VAV question

[the maestro]

what is the differential pressure sensor measuring exactly?

[NINAX]

what is the differential pressure sensor measuring exactly?

Check these out.

Flow Measurements (http://www.ddc-online.org/inout/inout_chapt02_ana_06flow.aspx)

Terminal Unit Airflow Measurements (http://www.krueger-hvac.com/lit/pdf/airflow_measure.pdf)

[adrianfromoz]

The DP sensor in a VAV controller is measuring the velocity pressure with repect to the static pressure. Hence the sampling tube has holes directly in the path of the airflow and at 90 degrees to airflow.

As velocity is proprtional to the square of velocity pressure the controller does the math and should spit out velocity.

[gnomethang]

The DP sensor in a VAV controller is measuring the velocity pressure with repect to the static pressure. Hence the sampling tube has holes directly in the path of the airflow and at 90 degrees to airflow.

As velocity is proportional to the square of velocity pressure the controller does the math and should spit out velocity.

As an addendum to that, some flow grids do not offset at 90 degrees but are slightly different to allow for a greater differential. This means that the box constant will be different and may need to be calculated from measurement over a couple of points. Depending on your VAV controller there is usually a 'Fudge Factor' that can absorb this in some way shape or form. Grids from some manufacturers (e.g. Bar-Col in the Netherlands) will declare the KV as manufactured.