Capacitance and Radio Frequency (RF) Admittance

Point level control (using contact and proximity sensors) and continuous level transmission for liquids, granular solids, and liquid–liquid interface Design Pressure Routinely to 1000 PSI (7 MPa), others to 5000 PSI (35 MPa), specialized applications to 20,000 PSI (140 MPa) Design Temperature 500°F (260°C) maximum with insulated sensors; 1000°F (540°C) bare metal, sealed to 200 PSI (30 kPa); 2000°F (1100°C) bare metal at atmospheric pressure Excitation Less than 10 V @ 10 kHz to 1 MHz Wetted Materials Type 316 SS and TFE for common models, with options for CPVC, FEP, PE, PEEK, PFA, PP, PVDF, urethane, Hastelloy, Inconel, Monel, nickel, titanium Span 2 to 3 in. (50 to 75 mm) of insulating liquid to 1000 ft. (300 m) for immersion probes and 0.1 in. (2.5 mm) to 10 in. (250 mm) with proximity sensors

3.3 Capacitance and Radio Frequency (RF)

Admittance

D S KAYSER (1982) B G LIPTÁK (1969, 1995) J B ROEDE (2003)

Service Point level control (using contact and proximity sensors) and continuous level

trans-mission for liquids, granular solids, and liquid–liquid interface

Design Pressure Routinely to 1000 PSI (7 MPa), others to 5000 PSI (35 MPa), specialized applications

to 20,000 PSI (140 MPa)

Design Temperature 500 ° F (260 ° C) maximum with insulated sensors; 1000 ° F (540 ° C) bare metal, sealed

to 200 PSI (30 kPa); 2000 ° F (1100 ° C) bare metal at atmospheric pressure

Excitation Less than 10 V @ 10 kHz to 1 MHz

Wetted Materials Type 316 SS and TFE for common models, with options for CPVC, FEP, PE, PEEK,

PFA, PP, PVDF, urethane, Hastelloy, Inconel, Monel, nickel, titanium

and 0.1 in (2.5 mm) to 10 in (250 mm) with proximity sensors

Inaccuracy Horizontal, less than the diameter of the probe rod

Vertical, less than 0.1 in (2.5 mm) for bare single points in conducting material, roughly 1% of maximum active length for all insulated probes in conducting or interface service, 3% in insulating liquids (or 0.5% with dielectric compensation), roughly 5% for granular insulating solids with constant density and composition, and

2 to 5% for conducting granular solids

Dead Band Unmeasurable with analog instruments and horizontal probes; dependent on A/D

resolution with digital continuous instruments; small and application dependent on vertical single points, but optional dead band adjustment is available

Temperature Coefficient Extremely variable depending on (a) probe insulation and degree of probe-to-sheath

bonding in conducting materials, and (b) composition and density variation in insu-lating liquids

Damping and Time Delay Adjustable time delay of 0 to 30 sec is included on most single-point controls;

adjustable time constants up to 30 sec are available on most analog transmitters, and digital instruments offer zero to several minutes

Cost $200 to $800 for single-point controls; $500 to 1500 for two-wire level transmitters;

all with type 316 SS and TFE wetted parts; increased cost with exotic metals, hermetic seals, flange mounting, longer insertion length, digital output, dielectric compensa-tion, extended press and temp, and longer inactives

Vendors (partial list) ABB Process Automation Instrumentation Div ( www.abb.com/us )

Arjay Engineering Ltd ( www.arjayeng.com ) Babbitt International Inc ( www.babbittlevel.com ) Bindicator ( www.bindicator.com )

LT

CA To Continuous Receiver

To On-off Receiver CA

Flow Sheet Symbol

LS

3.3 Capacitance and Radio Frequency (RF) Admittance 431

BinMaster ( www.binmaster.com ) Delavan Inc.

