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Foreword ixAcknowledgement xiIntroduction xiii

Part I1. History of radar 12. Physics of radar 133. Types of radar 33

1. CW-radar 332. FM - CW 363. Pulse radar 39

Part II4. Radar level measurement 47

1. FM - CW 482. PULSE radar 543. Choice of frequency 624. Accuracy 685. Power 74

5. Radar antennas 771. Horn antennas 812. Dielectric rod antennas 923. Measuring tube antennas 1014. Parabolic dish antennas 1065. Planar array antennas 108Antenna energy patterns 110

6. Installation 115A. Mechanical installation 115

1. Horn antenna (liquids) 1152. Rod antenna (liquids) 1173. General consideration (liquids) 1204. Stand pipes & measuring tubes 1275. Platic tank tops and windows 1346. Horn antenna (solids) 139

B. Radar level installation cont. 1411. safe area applications 1412. Hazardous area applications 144

Contents

Correct mechanical installation isvery important for the efficient func-tioning of process radar level transmit-ters. Although the best echo processing

software can cope with some of theeffects of poor installation, there is nosubstitute for correct installation at thedesign stage.

6. Radar level installations

A. Mechanical installation

Fig 6.1 Fig 6.2

Correct installation

Incorrect installation

1. Measurement of liquids - Horn antenna

Nozzle / socketThe most common installation for a

radar sensor is made in a vessel nozzleor socket. The instrument flange is thereference pane of the measuring range.

The horn antenna should alwaysprotrude into the vessel by at least10 mm. For example, a standard 6"

horn antenna is 205 mm long from theflange face.

If the vessel nozzle is significantlylonger than 195 mm, a waveguideextension should be considered so thatthe end of the horn is the requireddistance within the vessel.

115

116

Extended waveguide and curvedwaveguide hornIf a horn antenna radar has to beinstalled in a long tank nozzle, it can befitted with a waveguide extension. Thisis a parallel stainless steel tube betweenthe PTFE / ceramic waveguide in theflange and the horn of the antenna.Also, it is possible to bend the wave-guide extension of a horn antennathrough 90° for side mounting installa-tions. The minimum bend radius forthis type of antenna is 200 mm.

The orientation of the linear polar-ization of the radar is important with a90° bend in the waveguide. The direc-tion of the linear polarization should behorizontal if the bend turns verticallydown.

Extended and bent waveguides aresuitable for liquids with good reflectiveproperties. They should not be usedwith low dielectric liquids or solids.

Maximum level with a hornantenna radar

With a horn antenna installation it isnormally possible to measure liquidlevels right up to the end of the horn.Coating of a horn antenna does not pre-sent significant problems especially ata frequency of 5.8 GHz.

Fig 6.3 Horn antenna installations:standard, extended waveguide,extended waveguide with 90°bend


117

The PTFE rod antenna is well suitedto chemically aggressive products suchas acids and alkalis. It is used exten-sively within the pharmaceutical indus-try where mixtures of solvents, acidsand alkalis are commonplace.

The PTFE rod antenna radar levelsensors with Tri-Clamp fitting andcrevice free construction are availablefor applications in the food processingindustry and where hygienic vesselconditions are required.

The rod antenna is used for measur-ing liquids and slurries but not for drysolids applications. The sensor is mostcommonly mounted in a simple nozzleor socket.

Rod antenna radar transmitters aresupplied with simple screwed connec-tions such as 1 ½" BSP or NPT, flangedconnections 2" (DN50) to 6" (DN150),or hygienic Tri-Clamp.

It is essential that all of thetapered section of the rod antenna isinside the vessel and not in thenozzle.

Rod antennas are available withinactive extended lengths so they canbe installed in longer nozzles. Typicallythe extended versions are suitable fornozzles of up to 100 mm or up to250 mm.

Fig 6.4 Typical rod antennainstallations: the activetapered section must beinside the vessel.Extended rod antennasshould be used inlonger nozzles

2. Measurement of liquids - Rod antenna

Nozzle / socket.

