Operating Instructions

Appendix A: Technical reference
A
Note
Where a number follows the parameter name [for example, Master Reset (4.1.)] this is the
parameter access number via the handheld programmer. See Parameter Reference
(Page 107) 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 175). This distance is used as a basis
for the display of material level and mA output.
A.2 Echo Processing
A.2.1 Process Intelligence
The signal processing technology embedded in Siemens radar level devices is known
as Process Intelligence.
Process intelligence provides high measurement reliability regardless of the dynamically
changing conditions within the vessel being monitored. The embedded Process Intelligence
dynamically adjusts to the constantly changing material surfaces within these vessels.
SITRANS LR250 with mA/HART
Operating Instructions, 10/2019, A5E32220602-AG 223