Saturday, May 31, 2014

Robertshaw Capacitance Level Information

Flow Factor Robertshaw Capacitance Level Control

For a quote or information request, please click HERE.

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.


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.


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.


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
Continuous Level Measurement in Granular Products

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

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


Proximity (Non-Contact) Measurements

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


Friday, May 30, 2014

Capacitance Level Control Definitions


For a quote or part number ID, please click HERE.

Capacitance Level Definitions

High Level Fail Safe (HLFS):
Relay de-Energizes on High Alarm condition. Thus the alarm condition with power failure.  Example: To Keep tank from overfilling with power failure.

Low Level Fail Safe (LLFS):
Relay de_Energizes on Low Alarm Condition. Thus alarm condition with power failure. Example: To Keep pump from running dry and burning up with a ower failure.

Time Delay:
Used for Splashing or wavy processes to prevent cycling on and off.

Response Time:
How fast the unit goes to alarm condition after set point is reached.

C, NO, NC Contacts:
Contacts are normally shown in the non-energized alarm condition – no power, shelf condition.  Relay energizes in non-alarm condition.


This is the SET POINT. Where the desired actuation is to occur.

This is the distance, in Pico-Farads, inches or feet from ZERO to where the RESET is to occur. Sometimes called Deadband or Pump On – Pump Off.


The 0% Level point on the sensing probe.

The full level point on the sensing probe.

The distance in Pico-Farads, from the Zero Level point to the FULL Level Point (100%).

The Minimum amount of Capacitance present when no product is present in the vessel.
This is:
The Capacitance of the Probe Gland (and sheath and colling extension, if used) PLUS
The Capacitance generated between the probe and the vessel with air as the dielectric. In other words, the absolute minimum value of capacitance generated in that particular probe/vessel installation.

The Terminal Capacitance divided by the span capacitance. This ratio must not be exceeded where specified on product specification sheets (all instruments do not have this restriction).

Terminal Adjustment/Zero Adjustment Range/Maximum Zero Suppression:
This is the maximum “Zero Suppression” of the capacitance value at the zero point level in the vessel.  This value cannot be exceeded.

Capacitance Change:
The amount of capacitance difference between ZERO and 100% points in the continuous level measuring instruments; the amount of capacitance difference between product and no product in point level measuring instruments without differential; or the amount of capacitance difference between ZERO and Differential points in Point Level measuring instruments with differential adjustment.

This is the total capacitance value (not capacitance change) with tank of vessel at the 100% level (includes terminal capacitance). This value must not exceed the maximum span of the instrument.