Page 241 - MEMS Mechanical Sensors

230 Flow Sensors

Table 9.3 Data for Commercial Flow Sensors
Company Flow Range Sensitivity/ Response Time Fluid; Operating Maximum
Resolution Temperature Overpressure
Robert Bosch <1,000 kg/h — — Air; –40°C to —
GmbH [73] +120°C
HL Planartech- — — — Air; –40°C to —
nik GmbH [74] +120°C
Fraunhofer Insti- 2–700 g/s — 2 ms Air —
tute for Silicon
Technology [75]
HSG-IMIT [61] 10µl/h to 5 l/h 4 mV/K 5 ms Liquid —
0.01–50 slpm 1 4 mV/K 5 ms Gas —
Sensirion AG 150 nl/min to 50 nl/min 20 ms Water; +10°C to 5 bar
[62] ±1,500 µl/min +50°C
1 nl/min up to 50 50 ms Water 100 bar
µl/min
0.01–400 sccm 2 0.01 sccm 2 Nitrogen 2 bar
bypass: <100 Nitrogen; 0°C to —
l/min +70°C
Leister [77] 0.01–200 sccm 2 — 2 ms Gas; –10°C to 10 bar
+70°C
SLS Micro Tech- 0.01–1,000 0.3 mV/µl 230 µs Gas; –20°C to 3.5 bar
2
nology [78] sccm (with +120°C
bypass)
GeSiM [79] 1–70 µl/min 100 µV/(µl/min) — Water 40 bar
Mierij Meteo 0.2–25 m/s 0 to 360° 1 sec Air; –25°C to —
[80] +70°C
slpm = standard liter per minute.
1
2
1,000 sccm = 1 l/min.

pressure drop along a flow channel with known fluidic resistance, R , and calculat-
f
ing the flow Q from the fluidic equivalent to Ohm’s law: Q = ∆p/R . It is comparable
f
to measuring the current (Q) in an electric circuit by sensing the voltage drop (∆p)
over a fixed resistance (R ).
f
The sensor presented by Cho et al. [81] uses a silicon-glass structure with capaci-
tive read-out [Figure 9.22(a)]. Fluid enters the chip through the inlet at pressure p ,
1
flows through a channel and leaves the sensor with pressure p . If the flow channel is
2
small enough to create a resistance to the flow, a pressure drop ∆p appears across
the channel. The pressure above the membrane and the pressure at the inlet are kept
equal. The pressure difference is measured by a capacitive pressure sensor, which is
switched at 100 kHz.
Capacitive pressure sensing principles are also used in the devices described by
Oosterbroek [82, 83]. In addition, a hybrid piezoresistive readout was fabricated.
Two separate capacitive pressure sensors were used for the sensor shown in Figure
9.22(b). This enables the measurement of both pressure and volume flow rate. For
5
example, a 340-µm-wide channel has a resistance for ethanol of 1.7 × 10 –12 Ns/m .
The paper [83] also gives a detailed model to predict the sensor’s behavior. An
advantage of this sensor design is that the capacitor electrodes are not in contact
with the fluid, thereby avoiding any short circuit and degradation due to aggressive
fluids. Also, the sensor has a robust design using a glass/silicon/glass sandwich.