112

Velocity measurement technology

60

100

40

201510

0

m/s

to +140 °C

Thermalprobe up to

+70 °Cto

+350 °C

Pitot tubeto

+600 °C

NiCr-NiPitot tube

-40 to +600 °C

Vane probesup to +60 °C

Vane/temperature probes

Thermal hot wire probe formeasuring velocity, withdirection recognitionfunction

Probe selection

Thermal probes

Location selection

D

4 x D10 x D

Direction of flow

The principle of the thermal probeis based on a heated element fromwhich heat is extracted by thecolder impact flow. The temperatureis kept constant via a regulatingswitch. The controlling current isdirectly proportional to the velocity.When thermal velocity probes areused in turbulent flows, themeasured result is influenced bythe flows impacting the heatedbody from all directions. Inturbulent flows, a thermal velocity

Thermal probes are used foraccurate measurements in therange 0 to 5 m/s. Vane probes areideal for velocities from 5 to 40m/s. The measuring range of thePitot tube depends on thedifferential probe used. The new100 Pa probe can therefore be usedfor the exact measurement of the

flow speed from approx. 1 m/s to12 m/s. The Pitot tube yieldsoptimum results in the highervelocity range. An additionalcriterion when selecting the correctvelocity probe is the temperature.Thermal sensors can normally beused at up to approx. +70 °C.Special design vane probes can be

Measuring and application ranges of the velocity probes

sensor indicates higher measuredvalues than a vane probe. This canbe observed during measurementsin ducts. Depending on the designof the duct, turbulent flows canoccur even at low velocities.

You should measure in a straightpart of the duct, if possible. Theduct part should have a minimumof ten diameters of straight runbefore the measuring spot and

used to maximum +350 °C. Pitottubes are used for temperaturesabove +350 °C.

The flow measuring range 0 to 100m/s can be divided into threesections:– Low-speed velocity 0 to 5 m/s– Mid-speed velocity 5 to 40 m/s– High-speed velocity 40 to 100

m/s.

four diameters of straight run afterthe measuring spot. The flowprofile should not be interrupted inany way by flaps, dips, angles etc.

113

Velocity measurement technology

The vane probe is adjusted exactlyif the flow direction is parallel to thevane axis.

If the measuring probe is turnedslightly in the air current, the valueshown in the instrument changes.The measuring probe is positionedexactly in the air current if the valueshown is at max.

When measuring in a duct thereshould also be a minimum of tendiameters of straight run before themeasuring spot and four diametersof straight run after the spot for bestresults. By design, vanes are lessinfluenced by turbulence thanthermal probes or Pitot tubes.

Vane probes

Positioning in the air current

As part of approval measurements,indirect measuring procedures(grid measurements) are used tomeasure air flows.The following procedures aresuggested in VDI 2080/EN 12599:

Send for our technical manual

You will find lots of detailedinformation on air flowmeasurement in Testo´s HVACtechnical manual.

The measuring principle of the vaneprobe is based on the conversion ofa rotation into electric signals. Theagent which flows makes the vanerotate. An inductive proximityswitch “counts” the revolutions ofthe vane and supplies a pulsesequence which is converted in themeasuring instrument and is thenindicated as a velocity value.

Large diameters (Ø 60 mm, Ø 100mm) are suitable for themeasurement of turbulent flows (e.g.at outlet ducts) at smaller ormedium velocities. Small diametersare more suitable for measurementsin ducts in which case the ductcross-section must be 100 timesbigger than the probe cross-section

being impacted.The 16 mm probe has proven to bevery versatile. It is large enough tohave good starting qualities and issmall enough to withstandvelocities of up to 60 m/s.

Measuring velocity in ducts

– Trivial procedure for gridmeasurements in square cross-sections

– Centroidal axis procedures forgrid measurements in circularcross-sections

– Loglinear procedure for gridmeasurements in circular crosssections.

114

Velocity measurement technology

Even without the disturbing effectsof a grid in an aperture, the lines offlow are not directional and the flowprofile is irregular. Because apartial vacuum in the duct draws airout of the room in a funnel shapeeven a short distance from theaperture, there is no defined area inthe room over which ameasurement could be made.Therefore, only the duct or funnelmeasurement yields reproducibleresults. Measuring funnels ofvarious sizes are available for suchapplications.

These create defined flow

conditions at a known distancefrom the grid with a fixed volume. A velocity probe is positionedcentrally and secured. The volumeflow is calculated from the velocitymultiplied by the funnel factor(e.g. funnel factor 22).

