Greenheck AMD-xx-TD Transmitter - Manual (Page 4)

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The Vari-Green utilizes thermal dispersion technology
to measure airflow. As air moves across the surface of
an elevated temperature sensor, heat is transferred from
the sensor to the airflow in a mathematically defined
relationship between heat transfer rate and airflow
velocity.
Each Vari-Green sensing node utilizes a pair of precision
matched thermistors. One functions as a reference
sensor measuring the ambient air temperature at the
sensing node location, and the other thermistor is heated
to a preset temperature differential (∆T) above the
ambient air temperature. Airflow velocity is determined
using the measured ambient air temperature, the known
heat transfer characteristics of the heated thermistor,
and the power consumed to maintain the ∆T between
the two thermistors.
Using a combination of Computational Fluid Dynamics
(CFD) modeling and extensive full-scale wind tunnel
verification, the aerodynamically optimized cross section
of the sensing node aperture prevents error inducing
turbulence typical of sharp edged sensing tubes.
Using a combination of Computational Fluid Dynamics
(CFD) modeling and extensive full-scale wind tunnel
verification, the aerodynamically optimized cross section
of the sensing node aperture prevents error inducing
turbulence typical of sharp edged sensing tubes.
The aperture’s injection molded shape has been
engineered to minimize the angular flow effects
naturally present in any duct. The flared aperture
captures a representative sample of the ducted
airflow, while the contoured leading edges prevent the
creation of vortices. The center cross section of the
aperture functions in the same manner as a venturi. It
stabilizes and flattens the velocity profile at the point of
measurement ensuring the airflow maintains full contact
with the sensing thermistors. The flow conditioning
characteristic of the aperture compensates for turbulent
airflow with pitch and/or yaw angles up to ±30 degrees
and produces an accuracy of ±2% of reading for each
Vari-Green sensor.
1.1 – DESCRIPTION
The Vari-Green ® Thermal Air Flow Transmitter is used in conjunction with Vari-Green Probe(s). One Transmitter can
receive up to 32 individual flow sensors measuring a single ducted airflow or fan.
The Transmitter’s microprocessor averages flow and temperature signals from all sensors and outputs an analog
signal and local display of these averages. The Transmitter serves as the interface for all configuration and
diagnostic functions.
1.2 – THEORY OF OPERATION
1 – GENERAL INFORMATION