118

Rod antenna incorrectly installedin a nozzle

If the tapered section of a rod anten-na is installed inside a nozzle themicrowaves that are emitted from the

antenna ‘ring’ inside the nozzle causinghigh levels of noise and therefore mea-surement problems especially in thenear range.

Fig 6.5 Correct:Extended rod antennafor long nozzle.Normal noise curvewith good echo

Fig 6.6 Incorrect:Short rod antenna inlong nozzle. High‘ringing’ noise isproduced which isabove the echo signal


119

Rod antenna directly on thevessel opening

Rod antenna radar sensors, eitherscrewed or flanged, can be mounteddirectly into a hole in the top of a tank.

Maximum level with a rodantenna radar

As explained previously, it is ofprimary importance that the taperedsection of a rod antenna radar shouldbe entirely within the vessel. Theadverse effects of ‘ringing’ cannot bedealt with in software. Increased gainwill only increase the ringing.

The length of the tapered section ofa rod antenna means that we must con-sider the maximum level of liquid thatcould be present in the vessel. Ideally the liquid level should nottouch a rod antenna. However, this issometimes unavoidable and the follow-ing considerations must be taken.

Mechanical loadsIt should be noted that PTFE-rod

antennas can only withstand limitedmechanical loads. When subjected tolateral force, it may bend and deformor even break. Does the applicationinvolve strong agitation? Will the forceof the agitation cause damage to therod?

Coating of a rod antennaAs we have already explained, the

microwaves from rod antenna radarsare emitted from the tapered section ofthe rod. If the rod is submersed in vis-cous liquid and this product is allowedto coat the antenna, then the antennaefficiency will be compromised. Theradar will not function if there is a seri-ous amount of build up on a rodantenna. However, if light, low viscosi-ty liquids such as solvents or lowdensity water based products touch arod antenna, they will tend to be selfdraining and self cleaning. As a rule ofthumb, it is possible to measure clean,low viscosity liquids about half way upa rod antenna. However, the accuracyis compromised and no contact isalways better.

120

Fig 6.8 The effect of mounting a radar inthe middle of a parabolic dishvessel top

Fig 6.7 The ideal position is ½ radius invessels with a parabolic top

The following consideration shouldbe taken into account when installing a

horn or rod antenna radar in the top ofa vessel.

Mounting in dished topped vesselsA radar transmitter should not be

mounted in the centre of the dished endof the tank or too close to the vesselwall. The ideal position is approximate-ly a ½ vessel radius from the outerwall. Dished tank ends can act as para-bolic reflectors. If the radar sensor isplaced in the ‘focus’ of a parabolic tankend, the sensor receives amplified falseechoes. Whereas if the radar sensor ismounted outside the ‘focus’ of the par-abolic vessel top, amplified falseechoes are avoided.

Parabolic effectIf a radar level transmitter is

installed in the centre of a dishedtopped vessel, the unit will see large‘multiple echoes’ beyond the real firstecho. The effect of these multipleechoes can be clearly seen on an echotrace. The classic multiple echo form isshown in Fig 6.8. This same effect canoccur in a horizontal cylindrical tank.Multiple echoes can be ignored bypulse radar software. However thisproblem is more difficult for FM - CWradar. See chapter 4.

3. General installation considerations:Horn and rod antennas on liquids applications

r/2

r

echo curve


121

Fig 6.9 Profiles with flat interferingsurfaces or sharp edges cause largefalse echoes

Fig 6.10 Round profiles diffuse the radarsignals producing smaller falseechoes

Fig 6.11 A deflector causes signalscattering and reduces the ampli-tude of a false echo

False reflectionsFalse echoes from flat obstructions

or obstructions with a sharp edge,cause large false echoes. They producehigh amplitude reflections of the radarsignal. Rounded profile obstructionsdiffuse the reflection of the radar sig-nals and produce low amplitude falseechoes. Therefore they are easier tocope with than reflections from a flatsurface.

If flat obstructions in the range ofthe radar signals cannot be avoided, itis recommended that a deflector plate isused to reflect the false echo signalsaway from the transmitter. These scat-tered false echo signals will be low inamplitude. Therefore, they can be fil-tered out by the sensor software.

