7. LINEARISER
A sixteen segment, seventeen break-point (0 to 16)
lineariser may be selected in the ‘FunC’ section of
the configuration menu. The position of each
break-point is fully adjustable so that the slope of
the straight line between break-points can be set to
compensate for a non-linear flow meter. Each
break-point must occur at a current greater than
the preceding break-point and less than the
following break-point, in the range 3.8 to 21.0mA.
If this requirement is not observed when
configuring the lineariser the Rate Totaliser will
display 'FaiL' and the configuration adjustment
which produced the error message will be ignored.
Fig 9 shows a typical linearised Rate totaliser
characteristic.
Fig 9 shows a typical linearising characteristic
Selecting ‘Lin’ or 'bi-Lin' in the ‘FunC’ section of the
configuration menu activates the lineariser, this
does not change the configuration menu shown in
Fig 9, but the 'CAL' and 'SEt' functions are
extended as shown in Fig 10. As with a linear
Rate Totaliser, calibration may be performed with
an external current source using the 'CAL' function,
or with the internal reference using the 'SEt'
function.
The lineariser calibration is retained irrespective of
how the Rate Totaliser function ‘FunC’ is
subsequently changed. It is therefore possible to
select and deselect the lineariser without having to
reconfigure it each time.
The lineariser calibration may be reset to the
factory default settings without changing the Rate
Totaliser configure using the 'LtAb' function
described in section 6.19.
7.1 Lineariser calibration using an external
current source.
This method allows direct calibration of the
lineariser with an external current source and is the
preferred method when traceability is required. If
the exact system non-linearity is unknown, this
method also allows direct calibration from the
variable to be displayed.
The number of break-point required should first be
entered using the 'Add' and 'dEL' functions. In
both these sub-functions the Rate Totaliser initially
displays the current break-point and the total
number of break-points being used as shown
below.
Display
Description of function
'Add'
Add a break-point
Adds a new break-point before the
displayed break-point. The calibration
of existing break-points is not changed,
but the identification number of all
subsequent break-points is increased
by one.
'dEL'
Remove a break-point
Removes the displayed break-point and
joins the preceding break-point to the
following break-point with a straight
line. The identification number of all
subsequent break-points is decreased
by one.
To add a break-point use the ▲ or ▼ button to
select 'CAL' from the configuration menu and press
P
which will result in the
'Add' sub-function prompt
being displayed. To enter the sub-function press
P
which will reveal the current break-point and the
total number of break-points which have already
been entered. When adding a break-point the
insertion position can be selected using the ▲ and
▼ push buttons followed by
P
push button to
insert the additional break-point. In previously
uncalibrated linearisers each new break-point
should be added in front of the highest existing
break-point, if this sequence is not followed a 'FaiL'
message will occur in the 'Pt5' function. See
example in section 7.1.1.
21
