Figure 2-4 Single Level Internal Planar Probe
Figure 2-5 Single Level Internal Spiral Wire Probe
T
2
T
2
Figure 2-6 Twin-Level Internal Planar Probe
Spiral wire probes have the disadvantage of not being as
rugged as a solid disc probe.
Figure 2-7 Twin-Level Internal Spiral Wire Probe
fluid to fill and drain the cavity without trapping air bubbles
inside. The outer cylinder can also be made of a wire mesh.
The outer cylinder does not have to be coated in plastic,
even when used with water-based fluids. When used with
oils, the inner rod does not require insulation either.
2.2.3 S
IDE
-E
NTRY
P
ROBES
Another type is a side-entry probe (Figure 2-8), which
requires an entry point into the vessel wall, but may have the
advantage of accessibility in certain cases. These can be
made of simple metal rod, insulated in almost any plastic if
required.
2.2.5 B
ARE
M
ETAL
P
ROBES
Bare metal internal probes can be used, for example with
nonconductive fluids like oils, without difficulty. This applies
to all probe types described above.
Bare probes can also be used with aqueous fluids, but in
these cases a 1,000pF (1nF) ceramic NPO capacitor should
be inserted between the probe and the QT114 to block DC
current flows.
A bare internal probe used with conductive fluids and an
in-line blocking capacitor will generate a huge, robust
capacitive response that will not readily permit the use of a
2-level probe due to signal saturation. Even the slightest
amount of bare metal exposed to the fluid will usually
generate an immediate, large response with aqueous fluids.
2.2.4 C
OAXIAL
P
ROBES
Another type of internal probe is the coaxial probe (Figure
2-10); these are most useful with oils or similar fluids having
a low dielectric constant; the inner rod is connected to the
signal connection, and together with the outer grounded
cylinder forms a capacitor whose dielectric is either air or oil.
Keeping the gap between rod and cylinder to a minimum
increases the 'gain' of the electrode.
Coaxial probes are more expensive to make, and can have
problems with vibration if they are not constructed robustly.
The outer cylinder should be perforated at key spots to allow
-4-
QUANTUM [ QUANTUM RESEARCH GROUP ]