Delta Controls Corp ( www.deltacnt.com ) Endress + Hauser Inc ( www.us.endress.com ) FMC Invalco ( www.fmcinvalco.com ) GLI International ( www.gliint.com ) HiTech Technologies Inc ( www.hitechtech.com ) K-Tek Corp ( www.ktekcorp.com )

Lumenite Control Technology Inc ( www.lumenite.com ) Magnetrol International ( www.magnetrol.com )

Monitor Technologies LLC ( www.monitortech.com ) Monitrol Manufacturing Co ( www.monitrolmfg.com ) Omega Engineering Inc ( www.omega.com )

Penberthy ( www.penberthy-online.com ) Princo Instruments Inc ( www.princoinstruments.com ) Robertshaw Industrial Products Div., an Invensis Co ( www.robertshawindustrial.com ) Scientific Technology Inc ( www.automationsensors.com )

Systematic Controls ( www.systematiccontrols.com ) Vega Messtechnik AG ( www.vega-g.de )

INTRODUCTION

Characteristic of probe-type sensors, the RF probes operate

by applying a constant voltage to a metallic rod and

moni-toring the current that flows This current is proportional to

the admittance or capacitance (if conductivity is absent)

from the metallic rod to a second electrode Because the

tank wall is the most convenient second electrode, most

instruments monitor current to ground (which is usually

connected through the probe mounting) The obvious

differ-ence between conductance and RF probes is the frequency

of that constant voltage Whereas conductance types use DC

or low-frequency AC, the RF items usually operate in the

range of 0.1 to 1.0 MHz (although special applications can

operate at 15 kHz or even lower frequencies) RF probes are

connected to their associated electronic units with coaxial

cable except when the electronics are integrally mounted on

the head of the probe

The classic shortcoming of capacitance probes is false

HI level indications caused by conductive coatings that

con-nect the above-level sensing element to ground (or the actual

process level) Since 1970, solutions to this problem have

existed in all but the heaviest, high-conductance process

sit-uations In the case of single-point level switches, the answer

is electrogeometric In the continuous level transmitters, the

approach is purely electrical

Because of the solid, no-moving-parts construction, there

is very little to deteriorate or fail once an RF probe is installed

Compatibility with process liquids is the most obvious obstacle

to a satisfactory life span This is no problem with common,

activity and accelerated permeation experienced by

poly-mers can produce unexpected results Abrasion of metals

and insulators is another cause of shortened life that must

be anticipated Baffles to protect the sensor from high-velocity

solids, combined with judicious location, can minimize this danger

Probe failure in heavily agitated tanks can be avoided by attention to structural considerations In the case of rigid probes, the most likely cause of breakage is fatigue failure caused by eddies rapidly pushing the probe in one direction and then in the opposite A support, with an insulated bush-ing, near the tip of the probe greatly reduces the possibility

of such a failure Flexible cable types can wrap around an agitator and fail in minutes if not adequately anchored to the tank structure If they are anchored without removing slack, they can whip back and forth, causing insulation failure and eventual breakage Intermediate supports are possible in highly agitated service using insulated bushings Beware of thick, conductive coatings that can cause substantial errors

at each support point These “shortcuts” to ground can defeat the electrical coating rejection in the worst cases Correct structural design is the responsibility of the system designer, not the probe supplier Most suppliers can give rules of thumb for their probes and provide structural details of the construc-tion It may require a consultant who is skilled in fluid dynam-ics and mechandynam-ics to arrive at a sure configuration in a highly agitated vessel

Process instruments that depend on electrical character-istics of the process material are at a disadvantage, given that the electrical character is of little interest to most instrument users Fortunately, exact values of conductivity (g) are never required, and changes in relative dielectric constant (K) are more important than the precise value In most cases, classi-fication as conducting or insulating goes a long way toward successful application Within the conducting category, it is sufficient to know that, except for completely deionized water, aqueous solutions will be conductive On the insulating side, it is generally sufficient to understand that most liquids will have a K of 2 or greater The main exceptions are liquids that would be gases at room temperature (with the notable

