Hot-wire Catalog 238

8/12/2019 Hot-wire Catalog 238

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Probes for Hot-wire Anemometry


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Probes for Hot-wire anemometry

Introduction ..................................................................... 4

General information .......................................................... 4

Manufacturers responsibility............................................. 4

Copyright........................................................................... 4

The Dantec Probe System ............................................. 5

Probe construction............................................................ 5

Wire probes....................................................................... 5

Film probes ....................................................................... 5

Sensor configurations....................................................... 7

Probe body design............................................................ 9Probe supports ................................................................. 9

Shorting probes ................................................................ 9

Probe selection chart ...................................................... 10

Recommendations for use........................................... 11

Mounting and adjustment ............................................... 11

Disturbing effects ............................................................ 11

Maintenance and repairs ................................................ 12

Technical reference ...................................................... 13

Summary of technical data ............................................. 14

Quick guide to probe selection ................................... 15

Probes and probe supports ......................................... 16

Single-sensor probes with cylindrical sensors................ 17

Single-sensor probes with non-cylindrical sensors......... 18

Dual-sensor probes with cylindrical sensors .................. 19

Triple-sensor probes with cylindrical sensors ................. 20

Miscellaneous probes ..................................................... 20

Probe supports for single-sensor probes........................ 21

Probe supports for dual-sensor probes .......................... 21

Probe supports for triple-sensor probes......................... 22Shorting probes .............................................................. 22

Mounting tubes and guide tubes..................................... 23

Wires for probe repair ..................................................... 24

Hot-wire systems .......................................................... 25

Anemometers.................................................................. 25

Calibration unit ................................................................ 25


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Introduction

General informationThis catalog describes thecomplete line of DantecDynamics standard hot-wireand hot-film probes for usewith Constant TemperatureAnemometers (CTA).

The CTA anemometer istodays most widely usedinstrument for measurementand analysis of the micro-structures in turbulent gas

and liquid flows. The output ofthe anemometer representsthe instantaneous velocity ata point and forms the basis ofa statistical analysis describ-ing the flow conditions in thatpoint, for example mean velo-city, turbulence intensity etc.

Its main features are:

Fast response.Fluctuations up to 400 kHzor more can be measured.

High spatial resolution.Small eddies down tosome tenths of a mm canbe resolved.

High dynamic range.Velocities from a few cm/sup to several hundred m/scan be measured withalmost constant sensitivity.

Continuous signal.

Little disturbance of theflow due to small sensor

size.

The basic working principlemakes it possible to deter-mine fluctuations in any para-meter that, in addition to thevelocity, influences the heattransfer from the sensor, for

example density, pressure,temperature and composition,provided proper means aretaken.

Manufacturers responsibility

Dantec Dynamics is responsi-ble for the safety, reliability andperformance of the itemsdescribed in this catalog onlyif:

The specific environmental

conditions correspond tothe requirements stated forthe specific item in thiscatalog or on the probe orprobe support container.

Modifications or repairsare carried out by personsauthorized by DantecDynamics.

The items are used inaccordance with the recom-mendations given herein oron the probe or probe sup-

port container.

Copyright

Information contained in thisdocument is subject to changewithout notice.

This document may not becopied, photocopied, trans-lated, modified, or reduced toany electronic medium ormachine-readable form, inwhole or in part, without theprior written consent of DantecDynamics.

Publication no.: 238-6. 2012 by Dantec DynamicsA/S, Tonsbakken 16-18, P.O.Box 121, DK-2740 Skovlunde,Denmark.www.dantecdynamics.comAll rights reserved.


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Fig. 1. 5 m dia. platinum-platedtungsten wire, welded to theprongs.

Fig. 2. 5 m dia. platinum-platedtungsten wire, gold-plated at theends to provide active sensorlength of 1.25 mm.

Fig. 3. 70 m dia. quartz fiberwith nickel film, gold plated at theends to provide active sensorlength of 1.25 mm.

The DantecProbe System

Dantec provides a completeprobe system with a variety ofprobe types and configura-tions that cover most applica-tions in fluid dynamics.

The probe system includ-es probes, probe supportsand shorting probes.

Probe construction

In general, a probe consistsof the following:

Sensor, forming the heating element.

Sensor supports (prongsor substrate), carrying thesensor and leading cur-rent to it.

