9020184/E/497AIRFLOW'·Telescopic Pitot-Static TubeNote: to prevent the concentric flexible tube from the Pitot becoming 'crimped', ensure that the Pilotis replaced into its wallet/case nose end first. If the tube does become crimped, immerse the crimpedportion into hot water to restore it to its correct shape

I\1. Introduction

The telescopic pitot-static tube is anaddition to the established range of onepiece and jointed pitot-static tubesmanufactured by Airflow Developments \since 1955.The telescopic pltot-statlc tube, incommon with most Airflow pitot-static ltubes incorporates the modified semi- Iellipsoidal nose form recommended forflow measurement in 8S1042 Section2.1: 1983.The telescopic pitot-static tube has theadvantage of convenience andportability over one piece pitot-statictubes. When not in use it can bereduced to a length of 200 mm, whilst inuse, it can be extended to 980 mm. It isnot intended for use as a permanentinstallation and may be used up to amaximum temperature of 100·C forshort periods only.

2. Description of thetelescopic pitot-static tubeThe pitot-static tube consists of a 4 mmdiameter pitot-static head protruding,when extended, 70 mm from a sevensection telescopic tube. Concentricflexible tubes pass through thetelescopic section, protrude about 300mm from the end when the telescopictubes are fully extended, and terminatein a 'tee' piece from which connectionsto a manometer are made.A direction indicator is clipped to thelargest diameter section of thetelescopic tube. This can be adjusted toalign with the head direction prior to

insertion in a duct. Care must be takenwith alignment during insertion and withgeneral handling. .,..~

FIG. 1DUCT WALL

••• DIRECTION •••

- - OFFLOWtI • I,•. STATIC

TOTAL PRESSURE PRESSUREHOLE HOLES

_ DIRECTION _OF FLOW

t .tDUCT WALL I

APERTURE IN DUCT WALL

TELESCOPIC PROBE sDIRECTION INDICATOR

~

EY

\ • OUCT STATIC PRESSUREI (Ps)

I_I TOTAL PRESSURE (P,)

- VELOCITY PRESSURE, (Pv)

TOTAL PRESSURETAPPING(+)

MANOMETERS WILL READ Pv - P, - Po

3. Use of the pitot-statlctube

3.1 To determine the velocity orvolume flow in a ducted system itis usually necessary to carry outa 'traverse' with the pitot-statictube in a plain section of duct,and at right angles to the ductwalls. If possible, the sectionchosen should be at least sixdiameters (or equivalent)downstream of any bend orobstruction in the system. Drillholes at the appropriate points inthe duct wall to enable the pitot-static tube to be inserted andmoved successively to thevarious positions required. Forrecommended traverse positionssee fig 3 for circular ducts and fig4 for rectangular ducts.

3.2 Extend the telescopic pitot-statictube to the length necessary forthe tests to be carried out.To maximise accuracy alwaysextend the head to about 70 mmrelative to the smallest telescopictube even if the remainder of thetubes are not extended: (see fig 2)

HEADSTOWED

HEAD EXTENDEDFOR USE

70mm

L _

FIG. 2

3.3

When collapsing the pitot-statictube ensure that the flexible tubecan run freely from the other endof the telescopic tube.When carrying out a traverse it ispossible to extend the telescopicprobe in such a way so as to usethe junctions of the tubes asmarkers to determine theinsertion depth of the pitot head.

3.4 Connect the total and staticpressure tappings on the pitot-static tube 'tee' piece to the +and - connectors of a sensitivemanometer (see fig 1). Themanometer will then indicate thevelocity pressure (Pv), [Velocitypressure (Pv) = total pressure(Pt) - static pressure (Ps)]. Fromthe velocity pressure, the velocityand volume flow can be found.The slide rule supplied with eachpitot-static tube may be used tospeed up velocity calculations.

3.5 To obtain the best result ensurethat the tip of the pitot-static tubefaces directly into the flow in theduct. Reliable results will only beobtained if the flow in the duct issubstantially parallel to the walls.If swirl or cross flow is suspected,this can be checked by insertingthe pitot-static tube in severalpositions and turning the head inboth pitch and yaw to find theangle at which the maximumvelocity pressure is obtained. Ifthis is no more than about 15°from being parallel to the ductwalls, no great error will occur inthe results. See fig 5 for theeffect of yaw angle.

3.6 To measure the total pressure(Pt) or static pressure (Ps) only,connect the appllcable tappingon the 'tee' of the pitot-static tubeto the appropriate manometerconnector leaving the otherconnector open to atmosphere.

