PressureandLevelmeasurement...There are two basic strain gauge forms: 1 bonded 2 unbonded. Pressure...

Pressure and Level measurementControl Instrumentation–Lecture 11

Alpha Control Lab, Faculty of Information Technology, TUT

23.11.2016

Pressure andLevel

measurement

PressuremeasurementManometersMechanicalPressureSensorsElectricalMethods

LevelMeasurementFloatsPressuresensorsUltrasonicCapacitiveConductive

Pressure units

Pressure =forcearea

(1)

It is most commonly expressed in pounds per square inch (psi),pascals (Pa or (N/m2)) in metric units or bar. Bar is equivalentto 100000 N/m2, which were the SI units for measurement.

Process pressure measuring devices may be divided into threegroups:

1 Based on the measurement of the height of a liquidcolumn,

2 Based on the measurement of the distortion of an elasticpressure chamber,

3 Electrical sensing devices.

Pressure andLevel

measurement



Measurement types

Total vacuum which is zero pressure, as would be experienced inouter space.

Vacuum is a pressure measurement made between totalvacuum and normal atmospheric pressure.

Atmospheric pressure is the pressure on the earth’s surface due tothe weight of the gases in the earth’s atmosphere andis normally expressed at sea level as 101.36 kPa. Thepressure decreases above sea level and at an elevationdrops.

Absolute pressure is the pressure measured with respect to avacuum.

Gauge pressure is the pressure measured with respect toatmospheric pressure.

Differential pressure is the pressure measured with respect toanother pressure and is expressed as the differencebetween the two values.

pg = pabs − pat (2)

Pressure andLevel

measurement



Manometers

Manometers are most forms of liquid-columnpressure-measuring devices.

p =F

S=

mg

S=ρVg

S= ρgh (3)

Depending on the pressure range, water and mercury are theliquids most frequently used. Because the density of the liquidused varies with temperature, the temperature must be takeninto account for accurate pressure measurements.

Because of their inherent accuracy, manometers are used forthe direct measurement of pressure and vacuum. Nowadaysmanometers largely serve as standards for calibrating otherpressure-measuring instruments.

Pressure andLevel

measurement



Manometers

Pl Pr

0

1

2

1

PrPl

zerolevel

Figure: U-tube, inclined-tube and well-type

Pressure andLevel

measurement



Features

AdvantagesSimple operation and constructionInexpensive

DisadvantagesLow pressure range (water)Higher pressure range requires mercuryReadings are localizedSize

Pressure andLevel

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Elastic material

Measured pressure deforms some elastic material (usually metallic)within its elastic limit, the magnitude of the deformation beingapproximately proportional to the applied pressure.Many pressure sensors depend on the deflection of a diaphragm formeasurement.

Diaphragm is a flexible disc, which can be either flat or withconcentric corrugations and is made from sheet metalwith high tolerance dimensions.

AdvantagesProvide isolation from process fluidGood for low pressureInexpensiveWide rangeReliable and provenUsed to measure gauge, atmospheric and differential pressure

Pressure andLevel

measurement



Bourdon tube

The BT works on a simple principle that a bent tube will change itsshape when exposed to variations of internal and external pressure.As pressure is applied internally, the tube straightens and returns toits original form when the pressure is released.

The tip of the tube moves with the internal pressure change and iseasily converted with a pointer onto a scale. A connector link is usedto transfer the tip movement to the geared movement sector. Thepointer is rotated through a toothed pinion by the geared sector.

Pressure andLevel

measurement



Features

AdvantagesWide operating rangeInexpensiveFast responseGood sensitivityDirect pressure measurement

DisadvantagesLocalized measurement onlyNonlinear transducer, linearized by gear mechanismHysteresis on cyclingSensitive to temperature variationsLimited life when subject to shock and vibration

Pressure andLevel

measurement



Bellows

It is an expandable element made up of a series of folds orcorrugations called convolutions. When pressure is applied tothe bellows it elongates by stretching the convolutions and notthe end diaphragms. Bellows devices can be used for absoluteand differential pressure measurements.