122

Avoiding false echoesThe position of the radar sensor in

the vessel must be selected such that nostruts or in flowing material are in thedirect path of the radar signal.

The following examples and instruc-tions show common measurementproblems and how they can be avoided.

ShouldersVessel shapes which have flat shoul-

ders pointing to the antenna can influ-ence the measurement because of the

amplitude of the false echo. Deflectorsabove these flat shoulders deflect anddiffuse the false echoes and ensure areliable measurement.

Internal structures such as inlets formaterial mixing with a flat surfacepointing to the radar sensor, should becovered by a screen. False echoes canthen be ignored by the echo processingsoftware.

Fig 6.12 Deflector plate on a vesselshoulder

Fig 6.13 Deflector plate on internalstructure

Vessel installationsStruts, such as ladders bracing andprobes often cause false echoes. It isimportant to optimise the position ofthe radar level transmitter to avoid theworst effects of false echoes.

Internal bracing or welds can causestrong false echoes which can competewith the real product echo. Small

angled shields on bracing can avoid theworst false echo reflections. The falseechoes are diffused and filtered out as‘echo noise’ by the echo processingsoftware. If internal welds are cleanedup during manufacture, their affect onthe echo decision is minimised.


123

Fig 6.14 The sensor must bepositioned away from inter-nal structures such asladders

Fig 6.15 Angled reflectors oninternal structures such asbracing can reduce theeffects of the false echoes

Build-upIf the radar sensor is mounted too

close to the vessel wall, build-up on thevessel walls may cause false echoes.

Ideally, position the radar sensoraway from the vessel wall. The opti-mum position for dished top vessels isat ½ radius.

PolarizationAs already discussed in Chapter 2,

Physics of radar, the microwaves havelinear polarization.

Although polarization is of greaterimportance in stilling tube and bypasstube applications, it can also be signifi-cant when measuring in the top of avessel.

The amplitude of the false echoesfrom the internal structure of the tankcan often be reduced by rotating theradar transmitter through 45° or 90°.

The direction of the polarizationdepends on the direction of the encou-pling from the microwave module. Itshould be marked on the transmitter.

124

Fig 6.16 Avoid the effects of build up on the side of a vessel

For liquids applications, the radarlevel transmitter must be directed verti-cally down on to the liquid surface. If it

is angled, it will receive a weak productecho signal and possibly larger falsemultiple echoes.

In flowing liquidDo not mount a radar sensor above

or close to the liquid filling stream. Ensure that you detect the product

surface and not the in flowing material.


Orientation of the radar transmitter on liquids(horn or rod antenna)

Fig 6.17 The radar must bemounted vertically onliquids applications

Fig 6.18 Mount the transmitteraway from in flowingliquid

125

126

If the radar sensor is mounted tooclose to the vessel wall it can causestrong interfering signals. Build-up,rivets, or weld joints superimpose theirechoes on the real echo. Therefore,sufficient distance from the sensor tothe vessel wall must be allowed.

Different radar level transmittershave different ‘beam angles’, seeChapter 5 on Radar antennas.

With good reflection conditions onliquids without disturbing echoes in thevessel, it is recommended that the dis-tance of the sensor to the vessel wallcomplies with the inner emission cone.For liquids with false echo conditions itis recommended that the distance to thesensor complies with the outer emis-sion cone. See Fig 6.19. Use manufac-turers data.

Fig 6.19 Example 150 mm (6") horn antenna emission cone at 5.8 GHz

Sensor too close to the vessel wall


127

Radar level transmitters are widelyused in stand pipes, stilling tubes andbypass tubes. This type of installationmay be essential where there is foam,heavy turbulence, mechanically com-plex vessels and sumps or very lowdielectric liquids. Also, radar leveltransmitters have been used to replaceexisting sensors that utilise bypasstubes. This includes replacing displac-ers and float devices.

Foam generationDense conductive foam on the prod-

uct can cause faulty measurement.Under these conditions it is probablethat the radar will read the top of thefoam. Often, radar can see the liquidsurface through some lighter foams.However, the picture is not black andwhite and care must be taken in appli-cations where foam is generated. Theeffects of foam can be avoided by mea-suring in a stilling tube or bypass tube.It is best to discuss the application withthe manufacturer.

