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Probes for Hot-wire Anemometry
Probes for Hot-wire anemometry
Introduction ..................................................................... 4General information .......................................................... 4Manufacturer’s responsibility............................................. 4Copyright........................................................................... 4
The Dantec Probe System ............................................. 5Probe construction............................................................ 5Wire probes....................................................................... 5Film probes ....................................................................... 5Sensor configurations ....................................................... 7Probe body design............................................................ 9Probe supports ................................................................. 9Shorting probes ................................................................ 9Probe selection chart ...................................................... 10
Recommendations for use........................................... 11Mounting and adjustment ............................................... 11Disturbing effects ............................................................ 11Maintenance and repairs ................................................ 12
Technical reference ...................................................... 13Summary of technical data ............................................. 14
Quick guide to probe selection ................................... 15
Probes and probe supports......................................... 16Single-sensor probes with cylindrical sensors................ 17Single-sensor probes with non-cylindrical sensors......... 18Dual-sensor probes with cylindrical sensors .................. 19Triple-sensor probes with cylindrical sensors ................. 20Miscellaneous probes ..................................................... 20Probe supports for single-sensor probes........................ 21Probe supports for dual-sensor probes .......................... 21Probe supports for triple-sensor probes ......................... 22Shorting probes .............................................................. 22Mounting tubes and guide tubes..................................... 23Wires for probe repair ..................................................... 24
Hot-wire systems .......................................................... 25Anemometers.................................................................. 25Calibration unit ................................................................ 25
4
Introduction
General informationThis catalog describes thecomplete line of DantecDynamics’ standard hot-wireand hot-film probes for usewith Constant TemperatureAnemometers (CTA).
The CTA anemometer istoday’s most widely usedinstrument for measurementand analysis of the micro-structures in turbulent gasand 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 to some tenths of a mm can be resolved.
• High dynamic range. Velocities from a few cm/s up to several hundred m/s can be measured with almost constant sensitivity.
• Continuous signal.
• Little disturbance of the flow 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.
Manufacturer’s responsibilityDantec Dynamics is responsi-ble for the safety, reliability andperformance of the items described in this catalog onlyif:
• The specific environmental conditions correspond to the requirements stated for the specific item in this catalog or on the probe or probe support container.
• Modifications or repairs are carried out by persons authorized by Dantec Dynamics.
• The items are used in accordance with the recom-mendations given herein or on the probe or probe sup-port container.
CopyrightInformation 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.: 238-9. © 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 µm dia. plated tungstenwire, welded to the prongs.
Fig. 2. 5 µm dia. plated tungstenwire, gold-plated at the ends toprovide active sensor length of1.25 mm.
Fig. 3. 70 µm dia. quartz fiberwith nickel film, gold plated at the ends to provide active sensorlength of 1.25 mm.
The DantecProbe SystemDantec provides a complete probe 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 constructionIn general, a probe consistsof the following:
• Sensor, forming the heating element.
• Sensor supports (prongs or substrate), carrying the sensor and leading cur-rent to it.
• Probe body, carrying the sensor supports.
• Connector, providing elec-trical connection to the probe support or probe cable.
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 suspended between 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 highestpossible 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 configurations covering a wide varietyapplications.
Wire probesWires 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 toparticle contamination.
The sensor supports, orprongs, are made of stainlesssteel and tapered, providingan end surface of around 0.1mm in diameter to which thewires are spot-welded.
Miniature wire probesMiniature wire probes have 5µm diameter, 1.25 mm long plated tungsten wire sensors(Fig. 1). The wires are weldeddirectly to the prongs and theentire wire length acts as asensor. They are general pur-pose probes recommendedfor most measurements in
Gold-plated wires probes areavailable with one, two andthree sensors (single, X- andtri-axial arrays) in six differentconfigurations.
Miscellaneous wire probesTemperature probeFor measurements of quicklyfluctuating temperatures, awire probe with a 1 µm 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 temperatureprobe is available in one straight probe configuration.
