Engineer’s ClassVisit To

March, 2012

Samuel M. Herb, PE

Some Applications

Two Categories of Measurement• Direct measurements include

– Sight glasses– Dipsticks– Floats.

• Indirect include – Differential Pressure– Sonic– Nuclear Radiation– RF Admittance(Those that make a measurement of some other

variable, which is only related to level)

Level / Inventory Measurement

How far from fullHow far from emptyHow much material

WHERE

HOWMUCH

Two Types of Level Measurements....• Point Level

– High level & spill prevention alarms,– Low level & pump protection alarms,– Pump controls to turn pumps on & off

at certain distinct levels.

• Continuous Level- Process control - Inventory control - Inventory management

Point Level Applications

mPm

mPm

m P m

mPm

mPm

High Level

Low Level

Leak Detection

Pump Protection

Filling Line

Additional Point Level Applications

Interface Between Fluids

Will often have several combinations of sensors for sophisticated needs

Continuous LevelComposition Change

Point Level

Considerations – Process Conditions

• Pressure• Temperature• Agitation• Corrosion• Foaming• Explosion Hazards• Dangerous or Lethal Materials

Considerations –Material changes

• Material Density• Chemical Composition• Material Buildup• Electrical Properties

Some Level Technologies

Bubblers Conductivity Differential Pressure (D/P) Displacers Floats Gap Switches Load Cells Microwave

NuclearPaddle WheelsPlumb Bobs

(Yo Yo’s)RadarRF AdmittanceTuning ForksUltrasonic

Dip Stick Level Measurement

CALIBRATED GRADUATIONSFOR LEVEL VALUES

Basic Sight Glasses

CLOSE-LOOP SIGHT GLASSSCALE

OPEN-END SIGHT GLASS

(A) OPEN OR VENTED VESSEL (B) PRESSURIZED VESSEL

Sight Gauge

• Typical Price: $300 to $2500• Maximum Temperature: 700°F (370°C)• Accuracy: 0.25" (6mm) • Application comments:

– Excellent for thin fluids – Limited for interfaces, thick fluids, and foams– Must have same temperature as tank – Foam and boiling problems – Opaque coatings cause incorrect readings.

Float & Cable Arrangements

SCALE

WEIGHT

FLOAT

Tape Floats (& Servos)

• Typical Price: $$1000 to over $2500• Maximum Temperature: 300°F (150°C)• Accuracy: 1" (3mm)• Application comments:

– Excellent for continuous measurement of thin fluids– Limited with thick– Can ignore foams– Servo plumb bob is suitable for solids and interface– Mechanical hang up is biggest problem.

Float• Typical Price: $300 to over $2500 for continuous

measurement; $100 to $1000 for point level switches• Maximum Temperature: 500°F (260°C)• Accuracy:

0.1 to 3% span for continuous; 1" for switches • Application comments:

– Excellent for continuous measurement of thin fluids– Limited for interfaces and thick fluids– Ignores foam, but can be used to measure it– Point level switches are fair for thick fluids– Moving parts limit most designs to clean service– Only density-adjusted floats can detect interfaces

Application Box Score Color Codes

Point Level

Continuous

Liquids Granulars Slurries Interface

Liquids Granulars Slurries Interface

Green -OK Yellow- Caution

Clear- Not OK

Float Trade-offs• Advantages

– Unlimited tank height– Can achieve high accuracy– Low cost if not remote reading

• Disadvantages– Moving parts exposed to fluids– Limited pressure rating– Not very good in agitated vessels– Not suitable for granulars– High Maintenance

Continuous

InterfaceLiquids Granulars Slurries

Liquids Granulars

Point Level

Slurries Interface

Air Bubble System

INDICATOR / RECORDERAIR FILTER / REGULATOR

FROM AIR ORGAS SUPPLY

VISUAL FLOW INDICATORLEVEL

That pressure is related to change in height of liquid in vessel, AND density

PRESSURE

Bubbler• Typical Price: $300 to $2500• Maximum Temperature: No limit• Accuracy: 0.5 to 1% span assuming constant density

• Application comments: – Excellent for continuous level of thin fluids– Both conductive and insulating fluids– Will ignore foam– High maintenance– Needs clean gas supply

Bubbler Trade-offs• Advantages

– Low Cost– Simple Design– Easy to Install

• Disadvantages– Susceptible to plugging– Not suitable for closed or

pressure vessels– Requires frequent

maintenance– Requires compressed air

supply

pressure gauge

air in

Continuous

InterfaceLiquids Granulars Slurries

Liquids Granulars

Point Level

Slurries Interface

Diaphragm• Typical Price:

– $300 to $1500 for continuous – $100 to $300 for point level switches

• Maximum Temperature: – 350°F (175°C)

• Accuracy: 1 to 3% span for continuous,assuming constant density; 1 to 2" (25 to 50mm) for switches

• Application comments: – Continuous measurement is limited in thin fluids, – air in thick, will ignore foam; – Submerged sensors need reference to

atmospheric pressure;– Infrequently used switches suitable for granular

solids only.