Highly agitated product surfaceHeavy turbulence in the vessel

caused by strong agitators or violentchemical reactions influence radar levelmeasurement. A stilling tube or bypasstube of sufficient size allows reliablemeasurement even with strong turbu-lence in the vessel. This is providedthat the product does not build-upinside the pipe. Products which cancause slight build-up can be measuredby using a measuring tube of 100 mm(4") nominal bore or more. In a mea-suring pipe of this size, slight build-upis not a problem.

Very low dielectric constant liquidsNon-conductive and extremely low

dielectric constant liquids such as LPG(liquefied petroleum gas) can be mea-sured accurately and confidently usinga measuring tube or bypass tube. Asalready explained in Chapter 5, ‘Radarantennas’, the measuring tube effec-tively concentrates the microwaves giv-ing a large echo from the product sur-face. Dielectric constants as low as εr =1.5 can be measured.

General instructions for radarmeasurement in a tube

The stand pipes or stilling tube mustbe open at the bottom and must extendover the full measuring range (i.e.down to 0% level). Ventilation andsurge holes must be on one axis withthe radar’s linear polarization directedtowards the holes or slots. The directionof the polarization may differ betweenmanufacturers. On VEGA radars, thepolarization is in the direction of thetype plate or the casting nose.

As an alternative to the stand pipe orstilling tube in the vessel, a radar canbe installed outside the vessel in abypass pipe. The polarization must bedirected towards the process connec-tions which should be on the same axisas shown. (Fig 6.21).

4. Stand pipes, stilling tubes & bypass tubes

128

Fig 6.20 Stand pipe / stilling tubeshowing position of vent relativeto polarization direction andoverall length

Fig 6.21 Bypass tube showing directionof polarization

Fig 6.22 Installation on bypass tube:Radar level transmitters oftenreplace displacers or floatdevices

E E E


129

PolarizationIn summary, the sensor polarization

must be directed towards the pipe con-nection opening in a bypass tube or thebreather holes or slots in a stilling tubeor stand pipe. The holes or slots mustbe on the same axis.

Directing the polarization of theradar signals enables considerablymore stable measurements to be madein pipes because false echoes arereduced in amplitude.

Microwave velocity changeAs explained in Chapter 2 and

Chapter 5, with measurement in a standpipe or bypass tube the maximum mea-suring range is reduced between 5 and20 % . This is because the running timeof the microwaves in a tube is slowerand therefore the maximum range isreduced. For example in a 50 mm (2")tube a maximum range of 20 metresreduced to 16 metres and in a 100 mm(4") tube it is reduced to 19 metres.

Measuring tube radar measurement with inhomogeneous products

Fig 6.23 Slots must be used to ensure that inhomogeneous liquids mix correctly. Thepolarization must be aimed towards the mixing slots or holes

130

Adhesive productsWhen measuring adhesive products,

the inner diameter of the stand pipemust have a larger nominal diameter,for example 100 mm (4"), so thatbuild-up does not cause measuringerrors. In order to measure inhomoge-neous liquids or liquids that separateinto layers, the measuring tube musthave long holes or slots in it. Theseopenings ensure that the liquid is mixedand balanced at the correct level. Themore inhomogeneous the measuredproduct, the closer the openings shouldbe.

Again, for reasons of radar signalpolarization, the holes and slots mustbe positioned in two rows displaced by180°. The radar sensor should bemounted such that the polarization isdirected towards the rows of holes.

Surge pipe with ball valveA full bore ball valve can be used

with a radar level transmitter on a standpipe. The ball valve makes it is possi-ble for maintenance work to be carriedout without opening the vessel. This isparticularly useful with liquefied gasand toxic products. The full bore ballvalve must have the same diameter asthe pipe and be flush when it is in theopen position.

