Determination of the minimum detectable air
velocity by thermal flow sensors
Safir Issa1,2, Walter Lang1,2
1IMSAS
(Institute for Microsensors, Actuators and Systems), Microsystems Center
Bremen (MCB), University of Bremen, Otto-Hahn-Allee, Bld. NW1, D-28359
Bremen, Germany
2IGS (International Graduate School for Dynamics in Logistics), University of
Bremen, c/o BIBA Hochschulring 20, D-28359 Bremen, Germany
Content:
Introduction
Characterization of thermal flow sensors
Characteristic curves
Minimum detectable flow
Conclusion
Thermal flow sensors
Principle of sensing
Sensitive for low flow rates
Fast response time
Adaptable for wirless network
Applications: Medical, Automative
and airflow measurements
Measurement setup
Response time
model
experimental
Response time in ms
6
5
4
3
2
1
0
0
10
20
Velocity in m/s
30
40
50
Characteristic curves (1)
Ultra-low flow:
0,0025
R² = 0,9992
Range: 0 – 10
SCCM
R-sequired
values ≈ 1
Output Voltage in V
Characteristic
curve is linear
Diff_TS5
0,002
Sensitivity
increases with
“d”
Diff_TS10
0,0015
Diff_TS20
0,001
Diff_TS50
0,0005
0
0
Characteristic curves (2)
Low flow:
Range:
0 – 1000 SCCM
⇔ 0 – 5,5 m/s
Characteristic curve:
∆E = a ebu + c edu
R-sequired values ≈ 1
Sensitivity increases
with “d”
2
4
6
8
10
Flow in SCCM
12
Minimum detectable flow (1)
Noise resources:
Natural convection
Temperature noise
Thermal noise (Johnson noise)
⇒
Minimum detectable
velocity = U/S
≈ 0,05 mm/s
S=
∂Vdiff
∂u
u =0
0,14
Output voltage difference in mV
Noise level:
0,1198 mV ± 2,3 µV
(flow = 0)
Sensitivity of the sensor:
0,13
0,12
0,11
0,1
0
1000
2000
3000
Time in s
Minimum detectable flow (2)
Experimental
methods to generate
very low flow
4000
5000
0,18
position_1
Minimum detectable flow (3)
Water mass flow in g/s
3 different relative positions
for the two bottles
0,15
position_2
position_3
0,12
0,09
0,06
0,03
0,00
0
500
1000
a)
1500
Time in s
2000
2500
3000
1,2
b) Thermopiles output voltage
difference (mV)
Position_1
Position_2
Position_3
1
Sensor output in mV
a) Balance readings
( g/s ⇒ mm/s)
0,8
0,6
0,4
0,2
b)
0
0
500
1000
1500
Time in s
Minimum detectable flow (4)
⇒
Considering results and noise in zero flow ⇒
Minimum detectable flow velocity = 0.3 mm/s
2000
2500
3000
Conclusions
The studied thermal flow sensors have:
• Linear characteristic curves for ultra low flow
• Minimum detectable air velocity
• Theoretically 0.05 mm/s
• Experimentally 0.3 mm/s
Thank you for your attention