Interface Detection The SITRANS LG200, when used with the Model 7ML1301-6 co-axial probe, is a transmitter capable of measuring both an upper level and an interface level. The upper liquid must have a dielec-
tric constant between 1.4 and 5 and the two liquids have a dif-ference in dielectric constants greater than 10. A typical appli-cation would be oil over water, with the upper layer of oil being non-conductive with a dielectric constant of approximately 2 and the lower layer of water being very conductive with a dielectric constant of approximately 80. This interface measurement can only be accomplished when the dielectric constant of the upper medium is lower than the dielectric constant of the lower me-dium.
© Siemens AG 2008 Level instruments
SITRANS LG200
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Overview Introduction Guided Wave Radar transmitters combine TDR (time domain re-flectometry), 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 disconti-
nuity (created by media surface), part of the energy is reflected. The greater the dielectric difference, the greater the amplitude (strength) of the reflection.In the SITRANS LG200 transmitter, a waveguide with a charac-teristic impedance in air is used as a probe. When part of the probe is immersed in a material other than air, there is lower im-
pedance 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 critical in the application of TDR to vessel level measurement technology. The high speed EM energy (1000 ft/µs) is difficult to measure over short dis-tances and at the resolution required in the process industry. ETS captures the EM signals in real time (nanoseconds) and re-constructs them in equivalent time (milliseconds), which is much easier to measure with today’s technology.ETS is accomplished by scanning the waveguide to collect thou-sands of samples. Approximately 8 scans are taken per second; each scan gathers more than 30,000 samples.
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Modification 10/2008 Siemens FI 01 · 2009