User's Manual

SITRANS LR250 (FOUNDATION FIELDBUS)
Operating Instructions, 01/2014, A5E32221411-AB
229
Appendix A: Technical reference
A
Note
Where a number follows the parameter name [for example, Master Reset (4.1.)
] this is the
parameter access num
ber via the handheld programmer. See Parameter Reference
(Page
129) for a complete list of parameters.
A.1
Principles of operation
SITRANS LR250 is a 2-wire 25 GHz pulse radar level transmitter for continuous monitoring of
liquids and slurries. (The microwave output level is significantly less than that emitted from
cellular phones.) Radar level measurement uses the time of flight principle to determine distance
to a material surface. The device transmits a signal and waits for the return echo. The transit
time is directly proportional to the distance from the material.
Pulse radar uses polarized electromagnetic waves. Microwave pulses are emitted from the
antenna at a fixed repetition rate, and reflect off the interface between two materials with
different dielectric constants (the atmosphere and the material being monitored).
Electromagnetic wave propagation is virtually unaffected by temperature or pressure changes,
or by changes in the vapor levels inside a vessel. Electromagnetic waves are not attenuated by
dust.
SITRANS LR250 consists of an enclosed electronic circuit coupled to an antenna and process
connection. The electronic circuit generates a radar signal (25 GHz) that is directed to the
antenna.
The signal is emitted from the antenna, and the reflected echoes are digitally converted to an
echo profile. The profile is analyzed to determine the distance from the material surface to the
sensor reference point. See Dimension drawings (Page 205). This distance is used as a basis
for the display of material level and mA output.