Figure 2-4 Single Level Internal Planar Probe
Figure 2-5 Single Level Internal Spiral Wire Probe
T2
T2
Figure 2-6 Twin-Level Internal Planar Probe
Figure 2-7 Twin-Level Internal Spiral Wire Probe
Spiral wire probes have the disadvantage of not being as Coaxial probes are more expensive to make, and can have
rugged as a solid disc probe.
problems with vibration if they are not constructed robustly.
The outer cylinder should be perforated at key spots to allow
fluid to fill and drain the cavity without trapping air bubbles
inside. The outer cylinder can also be made of a wire mesh.
2.2.3 SIDE-ENTRY
PROBES
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.
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.
2.2.5 BARE
M
ETAL
PROBES
2.2.4 COAXIAL
PROBES
Bare metal internal probes can be used, for example with
nonconductive fluids like oils, without difficulty. This applies
to all probe types described above.
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.
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.
LQ
4
QT114 R1.04/1106
QUALCOMM [ QUALCOMM INCORPORATED ]