Probe body, carrying thesensor supports.

Connector, providing elec-trical connection to theprobe support or probecable.

Probes may have one, two orthree sensors for use in one-,two- or three-dimensionalflows. Each sensor requiresits own anemometer bridge.

The sensor may either bea thin wire suspendedbetween two prongs or a thinmetal film deposited on anelectrically insulating sub-strate. Film sensors can becylindrical (fiber-film probes)or non-cylindrical (film pro-bes). Wire sensors are usedin gases and in non-condu-cting liquids, while film sen-sors are primarily designedfor use in water and otherconducting liquids.

The sensor materials areselected to provide maximumflow sensitivity and highest

possible mechanical strengthwith a minimum of thermalinertia. The size of the sensorand its mounting are selectedto give minimal disturbance ofthe flow.

Most probe types areavailable with different prongor substrate configurationscovering a wide varietyapplications.

Wire probes

Wires are used as sensors inprobes for measurements inair and other gases at veloci-ties from a few cm/s up tosupersonic velocities. In addi-tion, they may be used innon-conducting liquids at lowvelocities.

Wire sensors have highflow sensitivity and the high-est frequency response. Onthe other hand, the mechani-cal strength is limited andthey are quite sensitive to

particle contamination.The sensor supports, or

prongs, are made of stainlesssteel and tapered, providingan end surface of around 0.1mm in diameter to which thewires are spot-welded.

Miniature wire probes

Miniature wire probes have 5m diameter, 1.25 mm long

platinum-plated tungsten wiresensors (Fig. 1). The wires

are welded directly to theprongs and the entire wirelength acts as a sensor. Theyare general purpose probesrecommended for most mea-

Gold-plated wires probes areavailable with one, two andthree sensors (single, X- andtri-axial arrays) in six differentconfigurations.

Miscellaneous wire probesTemperature probe

For measurements of quicklyfluctuating temperatures, awire probe with a 1 m dia-meter, 0.4 mm long platinumsensor is available. The wireis spot welded to stainlesssteel prongs. It is used with aspecial constant current ane-mometer bridge and operatedas a cold-wire probe with ameasuring current a fractionof a mA. The temperature

probe is available in onestraight probe configuration.

Film probes

Film probes are used formeasurements in liquids atlow and medium velocitiesand in gases. They are con-siderably more rugged thanwire probes and less sensitiveto contamination. Sensors arenickel films placed on quartzsubstrates of different shapes.

They are deposited by cat-hode sputtering, which is atechnique where the filmforms in a continuous processand results in a homogenousthin film of high purity andgood adherence to the sub-strate.

The films have high tem-perature coefficients of re-sistance and possess highmechanical and electricalstability.

The films are protected by

a sputtered quartz coating,0.5 m or 2 m in thicknessfor air and water applicationsrespectively. This layer pre-vents against electrolysis,when used in liquids, and pro-tects against wear and oxidi-zation in gas applications.

surements in one- or two-dimensional flows of lowturbulence intensity. The

accuracy of turbulence meas-urements may be reducedbecause of interference fromthe prongs. On the otherhand, the more rigidconstruction makes themmore suitable for high speedapplications without the riskof self-oscillation.

The probes are the cheap-est in the Dantec programand are straightforward torepair. Miniature wire probesare available with one or twosensors (single, X- and paral-lel arrays) in five different con-figurations.

Gold-plated wire probes

Gold-plated probes have 5m diameter, 3 mm long plat-inum-plated tungsten wiresensors. The wire ends arecopper- and gold-plated to athickness of 15 to 20 m,leaving an active sensor, 1.25mm, on the middle of the wire

(Fig. 2). They are designedfor measurements in high-tur-bulence flows of one-, two-and three-dimensions.

The plating of the endsserves the dual purpose ofaccurately defining the sens-ing length and reducing theamount of heat dissipated bythe prongs. This results in amuch more uniform tempera-ture along the wire than is thecase for miniature wires.

Another advantage is lessflow interference from theprongs at the point of measu-rement due to the widerprong spacing. Both increasethe accuracy at high turbulen-ce levels.


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Film probes withnon-cylindrical sensors

(f) Conical probes.

Measures mean velocity andvelocity fluctuations in one-dimensional flows. Mountswith the probe axis parallelwith the direction of the flow.

g) Flush-mounting probes.

