SITRANS LG200 is a loop-powered two-wire level transmitter based on the concept of Guided Wave Radar.
Principle of operation
Guided Wave Radar combines TDR (time domain reflectometry), ETS (equivalent time sampling) and modern low power circuitry.
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.
Equivalent Time Sampling (ETS)
ETS (Equivalent Time Sampling) is used to measure the high speed, low power EM
energy. ETS is a critical key in the application of TDR to vessel level measurement
technology. The high speed EM energy (1000 ft/µs) is difficult to measure over short
distances and at the resolution required in the process industry. ETS captures the EM
signals in real time (nanoseconds) and reconstructs them in equivalent time
(milliseconds), which is much easier to measure with today’s technology.
ETS is accomplished by scanning the waveguide to collect thousands of samples. Approximately 8 scans are taken per second; each scan gathers more than 30,000 samples.
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