P R O B E
O V E R V I E W
Choosing the proper Guided Wave Radar (GWR) probe is the most important decision in the application process. The
probe configuration establishes fundamental performance characteristics. Coaxial, twin element (rod or cable) and sin-
gle element (rod or cable) are the three basic configurations used today; each with specific strengths and weaknesses.
COAXIAL
PROBES
The Coaxial probe is the most efficient of all probe configurations and should
be the first consideration in all applications. Analogous to the efficiency of
modern, coaxial cable, coaxial probes allow almost unimpeded movement of
the high frequency pulses throughout its length.
The electromagnetic field that develops between the inner rod and outer tube
is completely contained. See Figure 1. The efficiency and sensitivity of a coaxial
configuration yields robust signal strength even in extremely low dielectric
(
ε
r > 1.7) applications. The sensitivity of this “closed” design, however, also
makes it more susceptible to measurement error in applications of coating and
buildup.
Figure 1
Coaxial Probe
TWIN
ROD
PROBES
The relationship of the twin rod probe to a coaxial is similar to that of older,
twin-lead, antenna lead-in to modern, coaxial cable. The 300-ohm twin-lead
cable simply does not have the efficiency of the 75-ohm coax. The parallel
conductor design is less sensitive than the concentric coaxial. See Figure 2. This
translates to Twin Rod GWR probes measuring dielectrics of only
ε
r > 2.5.
The “open” design also allows more accurate measurement where coating/
buildup are possible. A film coating has little effect on performance. However,
bridging of material between the rods or buildup on the spacers can cause
improper measurement and should be avoided. Figure 2 also shows that the
electromagnetic field develops not only between the rods, but also expands
outward making it more sensitive to proximity effects of objects located in the
immediate area.
Figure 2
Twin Rod Probe
NOZZLES
The 7XB Twin Rod probe may be susceptible to objects that are in close prox-
imity. The following rules should be followed for proper application:
1. Nozzle should be 3" (80 mm) diameter or larger.
2. For nozzles < 3" (80 mm) diameter, the bottom of the inactive section of the
probe should be flush with the bottom of the nozzle or extend into the vessel.
OBSTRUCTIONS
(METALLIC)
A
B
7XB Twin Rod probes should be installed so the active rod (below the
4" (100 mm) inactive sheath) is > 1" (25 mm) from metallic objects such as
pipes, ladders, etc. Bare tank walls parallel to the probe are acceptable.
TURBULENCE
Figure 3
The bottom of the probe should be stabilized if turbulence will cause a deflec-
tion of more than 3" (80 mm) at 10' (3 m) of length. The probe should not
make contact with a metal tank.
3
MAGNETROL [ MAGNETROL INTERNATIONAL, INC. ]
