T
1
Figure 2-8 Single Internal Horizontal Probe
Figure 2-9 Twin-Level Internal Horizontal Probes
T
2
Figure 2-10 Coaxial Probe For Non-Aqueous Fluids
Figure 2-11 Twin-Level Coaxial Probes For
Non-Aqueous Fluids
container, electrically coupled to the fluid mass below, will
create a substantial capacitive response. Internal probes are
much more resistant to this effect since the fluid surface is
guaranteed to become mechanically disconnected from the
probe when the level drops. Coating the inner vessel surface
with a smooth plastic of polyethylene or PTFE often has a
very beneficial effect on this phenomenon.
2.2.6 S
CALE
B
UILDUP
Scale buildup on internal probes, bare or insulated, is not
generally a problem since the sensor is still measuring
capacitance, not conductance, and a reduction in
conductivity around the probe will have minimal or no effect.
Probe designs should be tested for this to be certain in all
specific cases.
A legitimate concern with bare metal probes is the buildup of
scale or other deposits at the entry point of the probe into the
vessel. Such deposits may create a conductive surface path
(especially if the vessel is made of metal) that may lead to
false-positive trips. If the shank of the probe at the entry
point is insulated enough so that conductive bridging cannot
occur, this problem should be alleviated.
2.3 SINGLE LEVEL SENSING
When sensing for a single trip point, the single electrode can
be a simple horizontal strip on the outside of a nonmetallic
vessel (Figure 2-2), or an internal probe having a substantial
horizontal 'plateau' at the trip point (Figures 2-4, 2-5, 2-8,
2-10). When the strip or plateau is ‘covered’ with fluid the IC
will detect on at least the OUT1 line; OUT2 can be ignored.
The trip point ideally occurs at the centerline of the internal
probe or external electrode; this can be trimmed with a
potentiometer if necessary (see Section 3.2). Making the
electrode narrow and long (horizontally) will help keep the
trip point localized within a narrow band.
2.2.7 V
ISCOUS
, C
ONDUCTIVE
F
ILMS
Highly viscous fluids, or those having a high surface tension,
and having substantial conductivity can fool some electrode
designs into thinking that there is fluid present when there is
not. This is a particular problem with external electrodes,
where the residual films of certain types of fluids inside the
-5-
QUANTUM [ QUANTUM RESEARCH GROUP ]
