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Appendix C: Technical Reference
SITRANS LG200 is a loop-powered two-wire level transmitter based on the concept of
Guided Wave Radar. Guided Wave Radar, or Micropower Impulse Radar (MIR), is a
revolutionary, new level measurement technology.
Micropower Impulse Radar (MIR)
MIR (Micropower Impulse Radar) combines TDR (time domain reflectometry), ETS
(equivalent time sampling) and modern low power circuitry.
A reflection develops
from the liquid surface.
Time Domain Reflectometry (TDR)
TDR uses pulses of electromagnetic (EM) energy to measure distances or levels. When a
pulse reaches a dielectric discontinuity (created by media surface), part of the energy is
reflected. The greater the dielectric difference, the greater the amplitude (strength) of the
reflection.
In telephone, computer, and power transmission industries, it is used to successfully find
wire or cable breaks and shorts. An EM pulse is sent through the wire, traveling
unimpeded until it finds a line break or short. A reflection is then returned from the break
enabling a timing circuit to pinpoint the location.
In the SITRANS LG200 transmitter, a waveguide with a characteristic impedance in air is
used as a probe. When part of the probe is immersed in a material other than air, there is
lower impedance due to the increase in the dielectric. When an EM pulse is sent down
the probe and meets the dielectric discontinuity, a reflection is generated.
7ML19985KA02
24 V DC, 4 to 20 mA
loop powered
SITRANS LG200 – INSTRUCTION MANUAL
transmit pulse
air: dk = 1
media dk>1.4
A small amount of energy
continues down the probe
in a low dielectric fluid
(hydrocarbons, for
example).
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