Film probesFilm 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 temp-erature coefficients of resist-ance and possess highmechanical and electrical stability.
The films are protected bya sputtered quartz coating,0.5 µm or 2 µm 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.
one- or two-dimensional flowsof low turbulence intensity.The accuracy of turbulencemeasurements may be redu-ced because of interferencefrom the prongs. On the otherhand, the more rigid con-struction makes them moresuitable for high speed appli-cations without the risk ofself-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 probesGold-plated probes have 5µm diameter, 3 mm longplated tungsten wire sensors.The wire ends are copper-and gold-plated to a thicknessof 15 to 20 µm, leaving anactive sensor, 1.25 mm, onthe middle of the wire (Fig. 2).They are designed for mea-surements in high-turbulenceflows of one-, two- and three-dimensions.
The plating of the ends serves 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.
6
Film probes with cylindrical sensors (fiber-film probes)Fiber-film probesFiber-film probes have cylin-drical thin film sensors andmay be used as a substitutefor wire probes in liquids or ingas applications where morerobust probes are needed.Fiber sensors are 70 µm dia-meter quartz fibers, 3 mmlong, covered by a nickel thinfilm approx. 0.1 µm in thick-ness (Fig. 3). The ends arecopper- and gold-plated.
The fiber is soldered ontothe prong ends. Fiber probesfor water applications havelacquer-coated solderingjoints and prongs insulatingthem electrically from the sur-roundings. Fiber probes areavailable with one, two andthree sensors (single, X- andtri-axial arrays) in six differentprong configurations.
Split-fiber probesSplit-fiber probes have twoparallel nickel films depositedon the same quartz fiber, 200µm diameter and 3 mm long(Fig. 8). The ends are copper-and gold-plated, leaving a 1.2mm sensing length. The filmis protected with a 0.5 µmquartz coating.
Split-fibers are intendedfor measurement of instanta-neous velocity and directionin two-dimensional gas flows.They may replace dual-sen-sor fiber probes (X-probes) inslowly fluctuating flows, whena high spatial resolution is re-quired, or when the angle ofattack exceeds the ±45°acceptance angle for conven-tional X-array probes. Split-
fiber probes are available inthree configurations for workin free-stream flows, pipeflows and boundary layers.The standard versions areonly for gas applications.
Film probes with non-cylindrical sensorsFilm probes with non-cylin-drical sensors have the nickelfilm deposited on quartz sub-strates of different aerody-namic shapes: wedges,cones, flat plates or spheres.
The sensor is defined as aline or a ring. Two sputteredsilver leads carry the currentfrom the cable, normally at-tached to the probe body, forward to the sensor. Thesensor is protected by a quartzcoating (0.5 µm in gases and2 µm in liquids), while theleads are insulated by meansof a lacquer coating.
Conical film probesThe sensor of these probes isplaced as a ring (diameterapprox. 0.6 mm) around acone on the tip of a 1.5 mmquartz rod. Sensor dimensionis 1.4x0.2 mm (Fig. 4).
As the sensor is placeddownstream from the very tipof the probe, velocity fluctuati-ons are damped through theboundary layer. The band-width is thus lower than thatof wedge-shaped film probes.The conical probe is less sen-sitive to contamination than afiber or a wedge probe, and itshould be preferred to thewedge types whenever possi-ble. It is available in a straightconfiguration in two versions
for applications in gas andliquid flows.
Flush-mounting probesThese probes have the sen-sor (0.8x0.2 mm) placed onthe flat end of a quartz cylind-er (Fig.5). They are intendedfor measurement of wallshear stress in both laminarand turbulent boundary lay-ers. They may also be usedfor determination of points oftransition and separation.
The working principle isbased on the similarity between temperature and vel-ocity profiles in the viscoussub-layer, and the probe isused in practice like any otherfilm probe. It mounts in a holein the wall of the body underinvestigation. It is available inone straight configuration inversions for liquid and gasapplications.
