Cascade Slave Flow Setpoint/PV = 0 but valve is still open ~1.5-2%

Hello, I have had this problem in numerous configurations for years, but decided today was a good day to ask the experts.

I have a d/p flow meter that is a CAS slave (0-15 MSCFH).  The CAS master is an analyzer that runs above setpoint 99% of the time calling for a 0 MSCFH output.  What happens is the turndown on the d/p allows me to read/control to ~ .25 MSCFH, but then drops to zero.  So my tuning set says "hey we are good at setpoint" and the vent valve position stays open ~.75 to 2.5% when I really want it to be on the seat completely closed. I know I could enable track & drive the output to zero if the flow is below .25 MSCFH, but then I would be in IMAN/LO.  Just looking for expertise, suggestions or recommendations.  Thanks!

  • Try setting the master OUT_LO_LIM and slave SP_LO_LIM to slightly negative values, the slave setpoint should then be slightly lower than the slave PV driving the valve to 0%.

    I believe DeltaV allows up to -10% of scale for these limits.