27
PROGRAMMING cont.
20
16
12
8
4
0
25
50
100
Flow Rate (GPM)
Output
Current
(mA)
The graph also shows the effect of signal offset. The output current is the sum of the
output signal and 4 mA of offset current. At zero flow rate, the output current is 4mA of
offset, and 0 mA of signal. At 25 GPM, the output current is 4 mA of offset and 4 mA of
signal (8 mA). When the flow rate doubles to 50 GPM, the output signal doubles to 8
mA, and the output current goes to 12 mA. When an output signal is linear (the graph
is a straight line), the linearization rate mode should be selected in block o1, and the
number of points selected in block o4 should be two. When linearization is selected
in o1, blocks o2 and o3 do not appear.
Some flowmeters will put out signals that are somewhat linear, but not quite. For
example, a flowmeter may put out a current of 4-20 mA over a flow rate range of 0-
100 GPM, but may have a graph that looks like this:
20
16
12
8
4
0
25
50
150
Flow Rate (GPM)
Output
Current
(mA)
9.5
13.4
This graph is not a straight line. The signal output at 25 GPM is 5.5 mA, or 1.5 mA
greater than the signal output for a linear flowmeter. This non-linearity must be
accounted for or there will be significant error in flow rate and totalization readings.
If you were to draw straight lines from one point on the curve to the next point, then to
the next point, then to the last point, the straight line between points would be very
close to the curve between points. This is what the Eclipse does when linearization