AdvantagesSimple constructionEasily maintainedInexpensive

DisadvantagesHysteresis on cyclingSensitive to temperature variationsPoor overrange protection

Pressure andLevel

measurement



Strain Gauge

Fluid pressure→ electrical signal response

The basis of electronic pressure transmitters: devices designedto measure fluid pressure and transmit that information viaelectrical signals such as the 4− 20 mA analog standard, or indigital form such as HART or FOUNDATION Fieldbus.Strain Gauges When a wire or other electrical conductor is

stretched elastically, its length is increased and itsdiameter is decreased.

⇒An increase in the electrical resistance of theconductor.

There are two basic strain gauge forms:1 bonded2 unbonded.

Pressure andLevel

measurement



Operating Ranges

AdvantagesWide range (7.5 kPa− 1400 Mpa)Accuracy up to 0.1%SmallStable and fast

DisadvantagesUnstable due to bonding materialTemperature sensitiveThermoelastic strain causes hysteresisRequire regulated power supplies

Vout +-

Diaphragm

Straingauge

pressure

Pressure andLevel

measurement



Piezoelectric Transducers

Generate a potential difference proportional to a pressure-generatedstress. Because of the extremely high electrical impedance ofpiezoelectric crystals at low frequency, these transducers are usuallynot suitable for measurement of static process pressures.

AdvantagesAccuracy 0.075%Pressure measurement up to 70 MPa

Small sizeRobust and fastSelf-generated signal

DisadvantagesDynamic sensing onlyTemperature sensitiveRequire special cabling and signal conditioning

Pressure andLevel

measurement



Level devices

The measurement of level can be defined as the determinationof the location of the interface between two fluids, separable bygravity, with respect to a fixed reference plane.

The most common level measurement is that of the interfacebetween a liquid and a gas. Other level measurementsfrequently encountered are the interface between two liquids,between a granular or fluidized solid and a gas, and between aliquid and its vapor.

A commonly used level devices are: float-actuated, displacer,and head devices, etc.

Pressure andLevel

measurement



Direct level sensing

Level gauges are perhaps the simplest indicating instrument forliquid level in a vessel. Sightglass is to liquid level measurement asmanometers are to pressure measurement: a very simple andeffective technology for direct visual indication of process level. In itssimplest form, a level gauge is nothing more than a clear tubethrough which process liquid may be seen.

AdvantagesVery simpleInexpensive

DisadvantagesNot suitable for automated control.Maintenance - requires cleaningFragile

Pressure andLevel

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Floats

The float material is less dense than the density of the liquid and floats upand down on top of the material being measured. An advantage of thefloat sensor is that it is almost independent of the density of the liquid orsolid being monitored.

Tank

Sightglass

Tank

Spring-loadedcablereel

counterbalanceweight

linearscale

Figure: Sight gauge, float with spring-reel, float with linear scale

The float is at one end of a pivoted rod with the other end connected tothe slider of a potentiometer. Changes in level cause the float to moveand hence move the slider over the potentiometer resistance track and sogive a potential difference output related to the liquid level.

Pressure andLevel

measurement



Features

AdvantagesLarge level measurementInexpensive

DisadvantagesMaintenance - requires cleaningMechanical wear

A variety of float-actuated level devices which transmit thefloat motion by means of magnetic coupling have beendeveloped. A typical magnetic-bond float gauge consists of ahollow magnet-carrying float which rides along a verticalnonmagnetic guide tube. The follower magnet is connectedand drives an indicating dial similar to that on a conventionaltape float gauge.

Pressure andLevel

measurement



Hydrostatic Pressure

The most commonly used method of indirectly measuring aliquid level is to measure the hydrostatic pressure at the bottomof the container. The depth can then be extrapolated from thepressure and the specific weight of the liquid can be calculated

P = hρg , (4)

where P - pressure, h - height, ρ - relative density, g - accelerationdue to gravity.Hydrostatic pressure transducers always consist of a membranewhich is connected either mechanically or hydraulically to atransducer element. The transducer element can be based onsuch technologies as inductance, capacitance, strain gauge oreven semiconductor.It is a requirement of this type of measurement that staticpressure is measured.

