D6F-series MEMS Flaw Sensor User’s Manual (A286)
6
4.2 Detecting principle of mass flow sensor
As shown in Fig.4, the constant current is flowing to the heater at the center of the chip and the
heater becomes hot. When there is no flow, the heat distribution around the heater is symmetric,
so Vu and Vd of the electromotive force from both thermopiles will be equal.
On the other hand, when there is a flow of gas on the sensor surface, the heat source is biased
on the downstream side according to the flow of gas. The electromotive force of the downstream
thermopile will be larger than the upstream thermopile (Vd > Vu). The output difference between
the two thermopiles is approximately proportional to the square root of the mass flow rate of the
gas through the sensor surface. The output sensitivity and the mass flow rate depend on the
composition ratio of the gas. Through amplification, it is possible to electronically detect the flow
rate of the gas. The flow velocity sensor is adjusted so that it can output a voltage that
corresponds to the flow velocity at the condition of 25
℃
, 101.3kPa from the mass flow rate.
When the flow direction is perpendicular to the thermopiles and heater.
Fig4. Sensing image of mass flow sensor using heat wire
Heat distribution in no flow condition
Vd
＝
Vu
The heat distribution is symmetric.
Vd
≠
Vu (Vd > Vu)
The downstream temperature is high
compare to the upstream temperature.
Heater
Downstream
Thermopile
Cavity
V
d
Vu
Heat distribution in flow condition
Vout = Voff +
（
Vd - Vu
）
× gain
Vout
：
Output voltage, Voff
：
Offset voltage
Vd-Vu
∝
√
(Flow rate)
Upstream
Thermopile
Flow Direction
