Siemens SITRANS F TRANSMAG 2, Operating Instructions Manual, Page: 17 / 126

3.4 Theory of operation
The flow measuring principle is based on Faraday’s law of electromagnetic induction.
0DJQHWLF)LHOG
)ORZ
U
i
= When an electrical conductor of length L is moved at velocity v, perpendicular to the lines of
flux through a magnetic field of strength B, the voltage U
i
is induced at the ends of the conductor
U
i
= L x B x v
• U
i
= Induced voltage
• L = Conductor length = Inner pipe diameter = k
1
• B = Magnetic field strength = k
2
• v = Velocity of conductor (media)
• k = k
1
x k
2
U
i
= k x v, the electrode signal is directly proportional to the fluid velocity
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
14 Ω which allows flow measurements on fluids 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.
Description
3.4 Theory of operation
TRANSMAG 2 with MAG 911/E
Operating Instructions, 03/2021, A5E00102775-AC
17