432 Level Measurement

except for ammonia, for which K> 15) and atmospheric

pres-sure Not only do these fluids tend to have K less than 2, they

tend to have much higher temperature coefficients of K than

insulators that are normally liquids

This section is divided into single-point and continuous

transmitter categories to reduce confusion The RF sensors

are somewhat unique in that single-point switches have a

substantial performance advantage over the continuous type

in terms of accuracy, temperature capability, coating

rejec-tion, self-checking, and reliability It seems that, in many

cases, the transmitters are thought to be the superior approach

to control, and they obviously are necessary to obtain a

pro-portional band In many cases, strategically placed

single-point units are a superior route to precise and reliable process

control Considering that any instrument can fail on occasion,

single-point probes provide highly reliable backup, with

self-test capability, and are superior to any transmitter

TYPES OF PROBES

The most basic probe configuration is a metal rod The rod

is insulated from a metal mounting element that connects it

to the process vessel via threads into a half coupling or a flange

that mates with one on a tank nozzle The insulated junction

of rod and mounting includes whatever seal is required

between the process and outside world The next step in

com-plexity includes an insulating coating on the rod, (Figure 3.3a)

that isolates it chemically as well as electrically from the

process Insulated probes should have the insulation securely

bonded to the metal rod over the entire range of service

tem-perature This bonding ensures that process pressure changes

will not compress air space and change the calibration Bond-ing is also important to minimize permeation, which is present to some slight degree with all polymers The addition

of a tight-fitting metal tube over part of the insulation, welded

to the mounting element (Figure 3.3b), will make the covered section of the probe “inactive,” because it will always see the same impedance to ground, regardless of the process material

on the outside A more sophisticated way to “inactivate” a

guard principal A tight-fitting metal tube, insulated both from the rod and the mounting element (Figure 3.3c), with a voltage identical to that on the rod, will not only deactivate that section of the probe but will also negate the rod to mounting capacitance These are sometimes referred to as

three-terminal probes, because there is now a rod connection,

a ground connection, and a guard (sometimes called a shield)

style connections An additional variation is the probe that carries its own intrinsic ground reference This can be a larger

3.3d), with bleed holes at the top to avoid compressing gas

as the liquid rises in a closed chamber Perforated tubes, insulated and bare ground rods, as well as structural cages (Figure 3.3e) are also available for various conditions of agitation, temperature, and chemical compatibility The sen-sors, which use a ground wire wrapped in a helix directly

on the probe insulation, are unstable, unreliable, and facilitate coating

A three-terminal probe, with a plate welded on for greater

sensor The field from the guard can even be used to direct

or focus the field from the sensing element and determine the region of sensitivity In proximity measurements, the

FIG 3.3a

Insulated two-terminal probe.

Center Rod Connection Process Seal

Mounting Flange

Insertion

Length

Metal Rod

Polymer Insulation

Polymer Plug

FIG 3.3b

An insulated, two-terminal probe with grounded inactive section.

Center Rod Connection Process Seal

Mounting Flange Inactive

Length

Insertion Length

Metal Rod Polymer Insulation

Grounded Inactive Section

Polymer Plug Weld

3.3 Capacitance and Radio Frequency (RF) Admittance 433

capacitance change (proportional to sensor area) is typically

very small but stable, because it is effectively looking at the

air capacitance between sensor and process material Even

minute capacitance changes due to temperature or stress in

the mounting area could be catastrophic with an unguarded

probe The distance between the probe and process material

should be as short as feasible The capacitance produced is

inversely proportional to this distance This means that beyond

10 in (250 mm), the capacitance change becomes extremely

hard to detect The error produced by splashing and

conden-sation buildup on the plate is fairly benign—never more than

the actual coating thickness, and usually less Spans of 2 or

3 picofarads (pF) have been used successfully Such small ranges are the result of level change in shallow pans, with limited space for proximity plates

The electronic guard principle can be employed in an infinite variety of geometries A typical example is the flat plate probe (Figure 3.3g), which mounts flush with the tank wall This is very effective in reducing abrasion, eliminating bending of low-level sensors under heavy mechanical loads, and eliminating sparks from static discharge The monitored current flows from the center plate, and the guard surrounds

FIG 3.3c

A three-terminal probe that employs the “electrical guard.”

FIG 3.3d

A two-terminal probe with intrinsic, concentric ground reference.

