Testo explains the importance of matching the correct velocity measurement instrument to the application
The range of portable velocity measurement instruments available is huge. Problems can arise when choosing the correct instrument as the instruments available differ greatly in their features and suitability of application. Velocity measurement instruments range from simple, self-contained velocity indicators through to highly complex reference and data logging instruments. The benefit of such choice is that there is a solution to almost any measuring problem. However, it is important to understand the role of the sensor and its influence in choosing the right tool.
There are mainly three types of velocity sensor: 1. Thermal probes – 0 to 5m/s used up to 70°C The principle of the thermal probe is based on a heated element from which heat is extracted by the colder impact of the airflow. 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 than a vane probe. This can be observed especially during measurements in ducts. Depending on the design of the duct, turbulent flows can occur even at low velocities. To obtain an accurate reading you should measure in a straight part of the duct, if possible. The duct part should have a minimum of 10 diameters of straight run before the measuring spot 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.
2. Vane probes – 5 to 40m/s used up to 350°C The measuring principle of the vane probe is based on the conversion of a rotation into electric signals. The airflow makes the vane rotate and 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 diameter vanes (Ø 60mm and Ø 100mm) 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 than the probe cross-section. By design, vanes are less influenced by turbulence than thermal probes or Pitot tubes. Again it is important to correctly position the probe. A probe is correctly positioned when the flow direction is parallel to the vane axis. When measuring in a duct there should also be a minimum of 10 diameters of straight run before the measuring spot and four diameters of straight run after the spot.
3. Pitot tubes – above 40m/s used up to 600°C A Pitot tube consists of an impact tube, which receives total pressure input fastened inside a second tube which receives static pressure input. The resulting differential pressure is a dynamic flow-dependent pressure that is then analysed and indicated. As with thermal probes, the Pitot tube is more likely to react to turbulent flows than a vane probe. Therefore, a free inlet and outlet path must also be ensured during Pitot tube measurements.