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Capacitance instrumentation is an example of measurement where the
process variable (level for example) causes a capacitance signal to
be
generated that the instrument acts upon to provide an output or
signal
proportional to the change in the process variable.
Electrical "capacitance" is the property (or measure) of
two conductors (or plates), electrically insulated from each other, to store
energy when a
difference of electrical potential exists between them. Under these
conditions, an electrostatic field exists between the conductors and
the
amount of stored energy is a function of the area, spacing,
electrical
potential and the material between the conductors.
The unit of measure of capacitance is the "Farad", however
the "pico-farad" is used in capacitance instrumentation. This is
abbreviated "P? and is equal to 1 x 10 to the -12 Farads. Most capacitance
instrument manufacturers specify the operating range, sensitivity, adjustments,
etc. of the instrument in pico-farads.
The amount of "capacitance" in any given capacitor is
determined by the area, spacing of the plates and the material between the
plates. This can be
seen in the equation for the capacitance of a simple parallel plate
capacitor as shown
Capacitance = 0.225 K x A/D
Where : K = Dielectric
constant of material between the plates
A = Area of the
plates
D = Distance
between the plates
The equation indicates that the capacitance is directly proportional
to the dielectric constant, directly proportional to the area of the plates,
and inversely proportional to the spacing or distance between the plates.
Dielectric constant (K) is a dimensional number to relate the amount
of capacitance generated when anything other than air is used as the material
between the plates. For example, if some material was inserted in the
"air" capacitor that caused the capacitance to double, the material
would have a dielectric constant of 2 or twice that of air. The dielectric
constant number, then relates capacitance generated to that of air.
All materials have a dielectric constant value. For practical
purposes, all gases (including air) have a dielectric constant equal to (1). If
then, the air in an air-filled capacitor was displaced by a liquid, the
capacitance increase of the capacitor would be directly proportional to the
change in dielectric constant (K) between the liquid and air.
This is the fundamental concept upon which all capacitance
instrumentation is based.
Generation of the Capacitance Signal
For most applications of capacitance instruments, the method used to
generate a capacitance signal consists of placing a rod type
electrode
(probe) in contact with, or in close proximity, to the process
material.
If, for example, it were a level application, the changing level of
the
material within the vessel would create a change in the measured
capacitance at the probe. The magnitude of the capacitance change is
determined by the geometry of the installation, probe type, vessel
and the product being measured.
When measuring a non-con0uctive material, the change in level is
indicated by a change in the dielectric constant (K) of the material
between the plates of the capacitor. As the level rises, the air or gas
normally surrounding the probe is displaced by material having a higher
dielectric constant. The grounded metal vessel acts as the ground plate, and
the mass of material between the plates is the variable. As the level changes,
the capacitance varies because the dielectric between the electrodes has
changed.
When measuring a conductive rather than the vessel wall acts as the
ground plate. In this case, the area (A) in material, the measured material
itself, contact increases as the level increases, resulting in a change in the
capacitance of the system. Insulated probes are used in this type of
application to separate the conductive material from the probe. The thickness
of the insulation is the distance (D) between the plates.
In proximity (non-contacting) type sensors, the distance (D) between
the plates decreases as the level rises. If the material is conductive, the
surface of the material forms one plate of the capacitor. If the material is
nonconductive, two capacitors are in series - one through the air, and one
through the measured material. For either conductive or non-conductive
material, the capacitance changes as the material approaches the probe, because
the distance or dielectric between the electrodes has changed.
Successful application of any capacitance instrument depends upon an
adequate change being generated by the process. The word
"change' is
emphasized since all capacitance measurement systems start with an
initial or "terminal" capacitance value and increase to
some final value as a function of the process variable. The change in the
capacitance is denoted delta C. Each instrument has a specified minimum
capacitance change upon which it will operate, therefore, an estimation of the
generated capacitance signal must be made to ensure satisfactory instrument
performance.