Center Rod Connection Guard Connection Process Seal

Mounting Flange

Insertion

Length

Guard Element Polymer Insulation

Polymer Insulation

Metal Rod

Guard Length

Center Rod Connection Process Seal

Mounting Flange

Insertion

Length

Metal Rod Polymer Insulation

Concentric Ground Tube

Weld Bleed

Holes

FIG 3.3e

A two-terminal probe with a cage, for grounding in viscous liquids.

FIG 3.3f

A three-terminal probe with plate for proximity sensing.

Center Rod Connection Process Seal Mounting Flange

Insertion Length

Polymer Insulation

Grounding Cage Welds

Polymer Plug

Metal Rod

Center Rod Connection Guard Connection Process Seal

Mounting Flange

Guard Element

Proximity Plate

Polymer Insulation

Polymer Insulation

434 Level Measurement

it, interrupting a path to ground through any coating on the

face of the probe

For the ultimate in seal reliability and hermeticity, plastic

welding offers unique benefits By plastic welding a polymer

flange facing to the probe insulation (Figure 3.3h), a

high-pressure seal is formed (and has been used to 5000 PSI [35

MPa] with the correct flange rating), with the normal process

seal acting as a backup This type of weld is not possible

using TFE, which does not melt, but most other polymers

are candidates It also precludes the use of thin probe

insu-lation The flange facing should be used as the only gasket,

because it must be pinched between the flange faces Only

raised face flanges provide the correct sealing, so RTJ and

flat-faced flanges are not applicable

MOUNTING AND TANK ENTRY

One of the most frustrating barriers to good RF probe

appli-cation is inadequate or misplaced tank access on existing

tanks Often, adding a nozzle or even a half-coupling is

impossible, because the tank is pressure coded, requires ardu-ous inerting, or cannot be taken out of service One very simple-minded and geometrically demanding approach is to use a bent probe inserted through a side entry so as to get its active area to the desired measurement location A flange mounting

is usually mandatory to avoid “screwing,” which would cause the bent probe to rotate inside the tank like an airplane pro-peller Its angular location, once the thread is tightened, would also be a question If a suitable nozzle is available, it must be short enough to get the bend in the probe “around the corner” before it binds If the measuring leg of the probe

is relatively long, it must not hit the opposite wall of the vessel before the bend allows it to pivot Another drawback

to the bent probe is the requirement that any insulation be flexible enough to tolerate the bending process without tearing or splitting This limits the selection to soft, thick insulation and therefore relatively low capacitance (60 to

100 pF/ft)

Many strategies allow us to avoid the bent-probe trap by using a little bit of imagination Every tank or bin should have a vent or pressure relief pipe It is usually possible to tee that pipe so that the vent or relief goes off to the side and

a probe has straight access into the process (Figure 3.3i) This does not affect the pressure code of the vessel, since it is external plumbing Other ways to avoid the “pretzel probe” include the following:

• Use a grounded inactive section to get through various impediments

• “Dog-leg” a pressure gauge to give the probe a straight shot

• Cut a hole in the building’s roof or ceiling in lieu of headroom

FIG 3.3g

A three-terminal flush mounting probe.

FIG 3.3h

Probe with plastic-welded process seal (Courtesy

AMETEK-Drexelbrook.)

Sensing Plate

Polymer Insulator Mounting Holes

Guard Element Gasket Surface

Connection Enclosure

Hermetic

Facing

Probe Insulation

FIG 3.3i

Vent pipe serving dual function for tank access.