Measures skin friction (wallshear stress) in both laminarand turbulent boundary lay-ers. Determines the points oftransition and separation.Mounts in a hole in the wallconfining the flow to bemeasured with the substrateend plane flush with the wall.

The sensor is oriented

perpendicular to the flowdirection.

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Sensor configurations

The Dantec Dynamics probesystem comprises probes

with one, two or three sen-sors for measurements inone-, two- or three-dimensi-onal flows. Normally, eachprobe type is available in anumber of configurationswith different prong or sub-strate bends. In this way it ispossible to select the correctprobe for almost any measu-rement situation.

Single-sensor probesWire and fiber probes with

cylindrical sensorsProbes with cylindrical sen-sors (wires and fiber films)are available in the followingconfigurations:

Mounts with the probe axisparallel to the direction of themean flow. The probe is

rotated to get the velocitycomponents.

(c) Right-angled prongs,sensor parallel to probe axis.

Measures mean flow velociti-es and flow fluctuations inplaces that are not readilyaccessible, e.g. in pipes.Mounts with the probe axisperpendicular to the directionof the flow.

(d) Right-angled prongs,sensor perpendicular to

probe axis.Used in the same applicati-ons as (c), except that thesensor is turned 90. Thismakes these probes suitablefor boundary layer measure-ments, e.g. in pipes, as well.Mounts with the probe axisperpendicular to the directionof the flow.

(e) Offset prongs, sensorperpendicular to probe axis.

Designed for use in bounda-ry layers. The shape of theprongs permits measure-

ments close to a solid wallwithout disturbance from theprobe body, which is out ofthe boundary layer. Mountswith the probe axis parallel tothe direction of flow.

(a) Straight prongs, sensor

perpendicular to probe axis.Measures mean and fluctua-ting velocities in free-streamone-dimensional flows.Mounts with the probe axisparallel to the direction of theflow.

(b) Straight prongs, sensor

at angle of 45 to probe axis.

Measures mean flow velociti-es, flow fluctuations andReynolds shear stress in sta-tionary two- and three-dimensional flows.

Fig. 7. Sensor arrangement ofX-probe.

Fig. 8. Tip of split-fiber probe. Fig. 9. Tip of triple-sensor probe.

(h) Glue-on probe.

Determines the points oftransition and separation andmay measure skin frictionand heat transfer numbers. Itis glued directly onto thewall. The sensor is orientedperpendicular to the flowdirection.


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Triple-sensor probes

Triple-sensor probes havethree sensors and are nor-

mally used in three-dimensio-nal flows.

Tri-axial wire and fiber probes

(q) Tri-axial sensor probes.

Tri-axial sensor probes have

three mutually perpendicularsensors, consisting of gold-plated wires or fiber films.The sensors form an orthogo-nal system with an accept-ance cone of 70.4. Theprong ends are all perpendi-cular to the sensors. Thisgives minimum prong inter-ference and increases theaccuracy, when the threeprobe signals are decompo-sed into velocity components.

Used for measurement of

the U, V and W velocitycomponents in an instationarythree-dimensional flow field.Provides information for cal-culation of the full Reynoldsshear stress tensor. Mountswith the probe axis in themain flow direction. Theresulting velocity vector mustbe within the acceptancecone.

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Dual-sensor probes

Dual-sensor probes are de-signed for use in two-dimen-

sional flows. The sensors arearranged in X-arrays or V-arrays, where they form anangle of 90 with one another,or they are placed oppositeeach other on a cylinder sur-face (split-fibers).

X-array wire and fiber probesAll X-probes measure twovelocity components simul-taneously in turbulent, instati-onary two-dimensional flowfields. They provide informati-on for calculation of Reynoldsshear stress. The flow vectormay not exceed 45.

(k) X-probe, straight prongs.

Used in free-stream applica-tions. Mounts with the probe

axis parallel to the direction ofmain flow, so that the pre-dominant flow vector attacksthe two wires under 45.

(l) X-probe, straight prongs,radial operation.

Used in places not readily ac-cessible, for example in pipes.Mounts with the probe axisperpendicular to the main flowand with the predominant flowvector 45 to the two wires.

(m) and (n) X-probe, right-angled prongs, radial operation.