TYPES OF CAPACITANCE MEASUREMENT
1. Continuous
a. Overall with indication and/or output
b. Interface with indication and /or output
c. Proxlmity with indication and/or output
d. a, b, or c with relay/alarm outputs
Note: Continuous interface (immiscible liquids only) measurement can
only be accomplished when one product is conductive and the other is
nonconductive (oil/water, toluene/water, etc.) A vertical mounted probe is used
and, in most cases there can only be two variables involved and tank should
always be full with no air. Exceptions are where the third variable is very
close in dielectric constant to the non-conductive product or where you can use
a sheath to eliminate the error introduced by the third variable. The sheath
will extend through the upper third variable (air for instance) and below the
lowest point the overall liquid levels will ever reach. In all interface
measurements you will be measuring the conductive product. Interface
applications should be considered by individual application. Emulsion layers
are also a problem.
2. On/Off Point
Level (s) with or
without Differential
a. Single or multiple point
b. Interface (no differential)
c. Proximity
Note: On/Off interface measurement uses a horizontal mounted probe
and the product being measured must differ in dielectric/conductivity than the
other products in the tank. Emulsion layers are a problem.
Principle of Short Stop Operation
In point level control, a conductive coating on a horizontally
mounted
probe often causes false relay trips when the material level has
risen
above, and then falls below, the detection point. The current passes
through this coating to the vessel wall, completing the circuit. The
point level switch continues to indicate - now incorrectly - that the level is
above the alarm point.
The Short-Stop feature ignores the effect of conductive material
buildup. A second element added to the probe is electronically maintained (or
driven) at the same voltage and frequency as the measuring tip. This means that
no current can flow through the coating to the vessel wall. When the actual
level reaches the bare tip, current can flow to the tank wall only through the
material being detected. In this way, the driven shield technology eliminates
erroneous signals due to current passing through the material coating the
probe.
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Probe Installation
Horizontally mounted rod type probes must be installed in the vessel
at the desired point of level detection. This type of installation provides the
closest control (smallest deadband) in that a small change at or near the probe
will produce a large capacitance change. On applications involving viscous
liquids or materials that have a tendency to "cling" or
"build-up" it is recommended that the probe be mounted on a slight
downward angle to permit draining of the material from the probe. The
instrument actuation point should be set so that the high level point is always
below the gland" If the horizontally mounted probe is to be installed in a
recess or nozzle, the use of a "sheathed" probe is recommended. This
type of probe has an inactive (sheathed) portion near the mounting gland so as
to eliminate false operation if material collects or packs within the recess or
nozzle.
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Vertically mounted probes can be mounted in either the top or bottom
of the vessel with the probe midpoint corresponding to the approximate desired
level detection point for On-Off instruments. This allows a variation in the
level detection point up and down the length of the probe by means of the
instrument zero adjustment.
Proportional instruments for level measurement require a probe
longer
than the intended measurement span. For probe lengths exceeding 10
feet, a flexible probe is recommended, primarily for shipping considerations.
Long flexible cable probes are offered with optional lower end fittings for
weights or tie-downs. If top mounted probes are mounted through a nozzle and
process has high vapor content or subject to condensate forming in the nozzle,
then a sheath should be considered.
Continuous Level Measurement
in Liquid Products
The Continuous measurement of level in a tank containing liquid can
be
made by a number of methods. One of the simpler and more reliable
methods is by capacitance techniques. The liquids and slurries which
can
be measured by capacitance covers a wide range of products,
including
aqueous solutions, hydrocarbons, acids, alkalies and slurries of
rock
products, paper products, etc. The "self-cleaning"
characteristic of Teflon
covered probes, as well as their excellent chemical resistance, make
the
capacitance method almost universal in its application to liquid
level
measurement. An advantage of capacitance techniques in liquid level
measurement is that the specific gravity of a product is not a
factor except in terms of viscosity (ability of the product to freely flow or
drain from the probe.
