Sensors; Thermal Measurement Principle - Burkert 8626 Instructions De Service

The setpoint value (w) is set electrically via a standard signal or a field bus. The
process value (x) measured by the sensor is compared in the controller with the
setpoint value. The correcting variable is sent as a plus-width modulated voltage
signal to the servo component. The pulse-duty factor of the voltage signal is varied
according to the control deviation determined.
The process value, in addition, is sent out via an analog electrical interface or a
field bus and is available to the user for monitoring purposes or further evaluation
(e.g. calculation of consumption by integration).
The thermal measurement principle guarantees that the MFCs control to the
required mass flow to a large extent independently of pressure and temperature
variations in the respective application.
The MFMs, in contrast to the corresponding MFCs, do not have a proportional
valve, so that these devices can only be used to measure the mass flow and not
to control it. The characteristics of the other components, described in the
following, in particular the sensors, are identical with those of the MFCs.
Sensor

Thermal measurement principle

The flow sensors employed work on a thermal (anemometer) measurement
principle.
They measure in each case the product of density an flow velocity and thus deli-
ver a signal related to the quantity of material flowing. For most applications the
relevant quantity mass flow is directly determined thereby, without additional
measurement of secondary quantities, such as density, and the signal can be
further processed in the controller as the process value
Depending on the flow rate range and the intended market for the devices, the
individual types contain sensors with three different variants of flow rate
measurement. In the following, the functioning and associated characteristics of
these sensors are briefly described.
NOTE
Please take into account that the relative sensitivity for different
gases differs for the three measurement principles and any
correction factors existing for one operating gas to another are in no
case transferable between sensor variants.
1 )
It is true that the units generallly used for characterizing the measurement range, „l
dimensions "volume/time", but because of the reference to a standard state (here p=1013 mbar and
T=273 K), we are actually dealing with mass flow rates specific to gas types. These are obtained (e.g. in „kg/h")
by multiplication of the standard volumetric flow rate by the density of the operating gas in the standard state ρ
6 - MFC/MFM
1)
.
3
/min" or „m /h" , have the
N N
.
N