VIASYS Vmax Encore, Техническое справочное руководство

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Chapter 2 System Description
2–16
L2707
Technical Reference Manual
Vmax Encore
MASS FLOW SENSOR CIRCUITRY
Depending on the test, the Mass Flow Sensor circuitry measures the flow. This circuitry is entirely
different from previous generations of these products.
Mass Flow Sensor
The Mass Flow Sensor uses two metallic filaments to sense gas flow rate: a sensing filament and a
reference filament. The reference filament monitors the temperature of gas while the sensing
filament is maintained at a preset temperature (approximately 86ºC) above the temperature of the
reference filament. As gas flows through the Mass Flow Sensor, the gas molecules remove heat
from the sensing filament. To maintain the sensing filament offset temperature, more or less power
is delivered to the sensing filament. This power change is an accurate measurement of the mass
flow rate through the Mass Flow Sensor. Unlike volumetric flow, mass flow measurements are
unaffected by changes in air pressure or temperature.
Figure 2-9 Mass Flow Sensor Circuit
The electronic circuit of the Mass Flow Sensor monitors the resistance of sensor filaments to
determine their temperatures, so it is important that only the resistances of the filaments be
measured. A new technique, Kelvin Sensing, and exclusive design are incorporated in the Vmax
Encore Mass Flow Sensor to eliminate the influence of connector and cable resistance in the
measurements.
The Vmax Mass Flow Sensor has eight metallic pins supporting the two filaments in the throat of
the sensor. Currents are delivered to the filaments by sensor cables and these supporting pins.
Resistances of the sensor cables, cable connectors, and the supporting pins can limit the accuracy
of the filament resistance measurements. The new Vmax Encore Mass Flow Sensor gets around
these limitations by employing an additional set of non-current carrying sensor cables and metal
pins to provide the filament voltages to the electronic data processing circuits inside the Vmax
module. This Kelvin Sensing arrangement provides filament resistance readings free of