# How to measure?

## Measure different flows correctly

In order to determine the correct measurement procedure for the flow velocity measurement, the following parameters and ambient conditions must be taken into account preceding the measurement task:

• Flow measurement range
• Air temperature
• Level of contamination
• Measurement location

#### The most common measurement methods

• Thermal anemometers (hot-wire, hot-bulb)
• Vanes (large and small)
• Pitot tubes (straight and Prandtl)
• Funnel measurement

### Thermal probes

#### Functional principle

• Heat is removed from a heated element by a colder flow impacting onto it.
• The temperature is kept constant by means of a control circuit.
• In turbulent flows, the measurement result is influenced, i.e. increased, by flows from all directions.

#### Area of application

• For exact measurements from 0 to 5 m/s
• For temperatures up to approx. +70 °C

### Vane anemometers

#### Functional principle

• The vane is set in motion by the air flow
• The rotational movement is then converted into electrical signals
• An inductive proximity switch "counts" the revolutions of the vane and supplies a sequence of pulses, which is displayed in the measuring instrument as a flow value

#### Area of application

• For exact measurements from 5 to 40 m/s
• For temperatures up to approx. +350 °C
• Turbulent flow and low to medium velocity: large diameter (Ø 60 mm; Ø 100 mm)
• Duct: small diameters (Ø 16 mm)

### Pitot tubes

#### Functional principle

• The Pitot tube opening records the entire pressure and transmits this on to connection (a) of the pressure probe.
• The purely static pressure is recorded via lateral slots and transmitted to connection (b)
• The resulting differential pressure is the velocity-dependent dynamic pressure. This is then analyzed and displayed.

#### Area of application

• For temperatures over +350 °C
• Depending on the differential pressure probe, measurements from 1 m/s are possible
• For measurements in dusty, dirty air