Two different versions areavailable, both of which areintended for radial operation

in pipes or ducts. One versionhas the sensor plane parallelwith the probe axis (m), whilethe other (n) has the sensorplane perpendicular to theprobe axis. The two versionsthus measure the U-V and U-W components respectively.Mounts with the probe axisperpendicular to the mainflow and rotated, so that thepredominant flow vector at-tacks the two wires under 45.

Split-fiber probes

Split-fiber probes may substi-tute for X-array probes incases where optimum spatialresolution is required, orwhen the flow vector variesbetween 90. They are avail-able in three different configu-

rations.(o) Split-fiber probe, straightprongs.

Used in free-stream applica-tions. Mounts with the probeaxis parallel to the direction of

main flow, so that the pre-dominant flow vector attacksin the prong plane andperpendicular to the fiber.

Two different versions areavailable, both of which areintended for radial operationfor example between com-pressor guide vanes. Oneversion has the sensor per-pendicular to the probe axis,while the other has the sen-sor parallel with the probeaxis. The two versions thus

measure the U-V and U-Wcomponents respectively.

Mounts with the probe axisperpendicular to the mainflow and rotated, so that thepredominant flow vector is inthe prong plane and attacksthe fiber under 90.

Parallel-array probe

(p) Parallel-array probe,straight prongs.

This probe is specially desig-ned for measurement of ext-remely small turbulence inone-dimensional flows. Thetwo wires are supposed tomeasure simultaneously thesame turbulence, whereafterthe electronic noise is filteredaway using a correlationtechnique on the two signals.Mounts with the probe bodyparallel to the flow direction.

Fig. 10. Temperature-compensated wire probe for slowtemperature fluctuations.

Fig. 11. Plug-in probe design. Fig. 12. Probe support for single-sensor probes.


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Miscellaneous dual-sensorprobes

Temperature-compensatedprobe

Temperature changes in themedium under investigationcan affect velocity measure-ments. In this dual-sensorprobe, one sensor operatesas a velocity sensor while theother operates as a tempera-ture sensor.

Probe body design

The probe bodies are desig-ned to provide a rigid, aerody-namic mounting of the sen-sors and sensor supportswith a reliable electrical con-tact further on to the probesupport or the probe cable.

Single- and dual-sensor wire

and fiber-film probesWire and fiber-film sensorsare all mounted on probebodies, normally made ofceramic tubes, equipped withconnector pins that connect

to the probe supports bymeans of plug-and-socketarrangements.

Dual-sensor probes havemarks (one and two dots) thatindicate the sensor number.Fiber probes are also markedwith symbols indicating ap-plications in gas or liquids(red dot = air, blue dot =water).

TTriple-sensor prriple-sensor probesobes

Triple-sensor probes aremounted on probe bodies ofstainless steel, 6 mm outsidediameter, ending in six gold-plated connector pins. Tri-axial probes have the prongsembeded in ceramic tubes

glued into a stainless steeltube that mounts axially in the6 mm body.

Film probesFilm probes (cones, flush-mounting etc.) have cableequipped probe bodies andconnect directly to the probecable without the need for aprobe support. The probebodies are made of chromi-um-plated brass and thequartz rods carrying the sen-sors are glued directly intothe probe bodies by means ofepoxy resin. The cable exten-

ding from the probe body isterminated in a detachableBNC connector.

Probe supportsAll plug-in probes are moun-ted using probe supports.Probe supports serve as theelectrical connection betweenprobe and probe cable andprovide a mechanical mountfor the probe at the sametime.

Probe supports for single-

and dual-sensor probes

There are three probe sup-port types available for single-sensor and dual-sensorprobes: short, long straightand long right-angled.

The supports consist of acoupling ring with an internalrubber ring that provides awater- and pressure-tightsealing, and one or two sets

of contacts embedded in acylindrical body that ends inone or two PTFE-coated cabl-es with detachable BNC con-nectors.

Outside diameters for probesupports are 4 mm and 6 mmfor single- and dual-sensor

probes, respectively. Thecables on dual-sensor sup-ports are marked with oneand two rings indicating theconnector number correspon-ding to the sensor number onthe probe.

Probe supports for triple-

sensor probesThere is one long straightprobe support available fortriple-sensor probes. It con-sists of a stainless steel tube,

6 mm outside diameter, withsix connector sockets in thefront. The connector is coded,so that the probe will alwaysbe properly oriented in thesupport.

