Orion Instruments Eclipse 705 - Manual (Page 41)

41 57-600 Eclipse ® Guided Wave Radar Transmitter
3.2.2 Interface Detection
The ECLIPSE Model 705, when used with the Model 7xT
coaxial probe, is a transmitter capable of measuring both an
upper level and an interface level. It is required that the
upper liquid 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.
ECLIPSE Guided Wave Radar is based upon the technology
of TDR (Time Domain Reflectometry). TDR utilizes pulses
of electromagnetic energy transmitted down a wave guide
(probe). When a pulse reaches a liquid surface that has a
higher dielectric constant than the 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 liq-
uid 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
(refer to figure at left). 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.
Knowing the time between the first and second reflections,
along with knowing the upper layer dielectric constant, the
thickness of the upper layer can be determined.
In order to properly process the reflected signals, the
Model 705 is specified for those applications where the
thickness of the upper layer is greater than 2 inches. The
maximum upper layer is limited to the length of the
Model 7xT probe, which is available in lengths up to 20 feet.
The Model 7x7 twin rod flexible probe allows interface
operation up to 40 feet.
Emulsion Layers
As emulsion (rag) layers can decrease the strength of the
reflected signal, the ECLIPSE Model 705 is recommended
for applications that have clean, distinct layers. The ECLIPSE
Model 705 will tend to detect the top of the emulsion layer.
Contact the factory for application assistance regarding
emulsion layers.
Low Dielectric
Medium
(e.g. oil,
ε
= 2)
Air
(
ε
= 1)
High Dielectric
Medium
(e.g. water,
ε
= 80)
Emulsion Layer
Reference
Signal
Upper Level
Signal
Interface
Level
Signal
Time
Interface Detection