Page 36 ST 800 SmartLine Pressure Transmitters User’s Manual Revision 10
1. Linear scaling of the displayed PV value
When “Linear” is selected for PV Scaling, the Display will scale the selected PV input
according to the following formula:
((PV value – input low limit) / input span) x (Scaling High – Scaling Low) + Scaling Low
If the PV Selection is Pressure, the input low and high limits are the LRV and URV
respectively. If the PV Selection is Percent Output, the input low and high limits are 0
and 100%. If the PV Selection is Square Root, the input low and high limits are 0 and
100 %Flow.
Note that this scaling only affects the value shown on the Display; it does not affect the
Loop Output.
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2. Square Root scaling of the displayed PV value
When “Square Root” is selected for PV Scaling, the Display computes %Flow from the
Differential Pressure. This calculation is independent of the Transfer Function setting in
the transmitter. This allows the user to output Differential Pressure via the 4-20 mA
loop output while displaying the equivalent flow value on the Display. In addition, the
Display value can be scaled to show the flow in flow units (gal/min, gal/h, etc.) by
entering the correct scaling limits via the Scaling Low and Scaling High parameters.
For example:
PV Selection: Differential Pressure
PV Scaling: Square Root
Scaling Low: 0.0
Scaling High: 2500.0
Display Units: gal/h
LRV: 0.0
URV: set as required by the process
The Display will calculate 0-100 %Flow from the Differential Pressure and then scale this
to 0 to 2500 gal/h.
Note that the Square Root calculation is referenced to the LRV and URV settings of the
transmitter and it uses the LRV and URV to calculate the %DP input into the Square Root
flow algorithm. For normal flow applications, it is assumed that the LRV is set to zero and
that zero pressure equals zero flow. If the LRV is less than zero, the Square Root calculation
will calculate the flow as bi-directional flow.
3. PILD Factor value Display
PILD Factor can be configured as screen PV selection when the PILD option is enabled.
It will not display units and square root indication while displaying in large, trend or bar
formats.
The large PV format provides the current Up Limit and Down Limit values. The PILD
Factor value is present only when PILD is enabled and while in Monitoring mode. Otherwise
the value will be replaced by a message indicating PILD is disabled or in Training mode.
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