The support is not water-tight. It has three PTFE-coated cables with detachab-le BNC connectors markedwith one, two and three ringsindicating the connectornumber, which again corre-

sponds to the sensor numberon the probe.

Shorting probes

Shorting probes are used toshort-circuit the probe sup-port or the probe cable. Thisis done in order to cancel theinfluence of the cable andsupport resistances, whenprobe resistance is measuredin connection with the setupof the anemometer bridge.

Three versions are avail-able for single, dual and tripleprobe supports, respectively.In addition a BNC shortingprobe is available for directshort-circuiting of the probecable.

Fig. 13. Probe support for dual-sensor probes.

Fig. 14. Probe support for triple-sensor probes.

Fig. 15. Probe support for sing-le-sensor probes. Sectional view.

Fig. 16. Shorting probe for sing-le-sensor probes.


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Mounting andadjustment

Avoiding ground loops andnoise pickup

BNC connectors on probes,probe supports and probecables must not be in contactwith any metallic part of testrigs or mounting systems.BNC connectors on dual-and triple-sensor supports

must not touch each other,as this will disturb the opera-tion of the individual CTAservo-loops in the anemome-ter electronics.

Grounding in liquids

The liquid must be groundedas close to the probe as pos-sible by means of e.g. anelectrode plate connected tothe signal ground of the ane-mometer (see CTA manuals

for further information). If nogrounding is made, the thinprotective quartz coatingmay break down due tovoltage differences causedby electric charges buildingup in the liquid - or directconduction if the liquid issomewhat conductive. Sucha grounding will also reducethe amount of elec-tricalnoise that can be coupleddielectrically from the liquidto the CTA circuits.

Probe orientation

The probes must be placedin the flow with the samesensor orientation as duringcalibration.

It is recommended that thewire and fiber probes aremounted with the prongsparallel with the flow when-ever possible in order toavoid vibrations in the prongsduring measurement.

Overheat adjustment

Recommended overheatratio for wires and fiberprobes in air is 0.8, giving an

over-temperature between200 and 300C, and for filmprobes in water 0.1, corre-sponding to about 30C.

Modern computer control-led anemometers, like theStreamLine, have a facilityfor automatic adjustment ofoverheat resistance.

Square wave test

It is important to perform thesquare wave test to makesure that the system will notoscillate during measure-ment and cause the sensorto burn out. The square wavetest must be made at thehighest velocity encounteredin the flow.

Disturbing effects

Varying fluid temperature

If the ambient temperaturechanges, it will introduce

systematic errors in the cal-culation of the velocity. Asthe heat transfer is proportio-nal to the temperature diffe-rence between sensor and

fluid, a change in ambienttemperature will result in achange in probe voltage. If

not accounted for it will bemisinterpreted as a changein velocity.

Sensor contaminationContaminants, like dust in airand chemicals in water, mayadhere to the sensor andchange the heat transferdrastically. The influence ofdirt increases with decreas-ing diameter; this meanswires are generally moresensitive than fibers.

In liquids, particle contam-ination may be a seriousproblem, especially for fiberprobes, and it may often benecessary to filter the liquidfor particles down to the sizeof a few micrometers. Eventhen, cleaning both fiber andfilm probes used in liquids isrecommended at regularintervals. Film probes usedfor a long time may accumu-late calcium carbonate de-

posits that reduce sensitivity.The deposition increaseswith sensor operating tempe-rature.

Chemical reactions

Chemical reaction in the formof electrolysis may occur withfiber and film probes used inwater if the liquid is notproperly grounded, or if thequartz coating has beendamaged. Electrolysis eatsaway the sensor film in the

vicinity of the damage. This

results in an increase in sen-sor resistance and decreasein sensitivity. If electrolysis is

allowed to continue, the sen-sor will eventually disappear.

Bubbles

In liquids, absorbed gasesmay form bubbles on theheated sensor. In this case,the anemometer should beswitched to Standby and thebubbles removed by meansof a soft marten hair brush.Bubbles can be avoided bykeeping the liquid still forsome time, so that the aircan escape from it. Also thesensor temperature shouldbe kept as low as possible toavoid formation of bubbles.At atmospheric conditionsthe sensor temperature inthe water should be keptbelow 60C. If the sensorelement is partly coveredwith bubbles temperaturegradients may actuallydamage the thin protectivequartz coating.

