Spiral wire probes have the disadvantage of not being as

rugged as a solid disc probe.

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.

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

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.

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.

- 4 -

Figure 2-7 Twin-Level Internal Spiral Wire Probe

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2

Figure 2-6 Twin-Level Internal Planar Probe

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2

Figure 2-5 Single Level Internal Spiral Wire Probe

Figure 2-4 Single Level Internal Planar Probe