Glue-on probeThis is a special version ofthe flush-mounting probe,where the sensor is depositedon a KaptonTM foil, 50 µmthick. The sensor is 0.9x0.1mm and connected to gold-plated lead areas (Fig. 6). It isprimarily intended for qualit-ative measurements of pointsof transition and separation. Itis glued directly onto the wallin the points of interest.Copper wires soldered to theleads constitute the electricalconnection between probecable and probe. If the probeis used for quantitative mea-surements it must be calibra-ted in situ, as it normally can-not be removed when firstglued in place.
Fig. 4. Tip of conical probe. Fig. 5. Tip of flush-mounting probe. Fig. 6. Glue-on probe.
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 of transition and separation.Mounts in a hole in the wallconfining the flow to be measured with the substrateend plane flush with the wall.
The sensor is orientedperpendicular to the flowdirection.
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Sensor configurationsThe Dantec Dynamics probesystem comprises probeswith 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 withcylindrical 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 isrotated 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 toprobe 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, sensorperpendicular 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, sensorat 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 of transition and separation andmay measure skin frictionand heat transfer numbers. Itis glued directly onto thewall. The sensor is orientedperpendicular to the flowdirection.
Triple-sensor probesTriple-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 havethree 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 three probe signals are decompo-sed into velocity components.
Used for measurement ofthe 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 probesDual-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 probeaxis 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 operationin 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 probesSplit-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 ofmain 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 thusmeasure 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 slow temperature 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 designThe 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 wireand fiber-film probesWire and fiber-film sensorsare all mounted on probebodies, normally made ofceramic tubes, equipped withconnector pins that connectto 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 probesobesTriple-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 probesThere are three probe sup-port types available for single-sensor and dual-sensor probes: 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 setsof 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-sensorprobes, 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 for triple-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 connector number, which again corre-sponds to the sensor numberon the probe.
Shorting probesShorting 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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Gol
d-pl
ated
wire
pro
bes
Min
iatu
re w
ire p
robe
s
Res
ista
nce
ther
mom
eter
Tem
pera
ture
-com
pens
ated
pro
bes
Gol
d-pl
ated
X-p
robe
s
Min
iatu
re X
-pro
bes
Par
alle
l-sen
sor
prob
es
Trip
le-s
enso
r pr
obes
Fib
er-f
ilm p
robe
s
Con
ical
pro
bes
Flu
sh-m
ount
ing
prob
es
Glu
e-on
pro
bes
Fib
er-f
ilm X
-pro
bes
and
Spl
it-fib
er p
robe
s
Fib
er-f
ilm T
riple
-sen
sor
prob
es
Gases and non-conducting liquids
Conducting liquids
Low and medium temp. up to 150°C
Contaminated flow
Extremely low velocities
Low and medium velocities
High velocities
Large velocity gradients
Varying temperature
Sufficient space
Little space
Very little space
Mean velocity
Instanteneous flow direction
Velocity fluctuations
Extremely low turbulence intensities
Low and medium turbulence intensities
High turbulence intensities
Extremely high-frequency fluctuations
Turbulent shear stress
Spatial turbulence components
Wall shear stress
Temperatures and temp. fluctuations
MEDIUM
Probe selection chart
WIRE PROBES FILM PROBES
Recommended
Applicable
Inapplicable
FLOWCONDITIONS
STATE OFMEDIUM
SPACECONDITIONS
QUANTITYTO BE
MEASURED
MiniCTA
MultiChannel CTA
StreamLine
Hot-wire calibrator
StreamLine calibrator
TYPE OF ANEMOMETER
TYPE OFCALIBRATOR
* * * * ** * * * *
*) 54T42 MiniCTA and 9054N0802/0812/0822 Multichannel versions only!
