Theory Of Operation - Siemens SITRANS F M MAG 3100 sensor Operating Instructions Manual

3.3

Theory of operation

The flow measuring principle is based on Faraday's law of electromagnetic induction.
U = When an electrical conductor of length L is moved at velocity v, perpendicular to the
i
lines of flux through a magnetic field of strength B, the voltage U
the conductor
U = L x B x v
i
● U
● L = Conductor length = Inner pipe diameter = k
● B = Magnetic field strength = k
● v = Velocity of conductor (media)
● k = k
U = k x v, the electrode signal is directly proportional to the fluid velocity
i
Operating principle
The coil current module generates a pulsating magnetizing current that drives the coils in the
sensor. The current is permanently monitored and corrected. Errors or cable faults are
registered by the self-monitoring circuit.
The input circuit amplifies the flow-proportional induced voltage signal from the electrodes.
The input impedance is extremely high: >10
with conductivities as low as 5 µS/cm. Measuring errors due to cable capacitance are
eliminated due to active cable screening.
The digital signal processor converts the analog flow signal to a digital signal and
suppresses electrode noise through a digital filter. Inaccuracies in the transmitter as a result
of long-term drift and temperature drift are monitored and continuously compensated for via
the self-monitoring circuit. The analog to digital conversion takes place in an ultra low noise
ASIC with 23 bit signal resolution. This has eliminated the need for range switching. The
dynamic range of the transmitter is therefore unsurpassed with a turn down ratio of minimum
3000:1.
SITRANS F M MAG 3100 sensor
Operating Instructions, 09/2012, A5E03005599-02
= Induced voltage
i
x k
1 2
1
2
Ω which allows flow measurements on fluids
14
Description
3.3 Theory of operation
is induced at the ends of
i
19