VMA1400 Series Overview and Engineering Guidelines Technical Bulletin 11
The DPT provides maintenance-free performance within the control
range of 1 to 18 m/s (200 to 3500 feet per minute [fpm]) when used as
recommended.
Pressure independent VAV terminal boxes use an airflow pickup
device, which amplifies the airflow velocity pressure between 1.5 to
3 times (varies by manufacturer). The airflow velocity and the gain of
the airflow pickup produce an accurately measured pressure
difference.
The VMA Autocalibration function helps reduce the temperature
effect error by zeroing offset errors. As the ambient temperature
swings relative to the temperature at which the Autocalibration
occurred, an offset error occurs. In the worst case, the offset error
ranges up to ±0.179 Pascal per °C (±0.0004 inch w.c. per °F).
When calibration occurs, this error becomes zero.
The error envelope becomes smaller with increased airflow pickup
gain and with smaller ambient temperature deviations at the
transmitter location.
Power Source
You can use one 24 VAC power trunk to power multiple VMAs.
Transformers of up to 100 VA can be centrally located and the
secondary run can be without conduit (if allowed by local authority)
and without concern about polarity. When using a single transformer
to power multiple VMAs, use a wire gauge large enough to handle the
current and minimize the voltage drop. The voltage drop depends on
the current draw, wire gauge, and wire length. For more details about
transformer wiring and calculating the number of VMAs per
transformer, see the Mounting and Wiring Variable Air Volume
Modular Assembly (VMA) 1400 Series Controllers Technical Bulletin
(LIT-6363125).
Note: The 24 V power transformer must be UL/Canadian
Standards Association (CSA) listed as NEC Class 2 Power Limited.
See NEC Article 725/Class 2 (30 VRMS maximum) and
(100 VA maximum).
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