11
Mounting andadjustment
Avoiding ground loops andnoise pickupBNC 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 supportsmust not touch each other,as this will disturb the opera-tion of the individual CTAservo-loops in the anemome-ter electronics.
Grounding in liquidsThe 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 manualsfor further information). If nogrounding is made, the thinprotective quartz coatingmay break down due to voltage differences causedby electric charges buildingup in the liquid - or directconduction if the liquid issomewhat conductive. Sucha grounding will also reducethe amount of elec-trical noise that can be coupleddielectrically from the liquidto the CTA circuits.
Probe orientationThe probes must be placedin the flow with the samesensor orientation as during calibration.
It is recommended that thewire and fiber probes aremounted with the prongsparallel with the flow when-ever possible in order to avoid vibrations in the prongsduring measurement.
Overheat adjustment Recommended overheatratio for wires and fiber probes in air is 0.8, giving anover-temperature between200 and 300°C, and for filmprobes in water 0.1, corre-sponding to about 30°C.
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 the highest velocity encounteredin the flow.
Disturbing effects
Varying fluid temperatureIf 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. Ifnot 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 transfer drastically. The influence ofdirt increases with decreas-ing diameter; this meanswires are generally moresensitive than fibers.
In liquids, particle contam-ination may be a serious problem, 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 is recommended 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 reactionsChemical 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 thevicinity of the damage. This
results in an increase in sen-sor resistance and decreasein sensitivity. If electrolysis isallowed to continue, the sen-sor will eventually disappear.
BubblesIn 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 kept below 60°C. If the sensorelement is partly coveredwith bubbles temperature gradients may actuallydamage the thin protectivequartz coating.
Vortex shedding andvibrationsVibrations in the probe mounting or even in the sup-ports can occur at high velocities, introducing noiseto the probe signal. Careshould be taken to take proper 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 µm wire probe.
Fig. 19. Calibration curves for clean and contaminated (dust) hot-wire probe.
Fig. 20. Wire magazine with 10 wires for gold-plated wire probes.
Maintenance and repairs
Control and testing atDantec DynamicsEach probe is thoroughly con-trolled and tested at DantecDynamics 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 sensorCleaning 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 cleaning sensors in order to avoid mechanical 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 cleanedwith acetone, so that they areabsolutely free from any traces 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 repairFiber 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 fiber probes are returned toDantec Dynamics for repair.
Film probe repairFilm 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 thesensitivity and frequencyresponse. Replacement ofthe probe is to be preferred.
12
13
Unit Tung- Pure PtRh Pt Ir Nickel sten platinum 10% Rh 20% Ir
Resistivity Ω · 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 Ω · W · 109 4.1 5.7 4.4 3.6 4.5
Summary oftechnical data
Sensor materialThe 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 applicationsmainly due to its high mecha-nical strength.
Sensor resistanceThe 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 varymore 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 20°C.
Temperature coefficient ofresistanceThe relation between resist-ance and temperature for ametallic conductor can beexpressed by means of the
temperature coefficient of resistance α0 (TCR):
RT = R0·(1 + α0·(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 α0refers to 20°C.
The TCR at another tempera-ture T1 may be calculated as:
α1 = α20/(1 + α20· (T1 - 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 σ (Ω · m)• High temperature coefficient of resistance α (%/K)
To provide small time constant:• Small density ρ (kg/m3)• Small heat capacity c (J/kg/K)
To reduce heat transfer to the prongs:• Small thermal conductivity λ (W/m/K)
To withstand the flow:• High tensile strength γ (N/m2)• High resistance against chemical attacks (oxidisation)
Table A. *) This value is for nickel in its bulk condition. When sputtered, the temperature coefficient of 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 the probe label.
Lead resistanceThe lead resistance RL is theinternal probe resistance defined 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 the probe 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 for stable 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 150°C. 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 is normally 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 pressureCTA 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 velocity Minimum velocityThe lower velocity limit is de-fined by the onset of naturalconvection. If a probe is
14
calibrated and used under thesame orientation with respectto the gravity field, it may beused at even lower velocities.
