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Measuring small and high flows and its direction

The AFE-01 consists of four platinum thin film resistors. The low ohmic resistance with a small area isused as heater.
Two high ohmic resistors on the left and right hand side of the heater are used to detect the flow-speed and flow direction.
A further resistance allows the measurement of the gas temperature.
The two resistors close to the heater can be connected in a bridge circuit. This leads in an output signal which is a function of flow speed and direction.
In no flow condition, these two resistors are heated up equally. If flow appears, one of them is cooled more than the other, depending on flow direction.
The temperature difference can be measured and depends on flow speed and direction. Response and heating up time of this flow sensor is very short due to the small thermal mass.
This kind of evaluation allows the measurement of very low flow speeds.
In order to detect higher flow velocities, the temperature sensor can be connected in a constant-temperature-anemometer.
Detection of flow direction
Small mass flows
Simple signal processing and calibration
No moved mechanical components
Excellent reproducibility
Excellent long-term stability
Easy adaptable for different applications or into housings
Best price-performance ratio
Differential pressure sensors
HVAC and building control solutions
Medical devices
Automotive industry
Device monitoring
Technical Data
Measuring principle Thermal
Measuring range 0 ... 50 m/s (in CTA-mode as electronic circuit example)
Response sensitivity 0.001 m/s
Accuracy < 2% measured value (depends on electronic and calibration)
Response time t63% < 0.5 s
Temperature range -20 ... +150 °C
Electrical connection Cu - wires enamel- insulated (typical length 25 mm)
Heater RH (25°C) = 34 ± 10%
Measuring elements Rs_i (25°C) = 425 ± 10%
Reference element RR (25°C) = 710 ± 10%
Required voltages Typical 2 … 5 V (depends on air velocity)
Substrate material Ceramic with low thermal conductivity
In general Higher requirements on request

Example – Characteristic for small flow measurement with direction output
The heater RH is fed by a constant voltage or a constant temperature. As shown in the scheme above, the two sensor-elements (RS left and RS right) can be connected in a bridge circuit.
With a corresponding supply VCC, the bridge balance V_Br = V1-V2 is depending of the mass-flow.
If the bridge balance is adjusted at flow = 0 to V_Br = 0, the sign gives the information about the direction of the flow. For this the resistor R1 has to be adjustable.

Flow range from 2.5m/s to 50m/s
A flow direction-independent signal output 1 and/or U_Bs is available likewise and represents the flow dependent entire heat transfer of the sensor into the medium.
This can be taken in order to measure larger flow ranges than
2.5m/s. The signal U_Br can be further used to detect flow direction.
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