Electronics or Connection Enclosure

Vent Added Tee

Probe Inactive

Section

3.3 Capacitance and Radio Frequency (RF) Admittance 435

col-lector or a side of the tank

• Add a stand pipe (sidearm) parallel to the tank (Figure

3.3j)

• Tee into a fill pipe (this needs detailed application

anal-ysis)

Ingenuity will always trump bent probes for performance,

cost, and efficiency

ELECTRONIC UNITS

Electronic units for single-point instruments are available in

line-powered configurations for 24 VDC; 120 V, 50 to 60 Hz

AC; and 230 V, 50 to 60 Hz AC They also can be obtained

with “universal power” capability that allows them to use any

of these plus 130 VDC The output from these units is

gen-erally a set of double-pole, double-throw relay contacts Some

of the DC-powered instruments use an NPN or PNP output

transistor to effect the switching There are also two-wire,

loop-powered versions that offer intrinsic safety and

auto-matic self-checking The signal from these instruments is a

high or low current within the 4- to 20-mA range Various

types of automatic calibration are available for these

instru-ments, but they all encounter certain conditions that prevent

them from being calibrated properly under every possible

condition Regardless, the calibration of single-point

instru-ments is hardly rocket science

Level transmitters are primarily loop powered, although line-powered items are available from some suppliers The traditional analog 4- to 20-mA instruments have been giving way to those with microprocessors on board Most of the digital instruments are capable of communication using one

of the HART, Honeywell, or fieldbus protocols This allows interrogation and modification of the instrument by means

of a digital communicator One of the most popular features

of these instruments is their ability to calibrate on any two points in the range It is possible to enter the correct output (in milliamps or level units) at an existing low level and enter

a high point days later The instrument locks the input/output curve into those two points The signal from the instrument

is either analog 4- to 20-mA or digital The digital mode allows more than one transmitter to use a single loop The limited power available at the transmitter, combined with the multi-tude of calculations being executed, causes digital instru-ments to be considerably slower than the analog ones Response times of 3 to 4 sec are possible, whereas analog instruments are capable of responses within 100 to 300 ms For small tanks with high fill and drain rates, the digital instru-ments might not be an option

A hybrid instrument is the “multipoint” control It is essentially an analog instrument with internal, adjustable pick-offs and multiple relay outputs It is useful for sump control where several pumps might be involved, but the absence of an analog output makes calibration lengthy Each pick-off must be adjusted with the level at the desired point

on the probe This type of instrument is specified for HI, HI-HI,

FIG 3.3j

Side arm (cage) mounting, when there’s no other way (Courtesy of Robertshaw Controls Co.)

Probe

Cage Isolation Valves

436 Level Measurement

and HI-HI-HI points at times This doesn’t make much sense,

because one failure kills the whole operation

SINGLE-POINT SWITCHES

The typical capacitance switch employs a vertical or

hori-zontal probe, either polymer insulated or bare metal,

project-ing some distance into the process vessel with the center rod

insulated from the mounting (Figure 3.3k) With any gas or

current flowing from the center rod to the metallic tank wall

When a liquid or granular solid covers the center rod, the

current from there to ground will increase This change in

current is detected by the electronic unit, which switches the

output When the process material drops below the probe,

the RF current decreases and, hopefully, returns to its

previ-ous low-level state If the process material is thick and

con-ductive, any coating remaining on the insulator between

probe and mounting will maintain a higher RF current level

and can cause the switch to continue signaling a high level

The answer to false high-level indications is based on the

classic electronic guard principle By interposing a third

metal-lic element (shield) with an identical RF voltage between the

flow from center rod to mounting The shield element

sup-plies whatever RF current the coating demands, but this

cur-rent is not included in the measurement “Length is strength”

in capacitance probes Longer active lengths translate to more

substantial capacitance changes in insulating processes, and

longer interelement insulators allow the instrument to reject

higher conductance coatings This means using horizontal

probes for switch points close to the bottom and top of a tank rather than a vertical 2 or 3 in (50 or 75 mm)

Conducting Process Materials

Conducting materials (aqueous, metals, and most forms of carbon) carry the ground potential of the tank walls right to the probe If bare metal, the probe will signal “high level” the instant it contacts the process If it is insulated, the switch-ing point will depend on the capacitance settswitch-ing of the instru-ment With a vertical, insulated probe, it is possible to adjust the level at which the switching takes place by varying the capacitance setting of the instrument A horizontal probe of either type allows no level adjustment other than by relocat-ing the mountrelocat-ing