The limit will then bereached when the naturalconvection overrules the forc-ed convection. This happenswhen the Reynolds numberRe becomes smaller than twotimes the Grashoff number tothe power 1/3:
Re < 2·Gr1/3
where Re = U·D/n and Gr = g·D3·β·(Tw-T0)/ν2. U isfluid velocity, D is sensor dia-meter, ν is kinematic viscosityof fluid, g is acceleration of gravity, β is coefficient of ther-mal expansion (equal to 1/Tfor a perfect gas) and (Tw - T0)is sensor over temperature.The limit may be reduced if
the sensor over-temperatureis lowered, as this will lowerthe Grashoff number.
Maximum velocityNormally, wires and fiber filmsensors are designed to with-stand the aerodynamic loadsoccurring in practice, even atsupersonic speeds. The uppervelocity limit in the data sheetfor wire and film probes is defined as the velocity that creates a stagnation tempera-ture of 220°C on the sensor.Special probe designs maywork up to considerably high-er velocities.
Frequency limitin CTA modeThe frequency limit fcmaxrepresents what may be ex-pected when the probe is
Platedtungsten
Platedtungsten
Platinum
Nickel
Nickel
Nickel
Nickel
Nickel
Gold-plated wire sensors
Miniature wire sensors
Wire sensors for temperaturemeasurements 3)
Fiber-film sensors
Split-fiber sensors
Conical film sensors
Flush-mounting film sensors
Glue-on film sensors
Sen
sor
mat
eria
l
Sen
sor
dim
ensi
ons
Thi
chne
ss o
fqu
artz
coa
ting
Sen
sor
resi
stan
ceR
20(a
ppro
x.)
Tem
pera
ture
coef
ficie
nt o
fre
sist
ance
(T
CR
)α
20(a
ppro
x.)
Max
. sen
sor
tem
pera
ture
Max
. am
bien
tte
mpe
ratu
re
Max
. am
bien
tpr
essu
re
Min
. vel
ocity
Max
. vel
ocity
Freq
uenc
ylim
it f m
ax(6
3% r
espo
nse)
Med
ium
5 µm dia.1.25 mmlong 1)
5 µm dia.1.25 mmlong
1 µm dia.0.4 mmlong
70 µm dia.1.25 mmlong 3)
200 µm dia.1.25 mmlong
0.75 mm ×0.1 mm
0.75 mm ×0.2 mm
0.9 mm ×0.1 mm
–
–
–0.5 µm
2 µm
0.5 µm
0.5 µm
2 µm
0.5 µm
2 µm
0.5 µm
– 2)
– 2)
– 2)
–––
70 bar
70 bar 4)
70 bar
70 bar 4)
–
0.20 m/s
0.20 m/s
–0.20 m/s
0.01 m/s
0.20 m/s
0.1 m/s
0.01 m/s
–––
Air
Air
Air
Air
Water
Air
Air
Water
Air
Water
Air
200 m/s
500 m/s
60 m/s
350 m/s
10 m/s
350 m/s
500 m/s
25 m/s
–––
90 kHz 5)
150 kHz 5)
2 kHz 6)
90 kHz
30 kHz
40 kHz
50 kHz
20 kHz
–––
3.5 Ω
3.5 Ω
50 Ω
6 Ω
6 Ω
6 Ω
15 Ω
15 Ω
15 Ω
15 Ω
15 Ω
0.36%/K
0.36%/K
0.35%/K
0.40%/K
0.40%/K
0.40%/K
0.35%/K
0.35%/K
0.35%/K
0.35%/K
0.40%/°C
300°C
300°C
–300°C
60°C 7)8)
300°C
300°C
60°C 7)8)
200°C
60°C 7)8)
200°C
150°C
150°C
150°C
100°C
100°C
100°C
100°C
100°C
100°C
100°C
120°C
1) Overall tungsten wire length is 3 mm. Wire ends are gold-plated to 25-30 µm dia., limiting sensor length.2) Depending on type of mounting.3) Overall fiber length is 3 mm. Fiber ends are gold-plated, limiting sensor length.4) At 20°C, decreasing with increasing temperature, 1 bar at 80° (applies to probe design listed).5) At 100 m/s.6) Constant current mode.7) At 1 bar atmosphere pressure8) Max. 150°C at elevated pressure. Avoid bubble formation.