Vortex shedding andvibrations

Vibrations in the probemounting or even in the sup-ports can occur at highvelocities, introducing noiseto the probe signal. Careshould be taken to takeproper means against themor even better to avoid them,for example by mounting theprobe with the prongs in theflow direction.

Recommendations for use

Fig. 17. Grounding in liquids. Fig. 18. Typical square wawetest for 0.5 m wire probe.

Fig. 19. Calibration curves forclean and contaminated (dust)hot-wire probe.

Fig. 20. Wire magazine with 10wires for gold-plated wire probes.


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Maintenance andrepairs

Control and testing atDantec DynamicsEach probe is thoroughly con-trolled and tested at Dantec

Dynamics before it is shippedto the customer. The controlincludes a visual inspection ofsensor dimensions and checkof mechanical strength andelectrical properties. Finallythe probes are tested in aCTA anemometer undernormal operating conditions.Film probes for use in con-ducting liquids are operatedin running water for severalhours. The insulation of theprotective quartz coating istested in a sodium chloridesolution (3%) by applying avoltage across it.

All probes have their technicaldata (sensor resistances,leads resistance, temperaturecoefficient of resistance andmaximum operating tempe-rature) written on a probelabel on the probe container.

Cleaning the sensor

Cleaning of wire probes isbest performed by a soft mar-ten hair brush dipped in 2-propanol alcohol or acetone.Fiber and film probes shouldpreferably be cleaned usingdistilled water. 2-propanolalcohol should be used in alimited amount as the lacquercoating may soften if exposedto alcohol for a longer time.

Acetone should never beused on film probes.

Probes used in water may geta deposit of calcium carbon-ate. This can be removed byusing a 15% acetic acid solu-tion. After washing in the acidthe deposits may be removedby a soft marten hair brush ora folded piece of lense tissue.

It is important to use amicroscope when cleaningsensors in order to avoidmechanical damage.

Wire probe repairAll wire probes can be repair-ed in case of wire breakage.The damaged wire should beremov-ed and the prong endspolished with fine-grade wet-grinding paper and cleaned

with acetone, so that they areabsolutely free from anytraces of grease. The newwire is then fastened by spot-welding. It is important thatthe wire is not tightened

between the prongs so thatany slight vibrations in theprongs later will not break it.Wires for miniature wires areavailable in spools for repairpurposes. Gold-plated wiresare available in wire magazin-es with 10 wires in each.

Fiber probe repair

Fiber probes can be repairedby soldering on a new fiber.This involves a rather compli-cated procedure includinglacquer coating of the solde-ring joints followed by a burn-in to stabilize the resistanceof the sensor. It is thereforerecommended that fiberprobes are returned to

Dantec Dynamics for repair.

Film probe repair

Film probes (cones, etc.) can-not normally be repaired. Ifonly a small hole has appear-ed in the quartz coating onprobes used in liquids, it maybe possible to cover thedamage with a dot of lacquer.This kind of repair should beconsidered a temporary solu-tion, as it will reduce the

sensitivity and frequencyresponse. Replacement ofthe probe is to be preferred.

2


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Unit Tung- Pure PtRh Pt Ir Nickelsten platinum 10% Rh 20% Ir

Resistivity m 108 7.0 10.2 18.9 32.0 6.6

Temp. coeff. of res. %/C 0.36 0.38 0.16 0.07 0.64*

Density kg/m3 103 19.3 21.45 19.95 21.61 8.9

Heat capacity J/kg K 33.0 31.4 35.4 32.0 105.0

Heat conductivity W/m K 178 69.0 50.1 25.5 90.5

Tensile strength N/m2 1010 2.50 0.30 0.60 1.32 0.65

Max. operating temp. C 300 1200 800 700 400

Available as wollaston wire no yes yes yes no

Can be welded if plated yes yes yes -Can be soldered if plated yes yes yes yes

Figure of merit W 109 4.1 5.7 4.4 3.6 4.5

Summary oftechnical data

Sensor material

The standard sensor materialsare selected on the basis ofthe most common applicati-ons. The following propertyvalues are of importance whenselecting sensor material:

As appears from the Table A,tungsten is a superior sensormaterial in most applications

mainly due to its high mecha-nical strength.