Coating is a serious consideration in all conducting mate-rials, and the guard-type construction is usually required With bare metallic probes mounted vertically, the plain capacitance probe will do as long as the process never reaches the mount-ing area and no crystallization or heavy condensation occurs When using the guarded probes, the guard should be long enough to project well into the vessel, beyond nozzles and potential wall buildup

Insulating Process Materials

The insulating materials (oils, solvents, resins) are subtler in their effect on the switching circuit A horizontal probe pro-duces a sharp capacitance change over the thickness of the center rod Any portion that is in a nozzle should be inactive, either with the guard or a grounded inactive With a vertical probe, capacitance increases gradually as more of the active length is covered The switching point is adjustable even if the probe is bare metal Good practice requires that the probe

be at least 2 in (50 mm) into the process material at the desired switching point Attempting to get switching at the tip of the probe will lead to unreliable performance, because the slightest change in probe mounting or electronics can cause a constant high-level signal

Dielectric constant (K) is relative to the absolute dielec-tric of a vacuum (K of gases barely differs) This means that the capacitance (and the proportional RF current) in air will

be doubled in gasoline (K = 2) It will be multiplied by 20

in ethanol (K= 20) It will only be raised by 60% in liquid carbon dioxide (K= 1.6) Insulating coatings are a very minor problem Just avoid bridging to a grounded part

Plastic, Concrete, or Fiberglass Tanks and Lined Metal

The absence of metallic contact for a ground reference is seldom a problem using single-point RF probes in conducting process media The probe will indicate high level whenever

it touches the process Unlike metallic tanks (or those with metal pipes, pumps, or grounding rods), it must be tuned to

a value less than the capacitance-to-ground value of the tank Most tanks have at least 10 pF to ground, which is a perfectly adequate level for a reliable measurement Probes with a

FIG 3.3k

Bare metal, two-terminal probe.

Center Rod Connection Process Seal

Mounting Flange

Insertion

Length

Metal Rod Polymer Insulation

3.3 Capacitance and Radio Frequency (RF) Admittance 437

guard element should not be mounted horizontally in

ungrounded tanks with conductive contents The guard can

drive the entire process at the same voltage as the center rod,

hence there is no current flow and no high-level signal

In the case of an insulating medium in a fiberglass tank,

even metal pipes offer little help to the miniscule

capaci-tances The only sure answer is a vertically mounted probe

with its own metal ground rods or concentric tube This is

also a good precaution against RFI from walkie-talkies and

other sources, which can cause false high-level signals

Metal tanks that are rubber or plastic lined and

steel-reinforced concrete vessels, on the other hand, represent

excellent RF grounds The capacitance from process to metal

structure is very high as compared with the capacitance

pro-duced by an insulating medium This makes it look like a

short circuit to the RF current It is excellent for conducting

media, too, but it still requires the avoidance of horizontal

probes with driven guards Underground concrete or

fiber-glass tanks (except in desert conditions) present a similar

ground configuration

Concrete structures of cinder block with no vertical steel

reinforcing bars are completely ungrounded and perform

exactly the same as fiberglass tanks

Interface

Electrical sensing is the premier method of detecting the

interface between an insulating and a conducting process

medium The typical margin between the conductivity of

organic and aqueous phases is greater than 1000:1 The

mea-surement is completely independent of temperature and

den-sity variation Probes may be mounted vertically or

horizon-tally Vertical probes should be inactive down to about 6 in

above the desired interface control point This can be

accom-plished by use of the electronic guard, a grounded inactive,

or a short probe mounted from the rear (rear mount) on the

end of a suitable pipe The instrument will indicate high level

as soon as conductive material contacts the tip of the bare

probe In the case of heavy oil separators, it may be desirable

to use a probe with sharp edges machined into the tip

(Figure 3.3l) This will ensure good contact between water

and steel in spite of oil coating An interface detector with

bare metal rod should be tuned to the maximum capacitance

level of which it is capable

Horizontal probes should be bare metal with relatively

long insulators between probe and mounting (ground) Typical

proportions, to maximize the margin between insulating and

conducting phases, would be 12 in (300 mm) overall length

with a 10-in (250-mm) long insulator The use of sharp edges

machined on the tip of the probe is also advantageous in this

orientation This is one situation in which the electrical guard

is of no advantage (coatings are usually insulating rather than

conducting) and can actually be a drawback if the guard drives

the entire conducting phase at the same potential as the probe

This phenomenon is usually a product of insufficient

ground-ing, but it has been observed in metal tanks

GRANULAR SOLIDS

The best approach for establishing the high level in large silos is usually via a flexible cable probe with a long, thin weight at the bottom It can be mounted close to the fill point (Figure 3.3m) and negate most of the angle of repose that side-mounted sensors encounter The flexible aspect allows