Differential Pressure (D/P)

• Advantages– User familiarity– Easy installation/calibration– Reasonable Instrument Cost

• Disadvantages– Affected by product density– Bottom Mounted– Valving and piping increase cost– Multiple Fugitive Emission points

Continuous

InterfaceLiquids Granulars Slurries

Liquids Granulars

Point Level

Slurries Interface

Trade-offsSealed System Diaphragm

Differential Pressure• Typical Price: $300 to $2500 for continuous measurement;

$100 to $1000 for point level switches• Maximum Temperature: 350°F (175°C)• Accuracy: 0.25 to 1% span for continuous, assuming

constant density; 1 to 4" (25 to 100mm) for switches • Application comments:

– Excellent for continuous measurement of both thin and thick liquids and ignores foam;

– Risk of plugging by slurries requires diaphragm seals, repeaters, purging, or sealing legs;

– Point level switches are limited to clean liquids with constant specific gravity.

Interface Measurement with Displacers

(COURTESY OF MASONEILAN-DRESSER INDUSTRIES)

2

10

4

3POUNDS

2

10

4

3POUNDS

2

10

4

3POUNDS

1.4 lb 1.2 lb 1.0 lb

14-in. WATERLEVEL

7-in. WATERLEVEL

0-in. WATERLEVEL

2.25 in.DIAMETER14 in. LONG

(a) (b) (c)

DISTILLATE

WATER

WATERWATER

DISTILLATE

DISTILLATE

Displacer• Typical Price:

– $1000 to $2500 for continuous – $100 to $1000 for point level switches

• Maximum Temperature: 850°F (450°C)• Accuracy: 0.25 to 1% span for continuous,

assuming constant density;

0.2 to 0.5" (5 to 13mm) for switches• Application comments:

– Excellent for continuous measurement of thin fluids,– Only fair for interfaces and limited for thick fluids,– Ignores foam; – Point level switches are excellent for thin fluids,

but only fair for thick fluids and locating interfaces; – Neither are recommended for sludge or slurries;

vacuum and high viscosity can cause dynamic instability.

Displacer Trade-offs• Advantages

– Limited motion– Few moving parts

• Disadvantages– Calibration changes with density– Bottom mounted– Buildup on displacer

can cause calibration shift– Valving increases cost– High cost with long spans

Continuous

InterfaceLiquids Granulars Slurries

Liquids Granulars

Point Level

Slurries Interface

Capacitor Probe

LT

C

PROBE ONE:PLATE OF CAPACITOR

METAL WALL:OTHER PLATEOF CAPACITOR

It Starts With The Capacitor

AC

Conductive Plate

Conductive Plate

DielectricInsulating Material

AC

Factors that Affect CapacitanceDistance Between Plates

AC

AC

DistanceCapacitance

DistanceCapacitance

Factors that Affect CapacitanceArea of Plates

AC

AreaCapacitance

AreaCapacitance

AC AC

Factors that Affect CapacitanceDielectric of the Insulator

AC

DielectricCapacitance

DielectricCapacitance

AC AC

The Tank and Probe form a Capacitor

C =k Ad

Where:C= Capacitance in pFk= Dielectric Constant of material A= Area of the platesd= Distance between platesk air = 1

d

C

k oil = 2

A

The Tank and Probe form a Capacitor

C =k A

d

Where:C= Capacitance in pFk= Dielectric Constant of material A= Area of the platesd= Distance between plates

k air = 1

A

d

Ck oil = 2

Capacitance Limitations

• Changing Density• Liquids that Coat

Cut Monitor

Detects Change in Dielectric Constant of material… …such as contamination

getting into your product

Measuring with Changing Dielectric

Two Probes – one references “K”

Single Probe with two sensors – lower one measures “K”

BUT: RF Admittance

Uses RF signal to overcome limitations of Capacitance

RF Admittance Trade-offs• Advantages

– By far the most versatile technology– Wide range of process conditions– Works with all types of materials– On-off or continuous measurement