Sensor resistance

The sensor resistance figuresgiven in the technical data aretypical values. The actualvalues vary from probe toprobe due to manufacturingtolerances. Wire probes havemuch closer tolerances,normally around 10%, thanfilm probes, which can vary

more than 50% around thetypical value. The film proberesistance is determined notonly by the sensor geometrybut also by thickness and themetallic structure of the thinfilm resulting from the sputter-ing process.

Actual sensor resistance iswritten on the label on theprobe container for each indi-vidual probe. Sensor resistan-ces are always given at 20C.

Temperature coefficient ofresistance

The relation between resist-ance and temperature for ametallic conductor can beexpressed by means of the

temperature coefficient ofresistance 0 (TCR):

RT = R0(1 + 0(T - T0))

Higher order terms are neg-ligible for the normally usedsensor materials in a temper-ature range of a few hundreddegrees. The TCR value 0refers to 20C.

The TCR at another tempera-ture T1 may be calculated as:

1=

20/(1 +

20 (T

1- 20))

The TCR figure stated for awire probes is a typical valuecommon for all probes withthat wire type.

For film probes it may varywith the metal structure anddegree of annealing.

Technical reference

To provide high flow sensitivity: High specific resistivity ( m) High temperature coefficient of resistance (%/K)

To provide small time constant: Small density (kg/m3) Small heat capacity c (J/kg/K)

To reduce heat transfer to the prongs: Small thermal conductivity (W/m/K)

To withstand the flow: High tensile strength (N/m2) High resistance against chemical attacks (oxidisation)

Table A. *) This value is for nickel in its bulk condition. When sputtered, the temperature coefficientof resistance is typically reduced to between 0.4 and 0.5%/K.

The TCR is therefore measu-red for each individual filmprobe and written on theprobe label.

Lead resistanceThe lead resistance R

L

is theinternal probe resistancedefined as the resistancebetween the sensor and theconnector pins (or BNC con-nector) on the probe. All valu-es given in the technical dataare typical values for theprobe type. Deviations inactual lead resistance will in-fluence the over-temperaturebut will only have a secondorder effect on the overallmeasuring accuracy.

Sensor temperatureThe maximum sensor temp-erature indicates the level upto which the sensor will ope-rate stably. For wire probesthe limit is determined by theonset of oxidization, which ismost pronounced for smallwires. Film probes with thinquartz coating are annealedat Dantec Dynamics forstable operation up to thestated limit. If the films areoperated above that, the coldresistance will start to dropand, if the temperature isfurther increased, the film willburn off. Film probes withthick quartz coating are onlyannealed up to 150C. If usedat higher sensor temperatures,

the resistance will drop, andthe lacquer coating may startto detoriate.

When the sensor tempera-ture is used for calculation ofthe overheat ratio, use of asensor temperature some-what below the maximum inthe technical data is advised.This prevents the center por-tion of the sensor, which isnormally the hottest, exceed-ing the maximum allowabletemperature.

Ambient temperatureThe maximum ambient tem-perature states the limit up towhich the probe can be usedwithout damage. It is determi-

ned by the materials andassembly methods (for exam-ple glue) used.

Max. ambient pressure

CTA probes are normallyused around atmosphericpressure. The stated maxi-mum pressure is determinedprimarily by the mounting andtightening method (O-rings,plane seals etc.). A minimumpressure is not given as itdepends primarily on theapplication and the accept-ance of slip-flow conditions.

Fluid velocityMinimum velocity

The lower velocity limit is de-fined by the onset of naturalconvection. If a probe is


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Type of flow Medium Recommended probes

One-dimensional

Uni-directional Gas Single-sensor wire

Single-sensor fiber, thin coating

Conical film, thin coating

Liquid Single-sensor fiber,

heavy coating

Conical film, heavy coating

Bi-directional Gas Split-fibers, thin coating

Liquid Split-fibers, heavy coating,

special

Two-dimensional

One quadrant Gas X-array wires

X-array fibers, thin coating

Liquids X-array fibers, heavy coatings

Half plane Gas Split-fibers, thin coating

Liquids Split-fibers, heavy coating

Full plane Gas X-array wire, flying hot-wire

Three-dimensional

One octant Gas Tri-axial wire

70 cone Tri-axial fiber, thin coating

Liquids Tri-axial fiber, Special

90 cone Gas Slanted wire, rotated probe

Liquids Slanted fiber, heavy coating

One-dimensional

Uni-directional Gas Flush-mounting film,

thin coating

Glue-on film, thin coating

Liquids Flush-mounting film,

heavy coating

Glue-on film, special

15

Quick guide to probe selection

Wall flows (shear stress)


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Probes and probe supports


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SINGLE-SENSOR PROBES WITH CYLINDRICAL SENSORS

Miniature wire probes

Pt-plated tungsten wire, diameter 5 m,length 1.25 mm.

