Vane anemometers

Gup and Vane Anemometers. A number of flow meter designs use a rotating element kept in motion by the kinetic energy of the flowing stream such that the speed is a measure of fluid velocity. In general, these meters, if used to measure wind velocity, are called anemometers if used for open-channel Hquids, current meters and if used for closed pipes, turbine flow meters. 

Current Meters. Various vane designs have been adapted for open-channel flow measurement. The rotating element is partially immersed and rotates rather like a water wheel. Operation is similar to that of vane anemometers. 

The vane anemometer is an old invention. It can be likened to a small wind turbine with 4-10 rotating blades and a handle, as in Fig. 12.20. Earlier constructions were fully mechanical, where the spindle rotation was transmitted to a pointer through a series of gears. In modern vane anemometers, an electrical sensor records the spindle rotation and the signal is processed, giving the velocity on a digital display. Such an instrument usually is able to integrate the mean velocity over a time interval. 

The vane anemometer s physical dimensions are often quite large (compared with other local velocity measurement instruments). It does not strictly measure a local velocity at all, but rather provides a spatially integrated mean value. This is an advantage in many cases where the air volume flow rate has to be predicted using local velocities and an integration principle. 

The measurement range of a vane anemometer is typically between 0.3 and 30 m s E It may start rotating with slightly lower velocities, but due to the characteristic curve having a small nonlinear part in the low-speed end, the useful range is narrower. The actual precision depends on the quality of the instrument however, the inaccuracy may vary between 1% and 5% of the scale. The larger the vane, the higher the accuracy. 

The vane anemometer is not seriously affected by small deviations in alignment in the main flow direction. However, care is necessary since over 20° misalignment causes significant errors. With regard to providing a correction for fluid density, slightly different opinions exist. T35 Based on measurements, it is recommended- that the following density correction procedure be applied ... 

To calibrate larger sensors/instruments such as vane anemometers, a wind tunnel is required. A calibration wind tunnel consists of an open or closed tunnel, a fan to deliver the air, a nozzle to shape the velocity profile, and a mesh arrangement to uniform and reduce the flow turbulence. It may be necessary to control the air temperature in the tunnel by means of a heating/cooling sys-... 

The previous methods are mainly used to measure duct flow. When measuring flows on supply or exhaust terminals, different methods are used. The measurement on exhaust terminals is simple to carry out, as the velocity field near the terminal is relatively constant, with no steep gradients or swirls. In the case of a grill, traversing across the terminal surface using a suitable velocity instrument is a good alternative. A suitable instrument for most cases is the vane anemometer. 

F. Finaish, R. George, H. J. Sauer. Experimental study of tempetature, ptessure, humidity and/ or density effects on vane anemometers. ASHRAE Transactions, 101 (2), 1995, pp. 240-25 1. 

Figure 27.4 Vane anemometer (Courtesy of Airflow Developments)...

Vane Anemometer Another type of anemometer has a vane that moving air deflects. An indicator connected to the vane gives air velocity. 

Wind held measurements were made using two-axis cup-and-vane anemometers located at 11 stations and positioned 2 m above the ground. These were positioned both upwind and downwind from the release point. Fifty m upwind of the spill point were four levels of 1,000 O platinum RTD mounted in aspirated solar shields. Bivane anemometers were installed to measure atmospheric turbulence at three heights at two locations (50 m upwind of the spill point and 100 m downwind of the spiU point). The bivane data were not provided by LLNL to WRI for the DOE HSC database but are provided as plots in the Desert Tortoise Data Report (Goldwire et al., 1985). 

Wind field measurements were made with nine stations using two-axis cup-and-vane anemometers located 2 m above the ground, both upwind and downwind from the release point. Meteorological boundary layer measurements were made on a 20-m tower located 50... 

Vertical wind profile and turbulence were measured using three Gill bivane anemometers installed at a tower 600 m upwind of the release point. The data from these sensors were not transferred from LLNL to WRI. On this same tower, five levels of platinum RTD sensors at heights of 1, 2, 4, 8, and 16 m were installed to measure the ambient temperature and temperature lapse rate. Eighteen (18) stations using two-axis cup-and-vane anemometers (Met-One) mounted 2 m above the ground were used to determine the characteristics of the wind field before, during and after each release. 

In addition to the 24-m meteorological tower, a R. M. Young Model RE-5701 anemometer was positioned 5 m in front of the first gas concentration sensor array on the centerline at a height of 0.5 m. Seven cup-and-vane anemometers mounted at a 2-m height measured wind speed and direction and were placed around the test grid to provide measurements of the wind field. 

Figure 21.7 Vane anemometer. (Courtesy Abbott Birks)...

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