Output of PV vs Flow
One DCS supplier refers to the DCS output indication as implied valve position. Lacking a positioner on the valve there would be little chance that the relationship between the output indication and the actual valve travel could reflect the flow rate. If the control valve has a positioner adjusted with a linear proportional relationship between the input signal and output position - then you might be able to calculate the flow rate within about 10% - perhaps.
Consider adding a nitrogen flow meter. Check to see if GE Panametrics or Siemens Controlotron will rent a clampon ultrasonic meter for a two-week application trial. If successful, buy an ultrasonic meter and wire the signal to a spare pair within the existing junction box. You have to be lucky for spare pairs to exist, etc.
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