Reference Manual
00809-0100-4841, Rev. AA
May 2007
Rosemount 3490 Series
4-28
Standard 4-20mA input (Figure 4-15 on page 4-15):
P200 (PV units) must have been changed from "%" to the required units of
measurement.
The transmitter's 4-20mA output should be scaled to give a 4-20mA signal
over the full level range expected in the flow channel. If this is the case,
parameter P114 may be left at the default value of 1.0.
If the transmitter's 4-20mA output is not scaled over the full level range of the
liquid in the channel, P114 must be used to re-scale the signal ready for input
to the NLP calculation.
For example, if the maximum current input is below 20mA for the maximum
height of liquid in the channel, say 12mA, re-scale via P114:
P114 = (Current span Actual current span) = 16 (12-4) = 2.0
Refer now to the Section Non-linear Profile (NLP) calculation.
HART digital input (Figure 4-17 on page 4-21):
P200 (PV units) must be set to the required units of measurement.
The maximum value of level from the HART transmitter must be equal to the
maximum height of the liquid in the channel.
The level measurement must be re-scaled to the range 0 - 1.0 ready for input
to the NLP calculation. For example, if the level measurement range is
0 - 1.5m, parameter P114 = (1.0 1.5) = 0.667
Refer now to the Section Non-linear Profile (NLP) calculation.
Non-linear Profile (NLP) calculation
With P113 set to the NLP required and P114 correctly calculated, P115 will
have been automatically programmed with the correct NLP.
P116 is always programmed with the maximum flow expected in the channel,
which will occur at the maximum liquid level in the channel.
The PV value (D800) for the flow rate is derived by applying the normalised
transmitter input (range 0 to 1) to the profile, and then scaling by parameters
P114 and P116.
Figure 4-22 shows a typical open channel flow structure.
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