Siemens SITRANS WM300 MFA, Operating Instructions Manual

Page: 69 / 82

Technical Reference
A.1 Principles of Operation
SITRANS WM300 MFA
Operating Instructions, 08/2018, A5E45090709-AA
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The graph was plotted from tests using four ferrous blocks set equidistantly on a 406 mm
(16 inch) diameter circle on a non-ferrous disc.
The physical shape of the ferrous target generally becomes important at low velocities or
large gaps. At these points, tests indicate that a cubic shape gives the best results due to the
sudden change it causes in the magnetic field.
An increase in block size beyond 50 x 50 x 25 mm (2 x 2 x 1 inch) is generally not as
effective as minimizing the gap, except at very low velocities.
Milltronics manufactures probes to suit a wide variety of environments: low temperature, high
temperature, corrosive, and Class I, II, and III applications.
Pre-Amplifier (IMA and RMA)
The pre-amplifier accepts the voltage pulses generated by the probe and converts them into
NPN open collector outputs. The preamplifier comes internally mounted in the probe, or in a
module enclosure for remote mounting.
Internally mounted pre-amplifiers are called IMAs. Remote mounted pre-amplifiers are called
RMAs.
SITRANS WM300 MFA Operation
The SITRANS WM300 MFA provides a short circuit protected, +24 V DC unregulated supply
to the preamp.
The rate at which the pulses are received by the SITRANS WM300 MFA is compared to a
setpoint reference signal from the setpoint programmed.
The alarm relays will de-energize (failsafe operation) if the process variable (incoming
pulses) exceeds the upper setpoint or the lower setpoint. Once the process variable
(incoming pulses) is within the defined upper and lower limits, the Alarm Relay will reset
automatically or requires a reset manually depending on the state of Input I1 -Alarm O/P
Reset.
● Automatic reset: Input I1 is maintained Active Hi at all times. See Automatic Reset
(Page 32).
● Manual reset: Input I1 must transition from a Lo to Hi state. This method will maintain an
Active Lo input state. See Manual Reset (Page 32).