Airflow Sensors Microbridge Mass Airflow AWM Series AWM 1000/2000/3000 Series FEATURES 1 State-of-the-art silicon micromachining 1 Sensitive to low flows – 0.1 sccm to 20 SLPM 1 Adaptable for use with higher flows (See Application Note 2 page 128.) 1 Fast response time 1 Analog output 1 Low power consumption OPERATION The microbridge mass airflow sensor operates on the theory of heat transfer. Mass airflow is directed across the surface of the sensing elements. Output voltage varies 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 temperature sensing elements. The bridge structure provides a sensitive and fast response to the flow of air or other gas over the chip. Dual sensing elements positioned 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 interchangeability from one device to the next. 64 AWM 40000 Series 1 1 1 1 1 Repeatable response Laser-trimmed interchangeability Accurate, cost effective flow sensing In-line printed circuit board terminals Standard 0.100N (2,54mm) mounting centers 1 Accurate sensing of low pressure 0.001N to 4.0N H2O (.003 to 10mBar) The microbridge mass airflow sensor uses temperature-sensitive resistors deposited within a thin film of silicon nitride. They are suspended in the form of two bridges over an etched cavity in the silicon, 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 extremely fast response and high sensitivity to flows. Dual Wheatstone bridges control airflow measurement — one provides closed loop heater control, the other contains the dual sensing elements. The heater circuit minimizes shift due to ambient temperature changes by providing an output proportional to mass flow. The circuit keeps the heater temperature at a constant differential (160°C) above ambient air temperature which is sensed by a heat-sunk resistor on the chip. The ratiometric voltage output of the device corresponds to the differential voltage across the Wheatstone bridge circuit. AWM 5000 Series APPLICATIONS 1 Damper control for heating, ventilation, and air conditioning systems 1 Gas analyzers 1 Low vacuum control 1 Process control 1 Medical respirators and ventilators 1 Oxygen concentrators 1 Leak detection equipment 1 Vent hoods 1 Anesthesia control 1 Gas metering 1 Gas chromatography NOTICE Dust contamination may be possible in some applications, the effects of which can be minimized. By design, dust particles that may be present in the air stream will flow past the chip parallel to the chip surface. In addition, the microstructure chip produces a thermophoretic effect, which repels micrometer-sized dust particles away from the bridge structure. Dust adherence to chip edges and channel surfaces can be prevented using a simple filter. A disposable fivemicron filter used in series on the upstream side of the airflow device will provide adequate filtering in most applications. For a list of possible filter sources, see Filter Manufacturers, page 126. CAUTION PRODUCT DAMAGE AWM Series Microbridge Mass Airflow Sensors are NOT designed to sense liquid flow and will be damaged by liquid flow through the sensor. Honeywell 1 MICRO SWITCH Sensing and Control 1 1-800-537-6945 USA 1 F1-815-235-6847 International 1 1-800-737-3360 Canada