Measurements at suction apertures using a volume flowfunnel

The air vent greatly changes therelatively uniform flow inside theduct. Areas of higher flow velocityare created at the free vent surfacesand areas of low flow velocity andswirl at the grids. The flow profilesteadies at a distance from the griddepending on the grid design but isusually 20 cm. For best accuracy,

Supply/Returns

Max. values

Min. values

Mean values

Laminar flow

in the centre

of the duct

Measuring volume flow with a funnel

Velocity probe

x m/sFunnel

m3/h

a large diameter vane isrecommended. The area of the vanehelps to get an average reading ofthe turbulent flow from the grid.

v m3

h= x m/s * 22 v = Volume

x = Velocity22 = Funnel factor

Ambient air velocity is a veryimportant parameter in the thermalcomfort of people in rooms. testo400 supplies the current and meanair velocities. The maximumpermissible mean air velocitydepends on the air temperaturemeasured by testo 400 and theamount of turbulence calculatedfrom the air velocity.

Measuring ambient air velocity using testo 400 in accordance with DIN 1946 Part 2, ANSI/Ashrae 55-1992

The example shows a permissiblemean air velocity of 0.26 m/s withan air temperature measured at24.4 °C and an automaticallycalculated degree of turbulence of10%.

Air temperature °F

Mea

n ai

r vel

ocity

fpm

Mea

n ai

r vel

ocity

m/s

Air temperature °C

Degree of turbulence T

115

Velocity measurement technology

v = Velocity in m/s s = Pitot tube factor

= Air density in kg/m3

p = Differential pressure inPascal measured in Pitottube

Pitot tube

Measuring errors occur oftenbecause a mean density of 1200g/m3 is used in calculations. Whenmeasuring outer air flows, theactual air density can deviate by upto ±10% from the given meanvalue. Therefore an inaccuracy inthe air flow of up to ±5% canresult.

The testo 400 can compensate forthis by activating an automaticconversion for the Pitot tubepressure to velocity.

Absolute pressure offset

Barometric pressure

Metres above sea level

Temperature

Humidity

Absolute pressure in the duct

Density

Density factors

The correct air density can be easily input intesto 400

Static pressure

Total pressure

a

b

Multi-point averaging can then becarried out directly in m/s values.

It is important that the correct airhumidity is input in theconfiguration menu or that youmeasure absolute pressure,temperature and humidity with the0638 1645 absolute pressure probeand a temperature/humidity probe.testo 400 automatically calculatesdensity on the basis of themeasured values.

Static pressure

Total pressure

���v = s • ρ

ρ

2 • p

Pitot tube factor: s = 1.0 s = 0.67

MemoryLocationProbe

Special

Instr.PrintLanguage

MemoryLocationProbe

Special

Instr.PrintLanguage

MemoryLocationProbe

Special

Instr.PrintLanguage

Temp. 20.1 °CHumidity 50.0 %abs. Press. 911 mbar

1078.0g/m3

Density

Main menu

Special

Parameter

Parameter

Pitot

Parameter

Pitot tube factor

TemperatureHumidityabs. Press.Metre a.s.l.Barom. pressure

Diff. pressure

Density

The Pitot tube opening takes on thecomplete pressure and conducts itto connection (a) in the pressureprobe. The pure static pressure istaken up by a lateral slot andconducted to connection (b). Theresulting differential pressure is adynamic flow-dependent pressurewhich is then analysed andindicated.

As with thermal probes, the Pitottube is more likely to react toturbulent flows than a vane probe.Therefore, a free inlet and outletpath must also be ensured duringPitot tube measurements.

116

Velocity measurement technology

° ° ° °° ° ° °

Settings /Grid measurement

Duct cross section Geometrical data

Round

Square

Rectangular

Area

Long side 1.00 m

2.00 m

2.00

1.00

m2

Lateral side

Area

Correction factor

OK Cancel

Accuracy

Duct 1.0.0.0. (x)

Duct 1.0.0.1. (x)Duct 1.0.0.2. (x)Duct 1.0.1.0. (x)Duct 1.0.2.0. (x)Duct 1.0.3.0. (x)Duct 1.1.0.0. (x)Duct 1.2.0.0. (x)Duct 1.3.0.0. (x)Duct 2.0.0.0. (x)

x + + ++ + + +

1/1 (H 250 / V 500 mm)

+ Measurement points designatedaccording to measuring specificationSelected measurement point

x Previously measured measurement point

+

Mean

2.0 +- 0.8 m/s

14760 +- 5525 m3/h

Assessment of VAC systems on location

The “VAC module” option wasdeveloped for testo 400 to carry outquick and rational assessment ofVAC systems. This new optioncarries out measurements on sitequickly and efficiently andautomatically provides printouts.