3.7 The telescopic pitot-static tube isparticularly suited for use withelectronic manometers such asthose found in the Airflow APM,EDM or MEDM ranges. TheMEDM 500 model can give directreadings of pressure, velocity orvolume flow and can memoriseand average readings. Airflowalso manufacture a range ofliquid in glass manometers which

~~~~~~~~~-----.---- -----------

may be used with the;telescopicpitot-static tube. In common withall smaller pitot-static tubes, dueto the small size of the pitot headand flexible connecting tubes,telescopic pitot-statis,tubes willexhibit marginally slowerresponse times, thanllarqer pitot-static tubes when used withliquid in glass manometers.

3.8 The telescopic pltot-static tubecan be used with 2 mm boresilicon rubber tubing as suppliedwith the APM range or 5 mm bore.PVC tubing supplied rvith liquid inglass manometers. '

FIG. 3/~-, \

,~ ..~ ,+.-+.~'--t._f-,I~ ..~\I- \, I'It .--t .\-'

L L032:10

I 0.6790~• 0.8650--,

0.9680 iI~~~-------O ~

Log Linear rule lor traverse pomison 3 diameters In a Circular d,uci

''Ii.' '

0.0320

0.1350

Nole: Should dilficully In access make it rrnpossibte10Iraverse on more than Iwo diamelers: Iheseshould be rnutuauv at right angles and I,he numbersof points on each line must be increasep to toThe spacing should be as lollows: ' I

0019DO.077DO,153D0.217DO 361D ~ =O.639D O,783D O.847D O.923D 0 981D' fur° ~

'j)--<jr--(jt-"i>- -<jl---,:"'t I r. , , I I•.. -$-. -t+ -4-- -$_. ~~

+- +_~+ __<jl_ -+ . -.tj !t-+.~.++.-+-- J

-$--, '-E!>--¢ ----Ot)6IW~ • I 1

023S:V.~w_J FIG,4nSti3w--..J

1--~~-'--09J9W~---'...j

Pos.non 01auernanve measurmg pornts and Iraverselines reiauve 10 Side lengths lor rectangular duCIS

NO.OIpomts or Position relative 10 Inner walltraverse

lines

5 0.074,0.288,0,5,0.712,0.926,6 0,061. 0.235. 0.437, 0.563, 0.765, 0.939,

7 0.053,0,203,0366,0,5,0.634, 0.797,0.949.

Log Tchebvcneu Rule lor Reclangular Ducts

Nole: Distance belween measuring stations shouldI not exceed 200mm

FIG. 5EFFECT OF YAW ANGLE ON PRESSURE OUTPUTOF MODIFIED ELLIPSOIDAL NOSE PITOT TUBE,---,---..,----r----,---------, +00

+0' 15. _.+0.2 ffi._ o~

a:__ .-02 ~

IF>-04 ~

-06 -Is•• 0.8 ~

-10 ~

-12 is'0' 5~ 0 SO

YAW ANGLE'0'

4. Air Velocity CalculationsUsing S.I. ScalesThe standard formula for calculatingvelocity from velocity pressure isV = 1.291 ~ This is only correct fora standard air density of 1.2kg/m3

(at 16°C, 1000mb and 65%RH)For non-standard air conditions thisequation becomes:

V = 1.2911/1000 x .L x 100000 x PvV B 289 100000+Ps

V = velocity m/sB = barometric pressure mbarT = absolute temperature OK

(= t'C + 273 where t is airstreamtemperature)

Ps = static pressure PaPv = velocity pressure Pa

Th,100000, .

e expression IS a correction100000 + Ps '

for the static pressure in the duct andmay normally be ignored if Ps is lessthan 2500 Pa.

When calculating average velocity in aduct, the square roots of the velocity

F pressures should be averaged. Inpractice however, no great error will beintroduced by taking the square root ofthe average of velocity pressures whenthe majority of the readings do not varyby more than about ±25% from a meanfigure.

5. Air Velocity CalculationsUsing Imperial ScalesThe standard formula for calculatingvelocity from velocity pressure isV = 4000 ~ This is only correct fora standard air density of 0.0749 Ibs/ft3(at 68°F, 30"Hg and 65%RH)For non-standard air conditions thisequation becomes:

V = 4000 30 x T x 408 x Pv- -B 528 408+Ps

V = velocity ftlminB = barometric pressure ins HgT = absolute temperature OR

(= tOF+ 460 where t is airstreamtemperature)

Ps = static pressure ins wgPv = velocity pressure wg

Th . 408. .e expression ISa correction408 + Ps

for the static pressure in the duct andmay normally be ignored if Ps is lessthan 10 ins wg.When calculating average velocity in aduct, the square roots of the velocitypressures should be averaged. Inpractice however, no great error will beintroduced by taking the square root of

the average of velocity pressures whenthe majority! of the readings do not varyby more than about ±25% from a meanfigure.

6. Air IVelocity CalculationsUsing Metric ScalesThe standard formula for calculatingvelocity from velocity pressure isV = 4.05 ~ This is only correct fora standard air density of 1.2kg/m3

(at 20°C, 76'2mmHg and 65%RH) .For non-standard air conditions thisequation becomes:

V = 4.05 11/762 x lx 10363 x Pv\ B 293 10363+ Ps

V = velocity rn/sB = barometric pressure mm HgT = absolute temperature OK

(= tOe+ 273 where t is airstreamtemperature)

Ps = static' pressure mm H20Pv = velocity pressure mm Hp

Th . 10350. t'e expression ISa correc Ion10350 + Ps

for the static pressure in the duct andmay normally be ignored if Ps is lessthan 250 rnrn H20.

IWhen calcufating average velocity in aduct, the square roots of the velocitypressures shoutd be averaged. Inpractice however, no great error will beintroduced t1ytaking the square root ofthe average bf velocity pressures whenthe majority bf the readings do not varyby more than about ±25% from a meanfigure. '

TSllnstruments Ltd.

Stirling House, 373 Stirling Road, Cressex Business ParkHigh Wycombe, Buckinghamshire, HP12 3RT United Kingdomtel +44 (0) 1494459200 fax +44 (0) 1494459700e-meii answers@tsi.com web www.tsi.com