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Airflow readings drop out to zero when reading low amounts of flow on Xenta 102-AX

Airflow readings drop out to zero when reading low amounts of flow on Xenta 102-AX

Issue

nvoBoxFlow or nvoAirFlow drops down to zero or flutters back and forth from zero to the actual flow value when trying to maintain a low amount of airflow.

Product Line

TAC Vista

Environment

  • Vista Workstation
  • Xenta 102-AX
  • Xenta 102-AX Plug-in

Cause

The Xenta 102-AX application filters out any velocity pressure readings below a certain threshold in order to prevent nuisance readings that may adversely affect the operation of the controller.

Resolution

There is a parameter named UCPTpressCutOff that determines at what pressure level should readings be seen as valid and at below what pressure level should readings be rounded to zero. The default setting for this parameter is 0.016 inches of water column although this number is seen as 0.02 when viewed through Vista Workstation since it is truncated to 2 decimal places.

Whenever the velocity pressure reading detected by the Xenta 102-AX drops below this level it will change it to zero instead. Fluctuations in pressure readings at about zero are often a result of this filter. Check to see if the nvoAirPressure reading is at about this point just before dropping out to zero. Also, check to see that all of the VAV parameters are set correctly including duct area, pickup factor, and box constant.

While it is possible to lower the UCPTpressCutOff parameter there is no guarantee of the accuracy on any readings below the default setting.

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