bürkert 8626 Bedienungsanleitung Seite 12

Bypass sensor in CMOSens
(Types 8713 / 8703 / 8712 / 8702 / 8711 / 8701)
In this technology, the mass flow is measured in a specially shaped flow channel
whose wall contains at one point a Si chip with a membrane that has been formed
by etching. To this membrane are applied, in CMOSens
resistor and two temperature sensors, arranged symmetrically upstream and
downstream of the heater.
When the heating resistor is fed with a constant voltage, the voltage difference
between the temperature sensors is a measure of the mass flow of the gas
flowing in the channel over the chip.
gas flow
sensor element
Fig.: Schematic diagram of the bypass sensor in CMOSens
The cross-section of the flow channel is small enough that an adequate
measurement signal is generated already at flow rates of less than 1 cm
The upper measurement limit is reached when the originally laminar flow in the
channel becomes turbulent. Higher flow ranges can be obtained by placing a
bypass element in a larger channel which is connected in parallel. When the
division ratio remains constant, measurement of the partial flow, after suitable
calibration, allows the total flow to be calculated.
The low thermal mass of the temperature sensors and their direct contact with the
flow (apart from a protective layer) result in a very fast reaction of the sensor
signal to spontaneous changes in the flow. In this way, the MFC can compensate
changes in the setpoint or process value within a few 100 ms. Moreover, the
sensor has a high sensitivity down to the smallest flow rates as well as additional
correction and diagnostic possibilities via the signal from a further temperature
sensor on the chip.
12 - MFC/MFM
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