Fig 6.25 A full bore ball valve on astilling tube can be used toisolate a radar from the processwithout the need for a shutdown

Fig 6.24 Radar polarization must beaimed towards the mixing slotsor holes

EE


131

Diagram 1 (P132)Radar sensors for measurement onstand pipes or bypass tubes are used inflange sizes DN 50 (2"), DN80 (3"),DN100 (4") and DN150 (6").

This diagram (Diagram 1) shows theconstruction of a measuring tube (standpipe or bypass tube) with a DN 50 (2")flange.

The measuring tube must be smoothinside (average roughness Rz < 30).Ideally a continuous stainless steel (orHastelloy) pipe should be used withoutany joints in the measuring area.

If pipe extensions are required, theseshould be manufactured to length withweld neck flanges or with connectingsleeves. The transition between piecesof pipe should be smooth and no shoul-ders should be caused by welding.

The pipe and the flange should befastened with the internal bore alignedbefore welding.

The pipe must be smooth inside. Donot weld through the pipe wall.

Pipe roughness and joints must beremoved carefully or large false echoesmay cause a measurement problem andbuild-up will adhere to the uneven sur-face. All burrs must be removed fromholes and slots.

Diagram 2 (P133)This diagram (Diagram 2) shows theconstruction of a measuring tube for aradar level sensor with a DN 100 (4")flange.

Radar sensors with flanges of DN 80(3"), DN 100 (4") and DN 150 (6") areusually equipped with a horn antenna.Special offset rod antenna transmittersare available for DN50 (2") and DN80(3") flange connections.

Instead of the weld neck flange ofthe tube, a smooth welding flange canbe used for the transmitter connectionflange which is above the horn antenna.

The measuring pipe should besecured to the vessel bottom if there isagitation. Additional fastenings will berequired for longer measuring tubes.

A deflector plate at the end of themeasuring tube reflects the radar sig-nals away from the bottom of the ves-sel. This is designed to avoid largeechoes from the metal bottom of thevessel caused by the microwave pene-trating low dielectric products, such assolvents, when the level is low.

In some cases, the vessel bottomecho will interfere with the liquid levelecho. The deflector plate prevents thisproblem from occurring.

Construction of radar stand pipes

132

Diagram 1

Fig. 6.26


133

Diagram 2

Fig. 6.27

134

Fig 6.28 Measurement of reflective liquids can be carried out through low dielectricwindows or plastic tank tops

5. Measurement through plastic tank tops & low dielectricwindowsThe microwave signals from radar

level transmitters are capable of pene-trating low dielectric materials such asPTFE, polypropylene, glass and GRP(glass reinforced plastic). This ability isvery important for some applications.For example, where high purity isrequired for the semi-conductor indus-try or where substances are veryaggressive in the chemical industry, itis beneficial that the system remainsclosed for reasons of quality and safety.

A completely non-contact level mea-surement is possible for products withgood reflection properties where theradar measurement can be carried outthrough the top of plastic vessels.

Products with good electrical con-ductivity or with a dielectric constantof more than 10 can be measured inthis way.


135

As with light, microwaves followthe laws of reflection. As well as pene-trating the plastic tank top and reflect-ing off the conductive or high dielectricliquid, there is also a small reflectionfrom the plastic tank. If the tank ceilingis flat, the false echo from the plasticwill reflect directly back to the antenna.See Fig 6.29.

The performance of the radar isimproved if the plastic tank top issloped at an angle of 35° to 45°,and theantenna is about 400 mm (16") fromthe plastic ceiling of the tank.

The angle ensures that the reflectionfrom the plastic tank is carried awayfrom the antenna and if does not inter-fere with the real echo off the product.See Fig 6.30.

Pulse radar can also be used to lookthrough low dielectric windows inmetallic vessels. As with plastic tanks,the window has to be large enough tocope with the beam angle of the sensor,the window should be angled and the

antenna should not be too close to thewindow.Note: Laws on the use of radar leveltransmitters outside vessels variesbetween different countries.

