Probe selection

The flow measuring range 0 to 100 m/s can be divided into three sections:

- Low- speed velocity 0 to 5 m/s

- Mid- speed velocity 5 to 40 m/s

- High- speed velocity 40 to 100 m/s

Thermal probes are used for accurate measurements in the range 0 to 5 m/s. Vane probes are ideal for velocities from 5 to 40 m/s.

The depends on the differential probe used. The  new 100 Pa probe can therefore be used for the exact measurement of the flow speed from approx. 1 m/s to 12 m/s. The Pitot tube yields optimum results in the higher velocity range. An additional criterion when selecting the correct velocity probe is the temperature.

sensors can normally be used at up to approx. +70 ^°C. Special design vane probes can be used to maximum + 350 ^°C Pitot tubes are used for temperatures above 
+350 ^°C.

Thermal probes

The principle of the thermal probe is based on a heated element from which heat is extracted by the colder impact flow. The temperature is kept constant  via a regulating switch. The controlling current is directly proportional to the velocity.

When  thermal velocity probes are used in turbulent flows, the measured result is influenced by the flows impacting the heated body from all directions. In turbulent flows, a thermal velocity sensor indicates higher measured values then a vane probe. This can be ducts. Depending on the design of the duct, turbulent flows can occur even at low velocities.

Location selection

You should measure in a straight part of the duct, if possible. The duct part should have a minimum of  ten diameters of straight run before the  measuring sport and four diameters of straight run after the measuring spot.

The flow profile should not be interrupted in any way by flaps, dips, angles etc.

Vane probes

The measuring principle of the vane probe is based on the conversion of a rotation into electric signals. The agent which flows makes the vane rotate. An inductive proximity switch “counts” the revolutions of the vane and supplies a pulse sequence which is converted in the measuring instrument and is then indicated as a velocity value.

Large diameters ( Ø 60 mm, Ø 100 mm ) are suitable for the measurement of turbulent flows (e.g. at outlet ducts ) at smaller or medium velocities. Small diameters are more suitable for measurements in ducts in which case the duct cross-section must be 100 times bigger then the probe cross-section being impacted. The 16 mm probe has proven to be very versatile. It is large enough to have good starting qualities and is small enough to withstand velocities of up to 60 m/s.

Positioning in the air current

The vane probe is adjusted  exactly if the flow direction is parallel to the vane axis.

If the measuring probe is turned slightly in the air current, the value shown in the instrument changes. The measuring probe is positioned exactly in the air current if the value shown is at max.

When measuring in a duct there should also be a minimum of ten diameters of straight run before the measuring spot and four diameters of straight run after the spot for best results. By design, vanes are less influenced by turbulence than thermal probes or Pitot tubes.

Measuring velocity in ducts

As part approval measurements, indirect measuring procedures (grid measurements) are used to measure air flows.

The following procedures are suggested in VDI 2080/EN 12599:

- Trivial procedure for grid measurements in square  cross- sections

- Centroidal axis procedures for grid measurements in circular cross- sections

- Log linear procedure for grid measurements in circular cross sections.

Supply/Returns

The air vent greatly changes the relatively uniform inside the duct. Areas of higher flow velocity are created at the free vent surfaces and areas of low flow velocity and swirl at the grids.

The flow profile steadies at a distance from the grid depending on the grid design but is usually 20 cm. For best accuracy, a large diameter vane is recommended. The area of the vane helps to get an average reading of the turbulent flow from the grid.

Measurements at suction apertures using a volume flow funnel

Even without the disturbing effects of a grid in an aperture, the lines of flow are not directional and the flow profile is irregular. Because a partial vacuum in the duct draws air out of the room in a funnel shape even a short distance from the aperture, there is no defined area in the room over which a measurement could be made. Therefore, only the duct or funnel measurement yields reproducible results. Measuring funnels of various sizes are available for such applications.

These create defied flow conditions at a known distance from the grid with a fixed volume. A velocity probe is positioned centrally and secured. The volume flow is calculated from the velocity multiplied by the funnel factor (e.g. funnel factor 22 ).

Pitot tube

The Pitot tube opening takes on the complete pressure and conducts it to connection (a) in the pressure probe. The pure static pressure is taken up by a lateral slot and conducted to connection (b.) The resulting differential pressure is a dynamic flow- dependent pressure which is then analyzed and indicated.

As with thermal probes, the Pitot tube is more likely to react to turbulent flows then a vane probe. Therefore, a free inlet and outlet path must also be ensured during Pitot tube measurements.

v=s.√(2.p/ρ)

v=velocity in m/s

s=Pitot tube factor

ρ=Air density in kg/m3

p=Differential pressure in Pascal measured in Pitot tube