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Level Measurement
Smaller Than a Bread Box?
Joe Lewis
Managing Director, BlueLevel
Technologies, Inc.
If you have been in the process
measurement instrumentation
business as long as I have, you
remember when the size of some instruments
actually was similar in size or larger than a
bread box. But today, with the evolution and
invention of new sensing technologies, and the
continued advances in integrated circuits, the
relative size of level measurement sensors has
decreased dramatically. Is this an advantage?
Let me take a moment and highlight one material detection technology that tends to “slide
under the radar” (no pun intended) of most
people — the capacitive proximity switch.
The capacitive proximity switch is classified as
one of the family of proximity switches used to
detect the approaching presence of an object,
hence the word “proximity.” Usually proximity
switches are used to indicate mechanical movement for machine control or counting, and the
output is solid-state and used as an input to a
PLC. There are inductive and capacitive proximity switches. For our purposes, in the level sensing world we are interested in capacitive proximity switches because they can and are commonly used for material level sensing.
There are three primary advantages that you
look for in a capacitive proximity switch for
level detection (point monitoring for high, low
or intermediate level indication and control).
These are their compact size, low cost and suitability for digital input to a PLC.
Capacitive proximity switches commonly used
for the detection of material at pre-determined
points in bins, hoppers and tanks consist of a
simple RC-oscillator circuit with a multi-part
sensing electrode. These switches can be either
“shielded” or “unshielded.” Unshielded switches sense in multiple directions around the sensor face or “button,” while shielded sensors will
sense only directly in front of the sensor face.
Because of this, shielded sensors are typically
only used if a non-invasive installation is
required, as they can be installed flush with the
inside wall of a metal vessel. Unshielded units,
however, are best for level detection using invasive sensor mounting wells for liquid applications, and typically without mounting wells for
most powders and granular materials.
Capacitive proximity switches are definitely
smaller than a bread box, as the devices commonly used for level detection applications are
30mm in diameter and perhaps only 80-90mm
in length. This allows these units to be used on
small hoppers, like in plastic processing conveying systems and plastic machine hoppers, or in
very tight physical spaces where there is no
room for sensor heads that are four-inches in
diameter and the same or more in height.
The capacitive proximity level sensor is not
typically capable of switching high current
loads. Maximum loads of 300-500mA are common. This device will not be able to start
motors or other process equipment without an
interposing relay or starter. They are typically
used as sensor input to PLC’s. Some capacitive
proximity level switches operate on AC-voltage,
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these units typically are not connected to PLC’s,
but are used to pull in the coil of a relay or perhaps light LED’s. However, the DC-voltage
capacitive level switch is more common and
directly connects with PLC input cards using
either an NPN- or PNP-transistor output circuit.
The difference between NPN and PNP is more
technical than most of us need to know. It just
needs to be coordinated with the PLC input.
Most common with DC capacitive proximity
level switches is the PNP-configuration and
these outnumber NPN-configurations by at
least 2:1.
Capacitive proximity level switches have come
a long way, and reliable units are now commonly available for under $100. Some high-end
units do exist, such as a recent smart unit introduced to expand the liquid application base of
capacitive proximity switches. The smart sensor
claims to be able to distinguish between the
real liquid level and foam, film, or material
build-up. Time will tell if these smart units catch
on and replace any RF-admittance or other
probe-style units.
BlueLevel Technologies industry experts have
produced white papers on a wide range of subject matter related to level measurement and
monitoring of liquids and solids. All are available free of charge at
www.blueleveltechnologies.com/whitepapers.php. Subjects include
point level monitoring, continuous level and
inventory monitoring, safety instrumented systems and the use of level instrumentation within a wide range of industries.
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