Fig 6.29 Flat plastic tank top gives areflection directly back to theantenna

Fig 6.30 Angled plastic tank topimproves the measurement ofthe liquid inside

Fig 6.31 Optimum installationposition for a 5.8 GHz pulseradar looking through a lowdielectric window

Reflection of microwaves - plastic tank top

Measurement through low dielectric window

136

In some countries it is not permittedto allow FM - CW radar level sensorsto transmit outside an enclosed metalvessel. Therefore, if the benefits of alow dielectric window are to berealised, the antenna must be installedin a metallic spool piece above a plasticor glass window.

A refinement to the installation ofradar level transmitters looking through

low dielectric windows is the use of acone shaped PTFE seal. This can actas a dielectric lens focusing themicrowaves. Also, it is less prone toproblems of condensation because theliquid will tend to drain off the shapedseal. This type of installation maycause ringing noise.

Fig 6.32

Radar transmitter

Metallic spoolpiece

Cone shapedPTFE seal

Measurement through a low dielectric window


137

It is important to choose the correctthickness of dielectric material whenmeasuring through a window.

The possible interference from thewindow is composed of two differentechoes. The first echo is from the out-side face of the window material as themicrowaves penetrate the window.There is a 180° phase shift of themicrowaves when it is reflected fromthe first surface.

The second echo is an internalreflection as the microwaves leave thewindow material. This is reflectedwithout a phase shift. By choosing awindow material thickness that is a halfwavelength of the window material, thetwo out of phase reflections effectivelycancel each other out through destruc-tive interference. See chapter 2.

Fig 6.33 The optimum thickness of the window material must be a half wavelength of thewindow material

Optimizing the dielectric window

emitted wavereflection withphase shift from topsurface

plastic vessel ceiling

reflection withoutphase shift frominternal surface

D

transmitted signal

Reflections cancel each other through

destructive interference

reflection with phase shift

reflection without phase shiftoff internal face of window

{

138

The following table shows the opti-mum thickness for the most importantplastics and glasses that are suitable

window materials for radar sensors.

The optimum thickness is shown for5.8 GHz radar and 26 GHz radar.

Low dielectric windows for pulse radar transmitters : Frequency 5.8 GHz

Penetrated substance εε r optimum thickness D in mm

PE Polyethylene 2.3 17 (34; 51 …)PTFE Polytetrafluorethylene 2.1 18 (36; 54 …)PVDF Polyvinylidenfluoride ~7 8 (16; 24; 32 …)PP Polypropylene 2.3 17 (34; 51 …)Glass Borosilicate (Maxas, Duran) 5.5 11 (22; 33; 34 …)Glass Rasotherm 4.6 12 (24; 36; 48 …)Glass Labortherm 8.1 9 (18; 27; 36 …)Quartz glass ~4 13 (26; 39; 52 …)POM Polyoxymethylene 3.7 13.5 (27; 40.5; 54 …)Polyester 4.6 12 (24; 36; 48 …)Plexiglass Polyacrylate 3.1 14.5 (29; 43.5; 58 …)PC Polycarbonate ~2.8 16 (32; 48 …)

Low dielectric windows for pulse radar transmitters : Frequency 26 GHz

Penetrated substance εε r optimum thickness D in mm

PE Polyethylene 2.3 3.8 (7.6; 11.4…)PTFE Polytetrafluorethylene 2.1 4 (8.0; 12.0…)PVDF Polyvinylidenfluoride ~7 1.8 (3.6; 5.4…)PP Polypropylene 2.3 3.8 (7.6; 11.4…)Glass Borosilicate (Maxas, Duran) 5.5 2.5 (5; 7.5…)Glass Rasotherm 4.6 2.7 (5.4; 8.1…)Glass Labortherm 8.1 2 (4.0; 6.0; 8.0…)Quartz glass ~4 2.9 (5.8; 8.7…)POM Polyoxymethylene 3.7 3 (6.0; 9.0…)Polyester 4.6 2.7 (5.4; 8.1…)Plexiglass Polyacrylate 3.1 3.2 (6.4; 9.6…)

PC Polycarbonate ~2.8 3.6 (7.2; 10.8…)

Note: The optimum thickness can also be provided by the addition of several layersof identical material. The layers must fit flush without any gaps. Multiples ofthe half wavelength thickness can be considered too.