• Disadvantages– Application Sensitive [ALL TYPES ARE]– Insulating granular measurements

require special considerations– Changing dielectric measurements

requires special transmitter and sensor

Continuous

InterfaceLiquids Granulars Slurries

Liquids Granulars

Point Level

Slurries Interface

Capacitance / Radio Frequency• Typical Price: $300 to over $2500 for continuous measurement;

$100 to over $1000 for point level switches• Maximum Temperature: 2,000°F (1100°C)• Accuracy: 0.5 to 3% span for continuous;

0.125 to 2" (3 to 50mm) for switches• Application comments:

– Excellent for thin fluids & for measuring interfaces.– Fair to excellent in thick fluids and slurries.– Can ignore organic foam but can measure both aqueous

and organic foams.– As switch is generally very good for powder, chunky, or

sticky solids, but limited as continuous measurement of these.– As continuous measurement also cannot measure interface

between conductive layers or between liquids & solids.– As a switch, conductive coatings produce false high

without guard-type probe.– Short insertions can be a problem.

Conductivity Switch

• Advantages– Low Cost

• Disadvantages– Conductive,

non-coating liquids only– Insulating coating

Continuous

InterfaceLiquids Granulars Slurries

Liquids Granulars

Point Level

Slurries Interface

Uses Ions in Material

++ +

+–

–

––

––

Trade-offs

Conductivity Switch

• Typical Price: $100 to $1000• Maximum Temperature: 1800°F (980°C)• Accuracy: 0.125" (3mm) • Application comments:

– Excellent for conductive thin fluids– Fair for detecting interface– Limited for thick fluids and slurries or solids– Can detect conductive process materials– Thin insulating coatings produce false lows

while conductive thick fluids can show false highs– Ignores organic foams but can measure aqueous foams

Level Measurement by Weight

LOAD CELLS

INDICATORRECORDER ORTRANSMITTER

Load Cell Trade-offs• Advantages

– Non- Intrusive– Mass measurement– Ok for liquids, granulars, and slurries– Hi Accuracy in weight

• Disadvantages– Best specified when vessel is ordered– Usually requires free standing structure– Only weight measurement...not level– May be affected by ice, snow, and

wind loading

Continuous

InterfaceLiquids Granulars Slurries

Liquids Granulars

Point Level

Slurries Interface

Tuning Fork

• Advantages– No Calibration– Unaffected by electrical

and physical properties• Disadvantages

– Unpredictable with foam– Product Buildup– Moving Parts

Continuous

InterfaceLiquids Granulars Slurries

Liquids Granulars

Point Level

Slurries Interface

Material Dampens Vibration

Trade-offs

Vibrating Switch• Typical Price: $100 to $1000• Maximum Temperature: 300°F (150°C)• Accuracy: 0.25" (6mm)• Application comments:

– Excellent for powders and chunky solids.– Fair with thick fluids and limited with thin. – Ignores foam.– Excessive material build-up can prevent operation.– Sensitive to mechanical shock.

Rotating Paddle Switch

• Typical Price: $100 to $300• Maximum Temperature:

500°F (275°C)• Accuracy: 0.5" (13mm)• Application comments:

– Excellent for powders and fair for chunks of solids;

– Limited to detection of dry, non-corrosive, low pressure solids.

Material slows/stalls paddles

Paddle Wheel Trade-offs

• Advantages– Low Cost?

• Disadvantages– Very High Maintenance– Mechanical (Moving Parts)– Only for Granular Materials

Continuous

InterfaceLiquids Granulars Slurries

Liquids Granulars

Point Level

Slurries Interface

Plumb Bobs (YoYo’s) Trade-offs

• Advantages– Level Measurement independent of density– Wide range of granular size– Independent of moisture

• Disadvantages– Mechanical... moving parts– Higher maintenance– Plumb bob (weight) sometimes trapped

by the filling process– Moderate to high cost

Continuous

InterfaceLiquids Granulars Slurries

Liquids Granulars

Point Level

Slurries Interface

Weight on Flexible Cable

Radar

• Advantages– Hi accuracy– Ignores Vapor space changes

• Disadvantages– FCC Site License needed– Cannot measure interface– High cost

Continuous

InterfaceLiquids Granulars Slurries

Liquids Granulars

Point Level

Slurries Interface

Trade-offsReflects from surface

Radar• Typical Price: $1000 to over $2500• Maximum Temperature: 500°F (260°C)• Accuracy: 0.1 to 1% span• Application comments:

– Excellent for continuous measurement of conducting fluids, aqueous slurries, and powders;

– Limited for insulating fluids, aqueous foam, chunks or sticky solids;

– Low dielectric materials limit range; – Condensation or crystallization on antenna

can cause errors.