Gold-plated wire probes

Pt-plated tungsten wire, diameter 5 m,overall length 3 mm, sensitive wire length1.25 mm. Copper and gold plated at theends to a diameter of approx. 15 m.

Fiber-film probes

Nickel film deposited on 70 m diameterquartz fiber. Overall length 3 mm, sensiti-ve film length 1.25 mm. Copper and goldplated at the ends. Film is protected by a

quartz coating approx. 0.5 m or 2 m inthickness.

All dimensions in millimeters.

17


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SINGLE-SENSOR PROBES WITH NON-CYLINDRICAL SENSORS

Conical film probes

Flush-mounting film probes

Glue-on probes


8


18/25


DUAL-SENSOR PROBES WITH CYLINDRICAL SENSORS

Miniature wire probes Gold-plated wire probes Fiber-film probes

19


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TRIPLE-SENSOR PROBES WITH CYLINDRICAL SENSORS


0

MISCELLANEOUS PROBES

Resistance thermometer

Parallel-array probe

Temperature-compensated miniature wire probes


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PROBE SUPPORTS

4 mm dia. probe supports for single-sensor probes

6 mm dia. probe supports for dual-sensor probes


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PROBE SUPPORTS

Probe supports for triple-sensor probes

SHORTING PROBES


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MOUNTING TUBES AND GUIDE TUBES

Chucks and couplings

Watertight mounting tubes

Guide tubes


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2 m of platinum-plated tungsten wire, 5 m dia. 55A40

Wire magazine with 10 gold-plated wires, 5 m dia. 55P157

Wire magazine with 5 gold-plated wires for tri-axial 55P144probes

Spools of replacement wire of different materials and dia-meters and magazines with gold-plated wires are available,but must be ordered separately:

Special probesIn addition to the extensiverange of standard probesDantec Dynamics offersprobes and supports forspecial applications, special-ly designed to meet custom-ers requirements.

If you do not find the probeyou need in the standardprogram, do not hesitate tocontact your local DantecDynamics representative,

who will help you to find atechnical solution for yourmeasuring problem.

In order to make a quotationfor a special probe, DantecDynamics requires an outlineindicating the critical dimen-sions and a short descriptionof the application:

parameter to be measured

medium

velocity range

temperature range

pressure range

physical restrictions andconstraints

Wires for probe repair


24/25


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About Dantec Dynamics

Worldwide representation

From our six offices and more than 30 distributors worldwide we

approach our customers individually. We examine the specific needs

and find the best solution for you. For us you are a long-term partner

in improving efficiency, safety and quality of life.

A list of representatives is available at our website.

DENMARK (headquarters)

Dantec Dynamics A/S

[email protected]

FRANCE

Dantec Dynamics S.A.S.

[email protected]

GERMANY

Dantec Dynamics GmbH

[email protected]

JAPAN

Dantec Dynamics K.K.

[email protected]

UNITED KINGDOM

Dantec Dynamics Ltd.

[email protected]

USA

Dantec Dynamics Inc.

[email protected]

www.dantecdynamics.com

The specifications in this document are subject to

change without notice.

Dantec Dynamics is trademark of Dantec Dynamics A/S.

Publication No.: 238_v6

Fluid Mechanics

Particle Characterization

Combustion Diagnostics

Thermal Comfort

Microfluidics

Process Control

Strain, Stress & Vibration

Non-destructive Testing

Disatac Tachometers

Dantec Dynamics is the leading provider of laser optical measurement systems and sensors forfluid flow characterization and materials testing. Since 1947 we have provided solutions forcustomers to optimize their product and component testing. Dantec Dynamics provides qualitysolutions for an extensive list of customers in the areas of:

Documents

Hot-wire Catalog 238