Equivalent Time Sampling (Ets); Interface Detection - Siemens SITRANS LG200 Series Instruction Manual

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.

Interface Detection

The SITRANS LG200, Model 7ML1301-6 coaxial probe, is a transmitter capable of
measuring both an upper level and an interface level. The upper liquid must have a
dielectric constant between 1.4 and 5 and the two liquids have a difference in dielectric
constants greater than 10. A typical application 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 medium.
Upper Level Signal
Interface Level Signal
SITRANS LG200 Guided Wave Radar is based upon the technology of TDR (Time Domain
Reflectometry). TDR uses pulses of electromagnetic energy transmitted down a wave
guide or probe. When a pulse reaches a liquid surface that has a higher dielectric
constant than air (dielectric constant of 1) in which it is traveling, the pulse is reflected
and ultra high speed timing circuitry provides an accurate measure of liquid level. Even
after the pulse is reflected from the upper surface, some of the energy continues down
the length of the probe through the upper liquid. The pulse is again reflected when it
reaches the higher dielectric lower liquid. Since the speed of the signal through the upper
liquid is dependent on the dielectric constant of the medium in which it is traveling, the
dielectric constant of the upper liquid must be known to accurately determine the
interface level.
Page 82 SITRANS LG200 – INSTRUCTION MANUAL
air (dk = 1)
low dielectric medium (e.g.
oil, dk = 2)
emulsion layer
high dielectric medium (e.g.
water, dk = 80)
7ML19985KA02