Typical Applications
The probe can be either the rigid or flexible type. The rigid types
are
usually limited to 10 feet in length, while flexible types can be
made up to 100 feet or more. Figure 1 shows graphically the range of dielectric
constants with various products. The low dielectric materials are liquefied
gages such as LOX and nitrogen - typically these products range from 1.05 for
helium to 1.5 for oxygen. Hydrocarbons will range from 2.0 to 6.0 with the
heavier oils being the higher of the group. Next is the family of alcohols and
ketones which range from 12 to 35. The very high dielectrics are aqueous
solutions and many of the common acids or alkalies. These are in the range of
75 to 90.
Probe Selection and Location
Very low dielectric materials will produce low capacitance changes
when compared between air (vessel empty) and the vessel full capacitance. The
liquefied gases have such low dielectric constants that the total
capacitance
change with standard probes is insufficient to produce full span
outputs.
Therefore, the probe or installation must be modified to produce a
larger
capacitance change. In low dielectric liquids, that are free of
solids, this is
most easily accomplished by using a concentric probe. This probe
will
produce several times more change per foot of measured product on
low
dielectric materials than the standard probes.
The minimum range (span) of the Level-Tel instruments is 10 Pf for
full
output. It is therefore required that the measured material produce
a
capacitance change of at least 10 Pl but a much higher change is
preferred. The maximum range (span) of the Level-Tel varies with the
particular instrument, but ranges from 1OO0 Pf to 6000 Pf and
represents
many feet of material when the dielectric is low.
The higher dielectric materials, such as alcohols, aqueous solutions
and
acids and alkalies, produce large capacitance changes with only a
few feet
of measured material. Therefore, these products are measured most
satisfactorily with standard probes, either rigid or flexible. The
probes are
mounted so that they are located in a vertical position within the
vessel as
indicated in figure 2. The probe should be located so that the
product does
not flow on the probe during filling. In applications where
considerable
turbulence of the liquid occurs, it is suggested that a 'stilling
well" be
installed.
Continuous Level Measurement in
Granular Products
General
Continuous level measurement of granular products is one of the more
difficult measurements because of the physical characterisitics of
the
materials. Capacitance techniques offer one of the more accurate,
reliable
and simplest methods of continuous level measurement of granular or
powdered materials in silos, hoppers and bins.
Typical Applications
A probe, either rigid or flexible, is installed within the vessel in
such a
manner that the measured material will surround the probe. The
capacitance, as sensed by the probe, will vary directly with the
level
(height) of the material in the vessel. Figure 3 shows a typical
graph of
dielectric constant versus capacitance generated for some common low
dielectric products. Remember that the net change is the difference
between air and the product being measured. For example, wheat flour
has
a dielectric constant of approximately 3.0. With a flexible probe
mounted
concentrically in a 24" vessel, wheat flour will generate
approximately 9 Pf
per foot on the probe. With air surrounding the probe (vessel
empty), the
capacitance is approximately 3 .5 Pf/ft Therefore, the wheat flour
will
produce a net capacitance change of 5.5Pf/ft Avessel of 15 feet in
height,
having a "full" design point of 12 feet, would produce a
new capacitance
change of 56 Pf ( 12' x 5.5 Pf/ft.)
The Level-Tel transmitters have a measuring range (span) from 10 Pf
minimum to several thousand Pf maximum. The minimum full span of
level for wheat flour is approximately 1,8 feet (10 Pf divided by
5.5 Pf/ft),
while the maximum span could be well over 100 feet. Notice that the
span
can vary from a few feet to many feet dependent upon the material
being
measured.
A general rule of thumb is that low density products, those which
weight
only a few pounds per cubic foot, generally have low dielectric
constants
while high density materials (in the same family) will have higher
dielectric constants. The change in dielectric constant with density
for a
given family of products results from the particle size and shape,
which
affects the amount of air entrapment in the material. The greater
the air
entrapment, the lower the dielectric constant. Mineral oxides
generally
have higher dielectric constants than flours, cereals, etc. because
they have
higher densities.
