11
When using the internal calibrator, the following
procedure should be followed. The adjustments
may be performed with any input current between
4 and 20mA.
Step 1
Put the suppression / elevation, zero,
span and decimal point links in the
required position. Put the root / linear
link in the root position.
Step 2
Put the calibrator link in the 4mA
position and adjust the indicator zero
potentiometer to give the required
display at 4mA.
Step 3
Put the calibrator link in the 20mA
position and adjust the indicator span
potentiometer to give the required
display at 20mA.
Step 4
Repeat steps 2 and 3 until both
calibration points are correct. The
span and zero controls are almost
independent so it should only be
necessary to repeat each adjustment
twice.
Step 5
Return the calibrator link to the
'indicate' position. The indicator will
now respond to the 4/20mA input
current with the revised calibration.
8.3 Root extractor
The BA304ND can be supplied with a square root
extractor which enables the indicator to accurately
display the output from a differential flow meter in
linear engineering units between 10 and 100% of
full flow (4.16 to 20mA). The lineariser continues
to operate with reduced accuracy down to 2.5% of
maximum flow, alternatively clip-off can be
selected which will force the display to zero at
flows below 5% (4.04mA). The location of the clip-
off link is shown in Fig 7. Access to the link is
gained by carefully removing the indicator
assembly from the enclosure as follows. Remove
the terminal cover by unscrewing the two ‘A’
screws which will reveal two concealed ‘D’ screws.
Unscrew the four ‘D’ screws and carefully lift off
the front of the instrument enclosure. Fig 4 shows
the location of these screws. Finally remove
screws securing the electronic assembly.
Fig 7 Location of root extractor clip-off link
When calibrating a BA304ND fitted with a root
extractor the indicator zero potentiometer should
be adjusted to give the required display at 10% of
flow (4.16mA). The indicator zero potentiometer
should not be used to set the display to zero with a
4mA input. Zero suppression or elevation may not
be used, i.e. 4mA must correspond to zero flow.
For reference, the following table shows the output
current from a non-linearised differential flowmeter.
% of full flow
Current output mA
2.5
4.01
10.0
4.16
25.0
5.00
50.0
8.00
75.0
13.00
100.0
20.00
A root extractor can not be fitted to an instrument
with an internal calibrator.
8.3.1 Calibration example with root extractor
The BA304ND is required to display rate of flow in
gallons per minute, with a resolution of 0.1 gallons.
The differential flowmeter has an output of 20mA
at a flow rate of 140.0 gallons per minute
