. Adjacent segments
share common end points to ensure continuity.
The curve is defined by entering unique point-pair values for all five segments. For proper operation of the curve,
all eleven point pairs must be entered.
The first step when using a Custom Curve is to determine the Input and Units values that define it. If the curve is
a series of straight lines (such as a tank profile), fewer than five segments may be adequate to define the area to
be used.
For Custom Thermocouple curves, a three point Cold Junction Compensation curve must also be entered.
Note...
If any of the 11 Point Pairs are not needed to define the Custom Curve, each pair’s signal and unit values must
be increasingly greater than the largest signal and unit values which are included in those unique point-pairs
which are used (see point pairs eight to 11 in the example, below).
EXAMPLE
The volume of fluid in a tank is to be recorded by using a pressure transmitter mounted at the bottom of the tank.
The pressure signal vs. volume is defined by a curve made up of two non-linear segments and one straight line
segment, as shown below.
Note...
Because the Custom Curve can be selected as the linearisation function for any channel the Point Pairs carry
no range, units-of-measure or input units. When the custom curve is configured, all values in the signal input
and range low/high units have to be entered for the channel which is to use the curve.
CUSTOMER CURVE DECIMAL POINT POSITION: SIGNAL
Signal D P Pos.
X.XXXX
This decimal position display is for the signal input values. Selection of the decimal position is by using the up/
down arrow keys (see section 4.8.3 for possible settings).
