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Capacitance 101 Brochure0 pages
Capacitance 101: Understanding the
Industry's Most Versatile Point Level Control
Capacitance probes function like big capacitors.
A capacitor is an electrical device formed by two
conductors and separated by an insulator. The two
conductors are represented by the active portion
of the probe, and the wall of the metal vessel. The
probe energizes the conductors and measures the
capacitance that exists between them. The amount
of capacitance depends upon the size of the conduc-
tors, the physical space between the conductors,
and the dielectric of the material between them. The
dielectric constant of a material is a unitless number,
derived from the ratio of the amount of capacitance
produced when material is present, compared to the
amount produced when only air is present.
Capacitance is expressed in units called farads.
For our purpose, a farad is much too large. We talk
in terms of picofarads. A picofarad is one-trillionth
of a farad. When material makes contact with the
probe, the capacitance between the two conductors
changes. The unit perceives that change and sends
a relay signal to signify the presence or absence of
material. BinMaster capacitance probes use fail-safe
relays; so if the unit loses power, the relays fall to the
"safe" condition and send a signal indicating an alarm
condition.
Conductive materials that leave a residue once
they have fallen away require a coated or sleeved
probe. This non-conductive coating keeps the con-
ductive material from grounding the probe to the ves-
sel wall. Any conductive residue that builds up from
the vessel wall to the active portion of an uncoated
probe will short out the two conductors. It would be
like pressing the two conductors right up next to each
other, making it impossible to sense anything be-
tween them.
When you calibrate the probe, you are setting it
above the dielectric constant of the ambient envir-
onment (air), and below the dielectric constant of
Top-Mounted-
High Level
Top-Mounted
High Level —
Side-Mounted
-High Level
Side-Mounted
<- Mid Level
Side-Mounted
'<-Low Level
Top-Mounted
— Low Level
BinMaster PRO series capacitance probes are reliable and
affordable on/off level switches
your material. The dielectric constant of air is 1. The
BinMaster capacitance probes can sense material with
a capacitance of 1/2 picofarad above air.
A PRO-Shield on the BinMaster probes is de-
signed to overcome problems resulting from sidewall
buildup, or bridging between the sidewall and probe.
A shield is an energized portion of the probe not used
for sensing material. The first five inches of the shaft
as it extends out from the enclosure make up the
shield (see diagram).
The probe examines
a large area around itself,
not just the area imme-
diately surrounding it.
This allows the probe to
ignore buildup which can
occur on the probe as-
sembly. This also allows
use a sleeved probe.
Material Build-up
Active Probe
PRO-Shield—Ignores Buildup
"
