Penlon AV-S, Service Manual, Page: 88 / 94

Dual Wheatstone bridges control airflow
measurement - one provides closed loop
heater control, the other contains the dual
sensing elements.
The heater circuit minimizes shift due to
ambient temperature changes by providing
an output proportional to mass flow. The
circuit keeps the heater temperature at a
constant differential (160°C) above ambient
air temperature which is sensed by a heat-
sunk resistor on the chip.
The ratiometric voltage output of the device
corresponds to the differential voltage
across the Wheatstone bridge circuit.
Sensor flow characteristics
The graph shown below is a typical flow
versus resistance graph for the Honeywell
spirometer head units for the flow range
showing typical hysteresis between up and
down flow measurements (and
repeatability).
84
0 20 40 60 80 100 120 140
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
Flow
(L/min)
Resistance
(cmH
2
O)
APPENDIX