SITRANS Probe LR with mA/HART
86
Operating Instructions, 05/2019, A5E32337711-AC
Technical reference
A
A.1
Principles of operation
SITRANS Probe LR is a sophisticated radar instrument that uses advanced microwave pulse
technology
1)
to provide non-contacting continuous level measurement in liquids or slurries.
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 transmit 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). The echo is
detected by a receiver, and the transmit time is used to calculate level.
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 Probe LR consists of an enclosed electronic component coupled to an antenna
and process connection. The electronic component generates a radar signal 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 reference point on the instrument. This distance is used as a basis for the display of
material level and mA output.
1)
The microwave output level is significantly less than that emitted from cellular phones.
A.2
Transceiver
The SITRANS Probe LR transceiver operates under one of three sets of pre-set conditions
which affect the speed of the measurement response (P003).
Measurement response
P003
Max. fill/empty rate
P700/P701
Echo verification
P711
Failsafe timer P070
(time in minutes)
1
* 0.1 m/min
slow
2
100
2
1 m/min
medium
2
10
3
10 m/min
fast
2
1
The measurement response limits the maximum rate at which the display and analog output
respond to changes in measurement. P003 should be set to a measurement response just
faster than the maximum filling or emptying rate (whichever is greater).
When the echoes are received, the relevant echo algorithm (P820) is applied to determine
the true material echo.