51
The measuring principle of the return flow
recognition
The sensor element is embedded in the
mounting plate.
The sensor element comprises a diaphragm
with a heating zone and two symmetrically
arranged temperature sensors T
1
and T
2
.
The heating zone is set to an overtemperature
by means of a heating resistor and
temperature sensor T
2
.
If there is an incoming flow, the upstream part
of the diaphragm cools down along with the
temperature sensor T
1
.
The temperature of the upstream temperature
sensor T
2
is maintained due to the heated air in
the heating zone.
Temperature sensors T
1
and T
2
indicate a
temperature difference of ∆T.
In the case of a return air flow, the temperature
difference occurs at temperature sensor T
1
.
The amount and direction of this difference are
therefore dependent on the incoming flow.
• Advantage: the differential signal permits a
direction-dependent characteristic which
enables the Motronic to detect a return air flow.
T
T
1
T
2
1
0
SSP 198/36
Incoming flow
Temperature profile
without incoming flow
with incoming flow
Mounting plate
Diaphragm
Heating zone
Sensor element
Temperature difference
evaluation: ∆T = T
2
- T
1
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