Book - School of Science and Technology

472 Ductwork designALNORAJAFLOW(a)(b)Figure 16.33 (a) Rotating vane and (b) anemometer hoodand the speed is multiplied by the whole face area (regardless of free area) to obtain thevolume in m 3 /s. Readings are taken at various points and averaged. It is not so useful formeasurements in ducts, as the anemometer has to be introduced through a hole in the ductwall and is then difficult to manipulate.It must be admitted that the anemometer requires very careful handling, and in thehands of an unskilled operator entirely erroneous results can be obtained. Electronic typesare available and may be used for remote reading, where the indicating instrument isconnected by cable to the rotating vane. Various measuring head diameters are available,suitable for a range of measurement applications. Anemometer readings increase inaccuracy when used in conjunction with a calibrated catchment hood (Figure 16.33(b)).The hot wire anemometerOriginally developed for laboratory use, this type of instrument is particularly valuablefor measurement of low velocities such as occur in occupied spaces. The measuring headconsists of a fine platinum wire which is heated electrically and inserted in the air stream.Air movement cools the wire and, by calibration, current flow may be metered to indicatethe velocity sensed. As the instrument is not sensitive to the direction of flow, it is not usedfor measurement at grilles.Diaphragm air pressure gaugeThis instrument has a diaphragm and linkage to provide indication of static pressure on adial. Typically, the scale range is up to 0.25 kPa. It is often helpful to measure staticpressure in commissioning induction units, terminal control boxes and across componentsto check on the pressure loss.Digital instrumentsModern instruments are battery operated (optional mains power connections for longperiods of use are also available) and provide direct digital displays of the measured