Burkert Type 8741 - Manual (Page 12)

12
Systemdescription
Type 8741
5.4.2 mfc solenoid valve
The solenoid valve used for an MFC is a direct-acting, normally-closed
proportional valve. The nominal diameter of the solenoid valve is
determined by the required nominal flow-rate Q
nominal , the pressure
conditions in the process and the density of the operating medium.
note!
the sealing function of the proportional valve cannot be guar-
anteed for hard sealing materials.
When used within the specified pressure range, the proportional
valve that is used for the mass flow control also provides the
sealing function.
Exception:
If the seat seal is composed of a special, hard sealing material
(e.g. PCTFE), as for nominal valve diameters of 0.05 and 0.1 mm,
then the leakage rate of the proportional valve may differ from that
of a device with a soft seat seal (e.g. FKM or EPDM).
The maximum leak rate in both cases is 1 Nml/min (air).
A higher tightness may be possible, however it is not standard and
must therefore be requested.
5.5 operation of the sensor in the
mfm/mfc
The integrated flow-rate sensors in the MFM/MFC use the thermal
measurement process (anemometric and calorimetric) to measure
the mass flow. This essentially consists of a heating resistor and a
temperature probe. The operating medium which flows through the
device modifies the temperature difference measured between the
two resistors.
The thermal measurement principle allows the MFC to control the
required mass flow almost independently of the pressure and tem-
perature fluctuations in the application concerned.
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