Pressure andLevel

measurement



Features

AdvantagesSimple to assemble and installSimple to adjustReasonably accurate

DisadvantagesDependent on relative density of materialMore expensive than simpler typesExpensive for high accuracy applications

Pressure andLevel

measurement



Differential pressure

To minimize potential measurement errors electronicdifferential pressure measurement is used.

To obtain an electronic differential pressure measurement, twogauge pressure transmitters are used.

1 One transmitter is placed at the bottom of the tank tomeasure the height of the liquid column plus pressure (A).

2 The second transmitter is located at the top of the tankwhere it measures the head pressure only (B).

The two 4− 20 mA signals can then simply be subtracted(level = A− B) to obtain the actual level measurement.

One of the practical advantages of pressure sensors isundoubtedly the fact that they are so easy to calibrate.

Pressure andLevel

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Features

AdvantagesLevel measurement in pressurized or evacuated tankSimple to adjustReasonably accurate

DisadvantagesDependent on relative density of materialQuite expensive for differential pressure measurementMaintenance intensive

See also Bubble-Tube method [1] and [2].

Pressure andLevel

measurement



Ultrasonic Measurement

Ultrasonic level sensors work by sending sound waves in thedirection of the level and measuring the time taken for thesound wave to be returned. As the speed of sound is known,the transit time is measured and the distance can be calculated.

This noncontact form of measurement is ideal for manyindustrial applications for which corrosive conditions, changingproduct characteristics, and other factors make contacting levelmeasurement devices less suitable. The measurement systemconsists of a transducer or sensor, mounted on the tank or silo,and either a remote measurement transmitter or transmitterelectronics built into the sensor.

Pressure andLevel

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Features

AdvantagesNon contact with product

Suitable for wide range of liquids and bulk products

Reliable performance in difficult service

No moving parts

Unaffected by density, moisture content or conductivity

Reliable performance in difficult service

Accuracy of 0.25% with temperature compensation andself-calibration

DisadvantagesProduct must give a good reflection and not absorb sound

Product must have a good distinct layer of measurement and not beobscured by foam or bubbling

Not suitable for higher pressures or in a vacuum

The temperature is limited to 170 ◦C

Pressure andLevel

measurement



Capacitive Level Measurement

Principle: the electrical capacitance between two electrodesvaries with the dielectric constant of the material betweenthem. System consists of a rod electrode positioned verticallyin a vessel, the other electrode usually being the metallic vesselwall.

probe

insulation tankwall

environment

C

The electrical capacitance between the electrodes is a measure of theheight of the interface along the rod electrode. The rod is usuallyconductivity insulated from process fluids by a coating of plastic.The dielectric constant of the liquid must be known for this type ofmeasurement. The dielectric constant can vary with temperature sothat temperature correction may be required.

Pressure andLevel

measurement



Features

AdvantagesVery easy installationWide application rangeGood accuracySuitable for highly corrosive mediaWell-known and proven technology

DisadvantagesSensitivity to changes in the material propertiesBuildup

Pressure andLevel

measurement



Conductive Level Detection

Used for high and low level detection. The electrode orconductivity probe uses the conductivity of a fluid to detect thepresence of the fluid at the sensing location. The signalprovided is either ON or OFF.

1234

5

1 No contact with the probe ⇒ the electrical resistance willbe very high or even infinite.

2 Fluid covers the probe ⇒ the resistance in the circuit willbe reduced.

Pressure andLevel

measurement



Features

AdvantagesVery simple and inexpensiveNo moving partsGood for dual point control (level switching control) inone instrumentGood for high pressure applications

DisadvantagesLimited application for products of varying conductivityRestricted to conductive and non coating processesPossible electrolytic corrosion

Pressure andLevel

measurement



S. Medida. (2007) Pocket guide on industrial automationfor engineers and technicians. IDC Technologies. [Accessed:November, 2016]. [Online]. Available:http://www.pacontrol.com/download/Industrial-Automation-Pocket-Guide.pdf

G. K. McMillan, Process/Industrial Instruments andControls Handbook, 5th ed., D. Considine, Ed.McGraw-Hill Professional, 1999.

http://www.pacontrol.com/download/Industrial-Automation-Pocket-Guide.pdf

http://www.pacontrol.com/download/Industrial-Automation-Pocket-Guide.pdf

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