NOTES
SENSORTYPE
Summary of Technical data
exposed to a velocity (norm-ally 100 m/s in air) and opera-ted in an optimally adjustedCTA anemometer (closedloop). For wire probes thebandwidth may be calculateddirectly from the square wavetest as (1.3·τ)-1, where τ isthe time between start and first zero crossing of the re-sponse curve.
The bandwidth of film probes is in theory limitedonly by the servo loop, as thethermal inertia of the thin-filmsensor is neglectible. In prac-tice, however, the bandwidthis determined by the dampingeffect of the backing substrate,the protecting quartz coatingand the flow boundary layer.The maximum bandwidthsstated in the technical datafor fiber and film probes in air
are calculated from the squa-re wave test as (1.3·τ)-1. Thestated bandwidths serve asan indication of optimium ad-justment of the servolooprather than a measure of thereal bandwidth.
In water the boundary lay-er over the sensor plays apredominant role, and in pra-ctice fiber and film probesnever exceed bandwidths ofmore than 0.5 to 1 kHz inliquids. This is normally fullyadequate because of the lowfrequency content of mostliquid flows.
Secondary heat transferthrough the substrate makesthe amplitude characteristic offilms probes non-linear at lowfrequencies (below 100 Hz).
Type of flow Medium Recommended probes
One-dimensional
Uni-directional Gas Single-sensor wire Single-sensor fiber, thin coatingConical film, thin coating
Liquid Single-sensor fiber, heavy coatingConical film, heavy coating
Bi-directional Gas Split-fibers, thin coating
Liquid Split-fibers, heavy coating, special
Two-dimensional
One quadrant Gas X-array wiresX-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 wire70° 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 coatingGlue-on film, thin coating
Liquids Flush-mounting film, heavy coatingGlue-on film, special
15
Quick guide to probe selection
Wall flows (shear stress)
16
Probes and probe supports
17
SINGLE-SENSOR PROBES WITH CYLINDRICAL SENSORS
Miniature wire probesPlated tungsten wire, diameter 5 µm,length 1.25 mm.
Gold-plated wire probesPlated tungsten wire, diameter 5 µm,overall length 3 mm, sensitive wire length1.25 mm. Copper and gold plated at theends to a diameter of approx. 15 µm.
Fiber-film probesNickel film deposited on 70 µm diameterquartz fiber. Overall length 3 mm, sensiti-ve film length 1.25 mm. Copper and goldplated at the ends. Film is protected by aquartz coating approx. 0.5 µm or 2 µm inthickness.
All dimensions in millimeters.
17
SINGLE-SENSOR PROBES WITH NON-CYLINDRICAL SENSORS
Conical film probes
Flush-mounting film probes
Glue-on probes
All dimensions in millimeters.
18
All dimensions in millimeters.
DUAL-SENSOR PROBES WITH CYLINDRICAL SENSORS
Miniature wire probes Gold-plated wire probes Fiber-film probes
19
Plated tungsten wire, diameter 5µm, overallPlated tungsten wire, diameter 5µm, length
TRIPLE-SENSOR PROBES WITH CYLINDRICAL SENSORS
All dimensions in millimeters.
20
MISCELLANEOUS PROBES
Resistance thermometer
Parallel-array probe
Temperature-compensated miniature wire probes
21All dimensions in millimeters.