Inaccurate data calculations as wellas the time consuming entry of dateand time are eliminated with testo400. testo 400, with its VACmodule, is currently the onlymeasuring system worldwide withwhich a quick and objective

The velocity measuring instrument knows the duct dimensions

Preparation of measurement onlocation. All data related to themeasurement location are entered,prior to the measurement, in yourtesto 400 via PC. All you have to doon site is call up the currentlocation to access the information

available in testo 400. Themeasurement results are saved inthe location name selected by you.The volume flow is calculated usingthe duct data saved in theinstrument.

Measurement stipulation integrated in instrument

User-guided processing ofmeasurement stipulation inaccordance with standard. Themeasurement points are displayedin testo 400. testo 400 assigns therespective coordinates in the duct tothe selected measurement point.

Assessment of overall uncertainty on location

Overall uncertainty is made up ofthe irregularity of the velocityprofile, location inaccuracy,inaccuracy of the duct dimensions,accuracy of the velocitymeasurement system used and thenumber of measurement points. testo 400 takes all of theseinfluences into consideration. Inthis way, the overall uncertainty ofthe measurement can be assesseddirectly on location.

Printout of measurement results in standard layout

The PC software uses all of therelevant data in testo 400 andshows them in the measurementprotocol for each individuallocation. Time-consuming entry of

assessment of the functionality of aventilation system can be carriedout. Evaluations can be carried outwithout any additional calculations.The measurement stipulations arebased on internationally recognisedstandards; VDI 2080 in Germany,

Euronorm (EN) 12599/Draft andAshrae standards in the US.

all the readings and otherparameters are thus eliminated.Processing of the measurementprotocols is made much easier andquicker.

Duct cross-section

Mean value withmin/max

Measure-ment pointswithcoordinatesand averagevalue}

All values markedwith colour areautomaticallytaken over bytesto 400

Space for your company logo

Protocol14

Grid measurement according to VDI 2080, DIN EN 12599

Object: Jones Ltd. K: 2m/s 10.08 (MIN: 7,80 / MAX: 13.10)

Starting time: 25.08.1998 16:14:05

Date: 25.08.1998 Page 1/1

Finishing time: 25.08.1998 16:14:20

VAC system: CenterVentilator rpm: 500 rpmResponsible: Manfred Schulze

Instrument: testo 400 Ref. instruments:No calibration data included

Title: Müller_12.345/8

Duct dimensions: 1.000 x 2.000 (m)Grid: 4 x 4

Means of quadrants:

Condition of outside air Air pressure pa: 950 hPaTemperature ta: 27.4 °CHumidity RHa: 45.0 %RHAbs.pressure: 1013.0 hPaConditions in ductTemperature: 22.0°CSettings/T400Humidity: 35.0 %RH

Profile irregularity: % 17.0Uncertainty of location: % 7.8

Uncertainties:Uncertainty of air density: g/m3 1Accuracy of duct dimensions: mm 2

Volume flow: 72585.0 m3/hUncertainty (abs.): 6427.3 m3/hUncertainty (rel.): 8.9 %

Meas. area: 2,000 (m2)Meas. points: 16Hydr. diam.: 1.333 (m)

Comment: Ref value 50,000 m3/h, 22°C, center, exhaust

Probe: Pitot tubeLast calibration: 1.2.1998

for test report no.:637 / 0998

Meas. point

1 2 Mean

8.451 9.00 10.08

11.002 11.88

m/s 1 2 3 4

Distance (mm) 200 400 600 800

a 325 7.8 8.0 8.4 8.7

b 775 8.9 9.1 9.2 9.7

c 1225 9.9 10.5 10.8 11.1

d 1675 11.7 11.9 12.5 13.1

Uncertainties/Meas. system:Instrument accuracy: digit 1Probe accuracy: m/s 0.40

SignatureDate: 25.08.1998 Name:

M. Spencer

Air density: 1192.0 g/m3Mass flow: 06521.3 kg/hStandard volume flow: 67174.6 M3/h(N)