it to swing out of the way when struck by incoming solids The actual switching should take place on the weight that will be least vulnerable to abrasion In insulating materials,

at least 6 in of weight should be covered at the desired switching point Conducting materials, of course, will switch

as soon as they touch the tip of the probe Incoming material will not cause false high-level indication as long as it is in free fall This is because there is a very small air capacitor between each grain and all the others All this series air capacitance means that any deviation from normal air capac-itance is a negligible quantity If the incoming material is compressed (especially conducting materials), it IS possible that it could cause a trip, so the probe should be located accordingly Rigid probes, located near the fill, are suitable for smaller bins and lighter duty

FIG 3.3l

Oil shedding tip for interface with viscous organic phase.

FIG 3.3m

High-level probe for heavy granulars in large silos.

Metal Rod

Sharp Edges

Process Granular

Electronics or Connection Enclosure Half Coupling

Fill

Top of Silo

Insertion Length

Cable

Weight

438 Level Measurement

Horizontal probes mounted in the bin wall should be of

the “guarded” variety to avoid false high-level alarms—

especially for low-level service The primary concern is

bend-ing or abrasion, which becomes more acute as the depth and

density of the material increases Short, fat probes have much

better lifespan, and it is possible to use a flat-plate probe

(Figure 3.3g) mounted flush with the wall for optimal life

CONTINUOUS TRANSMITTERS

Probes are most commonly mounted vertically from the top

of the tank Angle mounting through the side of a tank is

possible, as is mounting up from the bottom When tank

penetrations are at a premium, it is often possible to “tee”

into a vent or drain line for probe mounting There are even

cases in which probes have been successfully “teed” into fill

nozzles, but this requires considerable application analysis

to predict the effect of incoming material The insertion

length of the probe should not extend beyond the desired

measuring range by more than 5% An active probe that is

not producing signal can still produce errors

Conducting Liquids

The main concept required to understand this class of

appli-cations is called saturation Assume that an insulated probe

is immersed in deionized water with minimal conductivity

(Figure 3.3n) The addition of drops of hydrochloric acid to the vessel will gradually increase the conductivity The output will also rise as more RF current flows from probe to ground Eventually, the conductivity will be high enough that the resistance (R) from probe to tank wall is negligible compared

to the capacitive impedance of the probe insulation (C5) Further increases in conductivity will make no observable difference in the RF current and, therefore, the output (i.e., the RF current has reached its saturation point) This is the concept that makes the RF transmitter an instrument rather than a lab curiosity A probe that is not saturated will have

a calibration that is a function of two variables (conductivity and level), just as a d/p transmitter calibration is a function

of density and level It is possible to adjust the threshold of saturation by adjusting insulation thickness (hence capaci-tance) and excitation frequency Raising the impedance of the insulation lowers the threshold, and vice versa

accom-plished A reasonable question might be, “Why not just use the 0.1µS/cm threshold at all times?” The answer is “coating rejection.”

The higher impedance that correlates with a low saturation threshold is less able to ignore high-conductivity coatings

In fact, a large mismatch may allow the coating to saturate the probe so that the output reflects the actual level plus coating length In general, the path to best conducting coating rejection lies in the highest probe capacitance and excitation frequency that will allow saturation by the process material

FIG 3.3n

Electrical representation of an insulated probe in conductive liquid (Courtesy of The Foxboro Co.)