Probe Selection and Location
The probe must be mounted to hang vertically in the vessel so that
it may
continuously sense the rise or fall of the material. The probe, if
the rigid
type, is entered and supported at the top by its gland assembly.
Typically,
rigid probes are limited to 10 feet in length. For vessels requiring
longer
probes, the flexible type is used and is either mounted by means of
its
gland or suspended by means of insulators, which are supported by
mechanical means within the vessel. Typically the probe is mounted
midway between the center and sidewall of the vessel, as shown in
figure
4. ln this way, it measures the average volume of material in the
vessel
even when the product has a large angle of repose. Flexible probes
are not
fastened or tethered to the bottom of the vessel in most granular
applications.
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Proximity (Non-Contact) Measurements
General
On many level control applications, it is impractical to use
immersed
probes as the sensing element. Products which have adhesive
characteristics can often be controlled within close limits through the
use
of a non-contacting probe which is placed above the product surface.
Many
products, having a tendency to congeal and harden when exposed to
air or
changes in temperature, can make use of this technique. Other
applications
include those where thickness of product must be maintained within
tight
specified limits. All of these applications can make use of
"proximity."
Typical Application Data
There are 2
major areas which must
be considered when using
proximity :
1. The size of
the probe (or flat Plate)
2. The dielectric
of the material being sensed
The first consideration is the size of the probe which can be
installed in the
vessel whose level is to be controlled. The area of the probe must
correspond to the material level control requirements. Reference
figure 5
shows the relationship between the area of the sensing probe and the
capacitance change which will result as a conductive material
changes its
level. Consider a typical application where it is desired to control
the level
of a conductive adhesive material : the material is in a heated vat
and is
used as a coating product for paper, fabrics, etc. It is desired to
control the
level to + or - 0.25" and the maximum free space available in
the vat will
permit a probe approximately 6 x 16 inches. This probe should be
spaced
from the sides of the vat and any rolls or mechanisms by at least
one inch.
The active area of the plate will be 6 x 16 : 96 square inches.
Referring to
figure 5, it will be observed that for a plate of 100 sq. in., the
level will
move from a minimum distance of 1 inch to a maximum of 2.5 inches to
produce a capacitance change of 13.5 Pf, (9 Pf to 22.5 Pf = 13.5 ).
The second curve, which shows the capacitance generated for a 25A
sq, in.
plate, would produce a change in capacitance of 17 Pf for a level
change of
approximately two inches. To produce a change of 10 Pf would require
a
total level change of approximatety 3 / 1 inch. Notice that the
portion of the
curve considered has been through the center of the curve. This
should
always be done for any particular plate area since it is in this
area that the
capacitance change is most nearly linear to level change. Other
curves are
easily calculated from the formula and it is suggested that for any
particular application, the curve be plotted for the probe area in
order to
determine the desired spacing capacitance generated.
The second consideration mentioned earlier involved the dielectric
constant of the material. When the material is a conductive fluid or
product, such as aqueous solutions, acids, alkalies, etc. , the
curves may be
used directly. When the materials involved have low dielectric
constants, a
factor must be applied. For example, if the material is a
hydrocarbon
having a dielectric of 3.0 to 4.0, the capacitance change will be
approximately L/6 of the change which would have occurred from a
conductive or high dielectric material. Thus, it will require a much
larger
change in level to produce a given capacitance change, BUT THE
MAXIMUM
DISTANCE (LOWEST LEVEL POINT) IS LIMITED. DO NOT ATTEMPT TO
REPEATABLY CONTROL A PRODUCT BEYOND APPROXIMATELY 6 INCHES
FROM THE PLATE AT ITS LOWEST POINT.