PROBE SUPPORTS
4 mm dia. probe supports for single-sensor probes
6 mm dia. probe supports for dual-sensor probes
22
PROBE SUPPORTS
Probe supports for triple-sensor probes
SHORTING PROBES
23
MOUNTING TUBES AND GUIDE TUBES
Chucks and couplings
Watertight mounting tubes
Guide tubes
24
2 m of plated tungsten wire, 5 µm dia. 55A40
Wire magazine with 10 gold-plated wires, 5 µm 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 offers probes and supports for special 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 and constraints
Wires for probe repair
Hot-wire Calibrator - Time-saving 2-point calibration of hot-wire probesThe Dantec Dynamics Hot-wire Calibrator is accurate,device for calibration of hot-wire probes in air from 0,5m/s to 60 m/s. By combiningcalibration at just two veloci-ties with a generic transferfunction, a calibration functionvalid for the entire velocityrange can be created.
The calibrator produces afree jet, where the probe is placed during calibration. Itrequires a normal pressurizedair supply.
In addition to two-pointmode, velocities in the entirerange can be set manually.
StreamLine Calibrator- Automatic calibration ofhot-wire probes
The StreamLine calibrationsystem is intended for computer- automated calibra-tion of probes in air from afew cm/sec up to Mach 1.
The flow unit creates afree jet and requires air froma pressurized air supply. The probe to be calibrated is placed at the jet exit. Theoptional pitch-yaw manipula-tor allows 2D and 3D probesto be rotated for calibration ofdirectional sensitivity.
25
Dantec Dynamics has nearly50 years of experience inmaking Constant TemperatureAnemometers. The probesand anemometers have beendesigned for perfect matching.
The product programmecomprises three lines of ane-mometers:
MiniCTA- Cost effective miniatureConstant Temperature Anemometer system forbasic flow studies
The MiniCTA system is a ver-satile anemometer that canbe used with DantecDynamics wire and fibre-filmprobes in airflows. It is espe-cially suitable for basic flowdiagnostics and its small sizefacilitates mounting close tothe probe or even for it to bebuilt into flow models. Quickcalibration of probes is possi-ble with an optional hot-wirecalibrator.
Multichannel CTASystem- Efficient and affordablesolution for mapping ofvelocity and turbulencefields in most air flows
StreamLine ResearchCTA system- For high-precision CTAmeasurements
Hot-wire Anemometry systems CTA Calibration systems
The Multichannel CTA offersan efficient and affordablesolution for mapping of veloci-ty and turbulence fields inmost air flows. Up to 16points can be monitoredsimultaneously, reducing theneed for traversing. A versionwith reference velocity trans-ducer allows for simultaneouscalibration of all probes in awind tunnel. This meansreduced experimental timeand lower costs.
The StreamLine systemoffers a complete concept ofhot-wire anemometry for effi-cient, reliable and cost-effecti-ve flow analysis in air (orother gases) and liquids.
StreamLine is computer-controlled and is integratedwith a fully automatic probecalibrator.The system isdesigned for high-precisionmeasurements and is DantecDynamics’ top-of-the-line CTAanemometer.
About Dantec Dynamics
Worldwide representationFrom our six offices and more than 30 distributors worldwide weapproach our customers individually. We examine the specific needsand find the best solution for you. For us you are a long-term partnerin improving efficiency, safety and quality of life.A list of representatives is available at our website.
DENMARK (headquarters)Dantec Dynamics A/[email protected]
FRANCEDantec Dynamics [email protected]
GERMANYDantec Dynamics [email protected]
JAPANDantec Dynamics [email protected]
UNITED KINGDOMDantec Dynamics [email protected]
USADantec Dynamics [email protected]
www.dantecdynamics.comThe specifications in this document are subject tochange without notice. Dantec Dynamics is trademark of Dantec Dynamics A/S.
Publication No.: 238_v9
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:
Dantec Dynamics, a Nova Instruments company
