![]() 80 (The accuracy of individual readings is likely to be ± 10%.) These measurements can identify problems caused by crossdrafts and badly adjusted back-baffles. If individual velocities are within about± 15% of the mean, there should be little cause for concern. The air velocity is measured in each of these areas and sometimes its variation with time. To make measurements, the working aperture of the cupboard is divided into equal areas by imaginary lines parallel to the sides of the aperture. This type of instrument will tolerate moderate contamination by dust, can be cleaned (with care), and is in common use in the field. Commercial instruments usually have two sensing elements to give temperature compensation. With a thermal anemometer the loss of heat from the sensing element is a function of the airstream velocity past it, resulting in a fairly fast response. In addition, the calibration of vane anemometers needs to be checked frequently. The instrument has a slow response to velocity fluctuations and cannot be used to investigate turbulence levels. ![]() The value measured is an average over a large area. It is relatively large and this may physically restrict its use in certain circumstances. The vane anemometer has a number of disadvantages. The latter type is becoming increasingly popular although the traditional vane anemometer is still widely used. There are two main types of instrument in general use for face velocity measurements: vane anemometers and thermo- or heated-element anemometers. Low face velocities make a fume cupboard sensitive to outside disturbances (for example drafts) whereas excessively high velocities can cause eddies in the wakes of operators and under sash handles which can lead to contaminant being drawn out of the cupboard. XIANYUN WEN, in Industrial Ventilation Design Guidebook, 2001 Face Velocity MeasurementsĪlthough it is generally accepted that face velocity is not sufficient to specify or describe fume cupboard performance, it is a relatively easily made measurement that is readily understood and widely quoted. These instruments are particularly useful for measurements at the face of a grille, and the recommended procedures for this are outlined in Chapter 16. ![]() Provided the instrument-to-duct diameter ratio is such as to accommodate the instrument at the traverse positions, this effect should not be more than 3%. With in-duct measurements, there will be a blockage effect, causing an overestimation of the velocity. Variations due to changes in air density are not significant for on-site measurements. Yaw error is less than 1% up to 12° of yaw. A calibrated instrument has an accuracy of about ± 1% over the speed range for which it was designed and with the instrument mounted in a steady, uniform airstream. The calibration correction v corr is applied to the indicated velocity v i to give the true velocity v a. The main advantage of these instruments compared with a Pitot-static tube is that velocities can be measured down to about 0.5 m/s the speed range depends on its manufacture, low, medium, and high velocity models being available up to a maximum of about 30 m/s.Ī vane anemometer requires a calibration chart, and this should be brought up-to-date regularly to ensure continued accuracy in the measurement of air velocity. Roger Legg, in Air Conditioning System Design, 2017 Vane AnemometersĪ vane anemometer consists of a number of light vanes supported on radial arms rotating on a common spindle.
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