7
5. PROGRAMMING & CALIBRATION
The BA558C is programmed and calibrated via
four front panel push-buttons. All the programming
functions are contained in an easy to use menu
which is shown diagrammatically in Figs 7A and
7B. Each function is summarised in section 5.3
which includes references to more detailed
information. Although this simple menu driven
system enables most adjustments to be made
without repeated reference to this manual, it is
recommended that at least the summary of the
programmable functions in section 5.3 is read
before attempting programming or recalibration.
When the instrument is fitted with alarms, the basic
menu is expanded to include the alarm functions
that are described in section 8.3 of this manual.
Note:
While the instrument is being programmed
or calibrated totalisation is suspended, any flow
occurring during this time will not be recorded.
5.1 Calibration structure
Fig 6 shows the BA558C calibration structure. The
4/20mA input current, which in flow applications
represents flow rate, is displayed on the five digit
rate display. Using the instrument 'CAL' or 'SEt'
functions this display may be calibrated to show
the flow rate in engineering units. Both the zero
(display at 4mA) and the span (display at 20mA)
are adjustable, although for flow application the
display at 4mA is usually zero.
The position of the decimal point in the rate display
is defined by the d.p. function. To simplify
calibration the BA558C uses floating point
arithmetic, moving the rate display decimal point
position will therefore affect totalisation.
The BA558C total display is calculated from the
rate display using two serial factors: the timebase
't-bASE' and the total scaling factor 'SCALE-t'. To
calculate total flow the timebase divides the rate
display according to whether the rate display
represents flow per second, per minute or per
hour. The total scaling factor 'SCALE-t' is a wide
range programmable dividing factor allowing the
total display to be in different units from the rate
display.
For example, if a BA558C is displaying rate in litres
per minute, setting 'SCALE-t' to 4.5461 produces a
total display in UK gallons. Alternatively, setting
'SCALE-t' to 4546.1 will produce a total display in
thousands of UK gallons.
Fig 6 Calibration structure
The total is calculated and the display updated
once per second. When total alarms are used for
batching applications, this resolution may define
the accuracy of the system.