139

Horn antennas are used on solidsapplications. This includes all pneu-matically conveyed products such aspowders, granular products and pellets.The rod antenna is not used for theseproducts but is used on liquids andslurries.

The product profiles of solids withina hopper or silo are rarely flat. A pow-der or granular product fills and emp-ties with different profiles. The angle ofrepose of the product depends upon thetype of product, the method of fillingand emptying, and the shape anddimensions of the silo.

Radar level transmitters, like ultra-sonic transducers, should be mountedin an off centre position with the anten-

na tilted at an angle towards the drawpoint at the bottom of the silo cone.

The end of the horn should be pro-truding by at least 10 mm into the ves-sel.

The transmitter is angled so that theradar beam bisects the product to pro-vide the optimum echo strength and anaverage contents value.

The radar transmitter should beinstalled away from the fill line andaway from any internal structure in thesilo that may produce large falseechoes.

Fig 6.34 Horn antenna is used for solidsapplications. The antenna is offcentre and tilted towards thedraw point of the silo to caterfor the angle of repose of thesolids product in the silo

6. Measurement of solids - Horn antenna

140

Fig 6.35 & 6.36 Solids installation showing angle of repose on typical filling andemptying cycle

Air or nitrogen purging of the hornantenna can be used on high tempera-ture solids applications or on applica-tions where the product being mea-sured coats and contaminates the anten-

na with thick layers of conductive dust.The flange is drilled laterally from

two opposite directions and tapped toreceive the air / nitrogen gas service.

Fig 6.37 Air / Nitrogen cooling orcontamination purging

Air / Nitrogen gas

High temperature and contaminating solids applications


141

The range of different radar leveltransmitters has expanded in recentyears. There are a number of differentwiring configurations available for safearea and hazardous area applications.These include 4 to 20 mA transmittersand fieldbus transmitters. The cost ofcabling must be considered whenchoosing a radar sensor.

The advent of the two wire, intrinsi-cally safe pulse radar has positionedradar as a ready replacement for themore traditional level transmitters suchas differential pressure and displacers.However, practical FM - CW transmit-ters still require the additional power ofa 4 wire device. In this section we lookat the range of possible wiring configu-rations for all types of radar.

1. Safe Area applications

a. 2 wire, 4 to 20 mA loop powered radar

b. 4 wire, 4 to 20 mA radar

Power 20/250 VAC / VDC

4 to 20 mA

Fig 6.38

Fig 6.39

4 to 20 mA, 24 VDC

c. HART protocol. Most two wire and 4 wire , 4 to 20 mA radar level transmit-ters are available with the HART protocol superimposed on the current loop.This allows the following:· Remote set up with HART hand held programmer· Set up via HART protocol with remote I/O into DCS· Possible multi-drop of 16 units using HART protocol

B. Electrical installation

142

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, 15

sens

ors

Ex

e, 5

sen

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3 tw

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ire

loop

s, m

ulti-

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, haz

ardo

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rea

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ing

and

incr

ease

d sa

fety

pow

er s

uppl

y on

eac

h lo

opV

BU

S

5 se

nsor

s on

eac

h 2

wire

loop

po

wer

ed b

y th

e lo

op

VE

GA

LOG

571

Ex

e w

iring

Pro

toco

l con

vers

ion

148

Fig

6.4

9

h.Fi

eldb

us (

VB

US)

, Ex

ia in

trin

sica

lly s

afe,

mul

ti-dr

op, 5

sen

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on

each

two

wir

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h 15

way

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ator

Pro

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VE

GA

LOG

571

VB

US

5 s

enso

rs o

n 2

wire

s in

trin

sica

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afe

VB

US


149

Fig

6.5

0

i.Fi

eldb

us (

PRO

FIB

US

PA),

Ex

ia in

trin

sica

lly s

afe,

8 s

enso

rs o

n 2

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drop

, via

seg

men

t cou

pler

toPR

OFI

BU

S D

P

8 se

nsor

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two

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men

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Pro

fibus

PA

Pro

fibus

DP

Part IIII

Other level techniquesApplications

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