L

I

A1

A2 B

Ce

Ce

C2 C4

C2+ C 4

C3 C5

C3+ C 5

C1

C4

C1

C2

C2

C4

C3

C5

Ka

Kp

R

R = Any Value

3.3 Capacitance and Radio Frequency (RF) Admittance 439

being measured A special case for coating rejection is one

in which the process liquid is conductive but can dry on the

probe, forming an impervious, insulating coating An

exam-ple is latex paint The liquid is quite conductive, but the dried

paint adds to the insulation thickness of the probe, decreasing

its capacitance and changing the calibration The solution is

to use a thick (low-capacitance) probe insulation so that thin

additions will have negligible effect on calibration By using

the highest possible frequency, conductive coating rejection

will also be maximized, and the resulting accuracy will be

the best possible compromise

mea-surement allows the electronic unit to measure two variables:

the capacitive component of RF current and the resistive

component of RF current The actual liquid level that

satu-rates the probe produces a pure capacitive current The

con-ductive coating, because it is relatively thin, has a much

higher resistance than the bulk liquid, so it will produce both

capacitive and resistive phase current By subtracting the

resistive component from the capacitive, the effect of

con-ductive coating on the output will be decreased In fact, once

the coating is longer than a nominal value, the two RF current

phases equate, and the effect of coating is precisely zero

Maximum coating error occurs when the coating length is

relatively short and its resistance therefore is low The

max-imum error is a fixed, predictable number of inches for a

given probe capacitance, excitation frequency, coating

thick-ness, and conductivity This means that percent inaccuracy

due to coating will be greater on short ranges than on long

ones In other words, length is strength

Insulating Liquids

Probes for measuring insulating liquids should generally

include their own parallel ground reference to guarantee a

uniform distance to ground through the process liquid

Lin-earity, sensitivity, and immunity to RFI are enhanced by a

concentric ground tube (Figure 3.3d) or other construction

In some cases, it is efficacious to use a metallic tank wall,

baffle, or ladder to furnish the required parallel ground

ref-erence This is frequently the case in pharmaceutical or

bev-erage applications where a concentric tube interferes with

any clean-in-place function It is also common in tall tanks

that require flexible cable sensors and with slurries, which

can accumulate solids and plug the ground reference

Changes in dielectric constant will cause a change in

calibration If composition is constant, temperature will be

the only concern for variation of K Because K is proportional

to the number of molecules between probe and ground,

out-put (as with a d/p transmitter) will be proportional to density

and, hence, the weight of process material For cases in which

constant composition and a reasonably narrow temperature

range are not possible, a transmitter with dielectric

compen-sation is available By using such an instrument, it is possible

to obtain a level signal that is independent of dielectric, and

even conductivity, when tanks are not dedicated This tech-nology demands two conditions:

1 A short inactive section at the bottom of the probe, approximately 6 in (150 mm)

2 Homogeneity of the process liquid (no stratification) The compensation is accomplished by making two inde-pendent measurements The first measurement is made with

a short sensor (composition probe) below the tip of the level probe (Figure 3.3o) This segment is assumed always to be covered by the process liquid Its capacitance is proportional

to the electrical character of the process material The second measurement uses the level probe Its output is proportional

to the level times the electrical character of the process fluid

By dividing the output of the level probe by the output of the composition probe, the transmitter output becomes inde-pendent of the electrical properties Please note that conduc-tive coatings can cause large inaccuracies in these particular transmitters Even so, conducting liquids that do not coat will

be measured with accuracy equal to the insulating ones For the sake of utility, both probe segments are usually provided, coaxially, in the same assembly It is also possible to use two completely separate probes, with the composition probe mounted horizontally below the tip of the level probe Inac-curacy can be limited to 0.5% over a wide range of electrical values and a wide temperature range

Continuous Liquid–Liquid Interface

Two elements are key to successful interface level measure-ment: immunity to total level variation and maximum margin between the signal contribution of the organic and aqueous

FIG 3.3o

A probe with a second element for dielectric compensation (Cour-tesy of AMETEK-Drexelbrook.)

Concentric Shield (Ground Reference)

Reference (Composition) Sensor

Insertion Length (I.L.)

5.5"

Measured Level Level Sensor