Laser• Typical Price: Over $2,500• Maximum Temperature: 300°F (150°C)• Accuracy: 0.25" (6mm) • Application comments:

– Excellent for continuous measurement of thick fluids, chunks and sticky solids;

– Limited for thin fluids, locating interfaces, working with foam, and powders.

Microwave

• Advantages– Flush mounted– Unaffected by physical

and electrical properties• Disadvantages

– Conductive coatings may cause problems

– Need emitter and receiver (2

installations)

Continuous

InterfaceLiquids Granulars Slurries

Liquids Granulars

Point Level

Slurries Interface

Interrupted Line of Sight Trade-offs

Microwave Switch• Typical Price: $300 to over $1000• Maximum Temperature: 400°F (200°C)• Accuracy: 0.5" (13mm)• Application comments:

– Excellent for conducting fluids and detecting interfaces– Limited insulating fluids and powders and chunks– Ignores organic foams and can measure aqueous foams– Fluids with low dielectric constant and thick coatings

are a problem– Microwave signals are absorbed almost entirely by water

causing problems with products that are water-based or have water content

Radiation Point Measurement

TWO GM-8GAMMASWITCHES

LOW LEVEL ALARMACTUATED WHEN BOTTOMSWITCH DETECTS INCREASEDRADIATION DUE TO FALLOF LIQUID

Continuous Radioactive Measurement

High AccuracyLower Accuracy

(A) (B)

Radiation [Nuclear]

• Typical Price: over $2500• Maximum Temperature: No limit• Accuracy: 1 to 2% span for continuous;

0.25 to 1" (6 to 25mm) for switches • Application comments:

– Excellent for continuous measurement of most fluids and solids,– Limited only in aqueous foams and slurries;– As a switch, is limited in detecting interfaces and aqueous

slurries,and can ignore foams but also can measure them;

– Requires NRC license and source disposal can be a problem;– Heavy coatings can limit reliability

Nuclear Trade-offs• Advantages

– Can be mounted external to vesselnon-intrusive

– Suitable for high temperature,pressure, and corrosive materials

– Non contact• Disadvantages

– High Cost– Requires licenses, approvals,

and inspections– People fear radiation exposure

Continuous

InterfaceLiquids Granulars Slurries

Liquids Granulars

Point Level

Slurries Interface

Thermal Dispersion• Typical Price: $100 to over $2500• Maximum Temperature: 850°F (450°C)• Accuracy: 1 to 3% span for continuous,

assuming constant density; 0.5" (13mm) for switches

• Application comments: – Excellent for thin fluids, – Fair for thick; – Ignores foams, but can measure them; – Foams detection is limited by thermal conductivity, – Interface detection by differential thermal conductivity.

How much HEATDissipates

AD

R

TO BREATHER EQUALIZER

BASE STRIP

RESISTANCEHELIXUNSHORTED

LIQUID SURFACE

HELIX SHORTEDBELOW SURFACE

Resistance Tape Level Sensor

Increased Level Squeezes Tape &Shorts Resistance

Resistance Tapes

• Typical Price: $1000 to $2500• Maximum Temperature: 225°F (110°C)• Accuracy: 0.1 to 1% span• Application comments:

– Excellent for continuous measurement of thin fluids,– Limited with thick– Fair in aqueous slurries– Will ignore foams– Limited temperature and pressure range– Large specific gravity changes affect accuracy

Magnetostrictive

• Typical Price: $1000 to $1,500• Maximum Temperature: 300°F (150°C)• Accuracy: 0.020% span or 0.030”

(0.762mm); whichever greater• Application comments:

– Excellent for continuous measurement of thin fluids

– Limited for thick fluids– Can be used with density-adjusted float

to locate interfaces– Ignores foams

Sign

als

Leve

l Cha

nges

Magnetostrictive Level Sensor

FLOAT

Product Level

ElectronicswithinProbe

Sound• Can be audible or not (Ultrasonic is not)• Created by Piezo-electric crystalMeasurement

Influences• Temperature 0.3% per degree F• Change in materials & gases• Stratified atmospheres

