705-5100-110 Datasheet, PDF (42/68 Pages) Magnetrol International, Inc. – Enhanced Model 705

3.2.3 Time Domain Reflectometry (TDR)
TDR uses pulses of electromagnetic (EM) energy to mea-
sure 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.
Although TDR is new to the industrial level measurement
industry, it has been used in the telephone, computer, and
power transmission industries for years. In these 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 Eclipse 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.
3.2.4 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 tech-
nology. 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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57-600 Eclipse Guided Wave Radar Transmitter