The use of rotating vane anemometers to measure inlet grille velocities.

The rotating vane anemometer is an instrument that is widely used in the field by maintenance engineers and inspectors. The anemometer consists of a vane that is held at right angles to an airflow. In modem instruments the speed of rotation of the vane is sensed and measured electronically and the air speed, which is a function of the speed of rotation of the vane, is indicated on a meter. The vane anemometer has a number of limitations: it is fragile and should not be used in corrosive or dusty atmospheres; it is not suitable for low velocities (below about 0.2 m/s); the vane tends to be large (usually about 100 mm in diameter) making it unsuitable because of access problems and blockage; it should be calibrated at frequent intervals. Because of the vane's size and inertia, the anemometer integrates the air velocity spatially and temporarily which can be an advantage for some applications. It is a portable instrument and can measure over a wide range of velocities. The vane anemometer is often used to make measurements on fume cupboards, booths and at inlet and outlet grilles to measure velocities and calculate volume flow rates. However its use at grilles does present a problem. Because of the nature of the flow, especially at inlet grilles, the velocity indicated by the anemometer is not the true velocity. Air emerges from the grille in an array of jets and it is found that the effect of this is to make the instrument over-read.

In the case of thermal anemometers and swinging vane anemometers, the ACGIH (1) manual of industrial ventilation gives correction factors to be applied at grille openings both under pressure and suction. However in the case of grilles under pressure (i.e. inlets) the range of applicability of the advice is very limited, particularly in relation to the open area i.e. 70% open area or greater. The free open area is used in calculations, with a velocity correction factor of 93%. Many grilles, especially at air inlets, but also on benches with perforated inlet and exhaust surfaces, can have open areas considerably less than 70%. In the case of rotating vane anemometers, no advice is given by ACGIH. An investigation was carried out at HSL to identify the problems in the use of vanes at inlet grilles and to determine the likely correction factors for a wide range of mesh geometries and open areas.