• Time of flight measurement• Distance = (Velocity * Time)/2• Will not work in vacuum

Sound Reflections• Source of echo is boundary area of two different materials• Smooth surfaces produce best echoes• Poor echo produced by

– Foam– Rough Surfaces– Agitation

Lost Echo• Scattering of sound by excessively rough surfaces • Acousticly “soft” surface, such as foaming• Exceeding the calibration range

Beam Angle

• Sound is not a parallel beam• The longer the distance,

the larger the sound footprint• Typical Angle of 12 degrees• Every foot of range

increases footprint by 1.25”

1 ft

1.25”

Ultrasonic Measurement

RECEIVER ANDAMPLIFIER

TIMED GAINCONTROL

WAVESHAPING

LOGIC ANDDISPLAY

GENERATOR ANDTRANSMITTER

TIMINGGENERATOR

TRANSDUCERS

TRANSMITTED BURST

RECEIVED BURST (ECHO)

ELAPSED TIME PROPORTIONALTO DISTANCE

OBJECT BEINGSENSED

Ultrasonic Ignores Obstructions

Software mapping techniques used to "blank out" or suppress unwanted interference signals creating picture of a "perfect" tank.

Ultrasonic Trade-offs• Advantages

– Non-contact technology– No moving parts– Easy Calibration

• Disadvantages– More Position sensitive

than other technologies– Can be affected by vapors,

structural members, foam– Limited temperature &

pressure range– Won’t work in a vacuum

Continuous

InterfaceLiquids Granulars Slurries

Liquids Granulars

Point Level

Slurries Interface

Ultrasonic Gap Switches

• Advantages– No calibration needed– No moving parts– Easy to install– Can ignore foam

• Disadvantages– Fooled by a plugged gap– Won’t detect aerated products

Continuous

InterfaceLiquids Granulars Slurries

Liquids Granulars

Point Level

Slurries Interface

Material fills gap & blocks sound

Ultrasonic• Typical Price: $100 to $2500• Maximum Temperature: 300°F (150°C)• Accuracy: 0.25 to 3% span for continuous;

0.125 (3mm) for switches • Application comments:

– Excellent for thin fluids– Ignores foams– Continuous measurement is fair for thick fluids

but switches are limited in that application– With continuous, dust, dew in vapor space

hurts performance and range is limited by foam and angled or aerated solids

– With switches, air bubbles and solid particles in the liquid will produce a "low" signal.

Micropulse TDR

• Sends million pulses per second along length of wire or rod.

(Time Domain Reflectometry)

• Impedance change reflects signals.

• Level = half time of travel

TDR Probe styles

• Single probes – (Rod or Cable)Easier to install – lower price.

• But they have larger pulse field– 12” radius, must be kept away from

internal obstructions in vessel.– Good sensitivity – Good accuracy (to K > 2 – 3)– Best ability to ignore material build-up

12”

TDR Probe styles

• Twin probes – (Rod and Cable)better sensitivity to lower “K” materials due to a concentrated electromagnetic field.– 4” radius, which can easier ignore

internal structures in vessel.– Good with lower “K” materials down to

about 2 and slightly below (K > 1.7)– Not good for coating build-up that may

bridge elements

4”

TDR Probe styles

• Coaxial probes – (only Rod style) Best ability to accurately sense the lowest “K” materials (down to K > 1.5)- Not affected by vessel obstructions.

Signal fully contained.- Not good with viscous or coating fluids.- Not suitable for solids/granular mtrls.- Length limits to 20 feet.

10mm Rod X 1” ID316SS tube 1.28” OD

Time Domain Reflectometry (TDR)• Typical Price: $1000 to over $2500• Maximum Temperature: 400°F (200°C)• Accuracy: 0.1 to 2% span• Application comments:

– Excellent for continuous measurement of thin fluids, conductingthick fluids, aqueous slurries, powders, and solid chunks

– Limited for sticky solids and locating interfaces – Ignores organic foams and can measure aqueous foams– Long nozzles are a problem– Range and accuracy on insulating material is greater

with high dielectric constant– Can have significant dead zones depending on mounting.

Level Measurement-Common Problems

• Plugged lines• Liquid/foam• Liquid agitation• Others

– Dry leg/wet leg– Zero elevation/suppression

Remote Configuration

PC Configuration

Complete

Real World uses Multiple Sensors

Point Levels

Continuous

Summary: Level Instrumentation• By surface detection• By interface detection• By hydrostatic pressure• By weight• By golly…