Airflow Sensors
Microbridge Mass Airflow
AWM Series
AWM 1000/2000/3000 Series
AWM 40000 Series
AWM 5000 Series
FEATURES
State-of-the-art silicon
micromachining
Sensitive to low flows 鈥?0.1 sccm to
20 SLPM
Adaptable for use with higher flows
(See Application Note 2 page 128.)
Fast response time
Analog output
Low power consumption
OPERATION
The microbridge mass airflow sensor op-
erates on the theory of heat transfer. Mass
airflow is directed across the surface of
the sensing elements. Output voltage va-
ries in proportion to the mass air or other
gas flow through the inlet and outlet ports
of the package. The specially designed
housing precisely directs and controls
the airflow across the microstructure
sense element. Mechanical design of the
package allows it to be easily mounted to
printed circuit boards.
The microbridge mass airflow sensor has
a unique silicon chip based on advanced
microstructure technology. It consists of
a thin-film, thermally isolated bridge
structure containing heater and temper-
ature sensing elements. The bridge struc-
ture provides a sensitive and fast re-
sponse to the flow of air or other gas over
the chip. Dual sensing elements posi-
tioned on both sides of a central heating
element indicate flow direction as well as
flow rate. Laser trimmed thick film and thin
film resistors provide consistent inter-
changeability from one device to the next.
Repeatable response
Laser-trimmed interchangeability
Accurate, cost effective flow sensing
In-line printed circuit board terminals
Standard 0.100 (2,54mm) mounting
centers
Accurate sensing of low pressure
0.001 to 4.0 H
2
O (.003 to 10mBar)
The microbridge mass airflow sensor us-
es temperature-sensitive resistors de-
posited within a thin film of silicon nitride.
They are suspended in the form of two
bridges over an etched cavity in the sil-
icon, shown below. The chip is located in
a precisely dimensioned airflow channel
to provide a repeatable flow response.
Highly effective thermal isolation for the
heater and sensing resistors is attained
by etching the cavity space beneath the
flow sensor bridges. The small size and
thermal isolation of the microbridge mass
airflow sensor are responsible for the ex-
tremely fast response and high sensitivity
to flows.
Dual Wheatstone bridges control airflow
measurement 鈥?one provides closed
loop heater control, the other contains the
dual sensing elements. The heater circuit
minimizes shift due to ambient temper-
ature changes by providing an output
proportional to mass flow. The circuit
keeps the heater temperature at a con-
stant differential (160擄C) above ambient
air temperature which is sensed by a
heat-sunk resistor on the chip. The ratio-
metric voltage output of the device corre-
sponds to the differential voltage across
the Wheatstone bridge circuit.
APPLICATIONS
Damper control for heating, ventilation,
and air conditioning systems
Gas analyzers
Low vacuum control
Process control
Medical respirators and ventilators
Oxygen concentrators
Leak detection equipment
Vent hoods
Anesthesia control
Gas metering
Gas chromatography
NOTICE
Dust contamination may be possible in
some applications, the effects of which
can be minimized. By design, dust parti-
cles that may be present in the air
stream will flow past the chip parallel to
the chip surface. In addition, the micros-
tructure chip produces a thermophoret-
ic effect, which repels micrometer-sized
dust particles away from the bridge
structure.
Dust adherence to chip edges and
channel surfaces can be prevented us-
ing a simple filter. A disposable five-
micron filter used in series on the up-
stream side of the airflow device will
provide adequate filtering in most appli-
cations. For a list of possible filter sourc-
es, see Filter Manufacturers, page 126.
CAUTION
AWM Series Microbridge Mass Airflow
Sensors are
NOT
designed to sense
liquid flow and will be damaged by liquid
flow through the sensor.
PRODUCT DAMAGE
64
Honeywell MICRO SWITCH Sensing and Control 1-800-537-6945 USA
1-815-235-6847 International 1-800-737-3360 Canada
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