Anemometer calibration

Z. Yue, T. G. Malmstrora. A simple method fot low-speed hot-wire anemometer calibration. Measurement Science and Technology, 9, 1998, pp. 1506-1510. 

Up to this point, the anemometer on the finish of the fast and accurate verification could be attached to anemometer calibration box with the above-mentioned corrective curve anemometer and the formula, to be used together with the anemometer in the normal usage period until the next correction. 

Whether the equivalent velocity method or the traditional one, only can be used in the valid period of the standard wind tunnel. To the standard wind tunnel itself, it must also be periodically corrected in accordance with the relevant standards. If there is damage in the use of its period, in the absence of rehabilitation and correction by the former, it shouldn t be used for the job of the anemometer calibration. 

Under normal circumstances, for the year-round change of air density in the mine area is very small, the anemometer can be corrected directly in the method introduced in the text. However, when the anemometer calibration and the test of equivalent velocity, or the usage and the correction of anemometer, are to the large difference in atmospheric conditions (such as the anemometer corrected in the plains used in high altitude), air density must be corrected accordingly in the use of the air velocity Equivalent weight method to verify the anemometer. 

A calibration facility must produce the desired velocity range for the meter to be calibrated. The air temperature should be kept constant over the test to ensure constant density. For thermal anemometers, velocity calibration only is not sufficient. They should also be checked for temperature compensation. In the case of omnidirectional probes, sensitivity to flow direction should be tested. In the case of low-speed (thermal) anemometers, their self-convection error should be measured, and, for instruments measuring flow fluctuation (turbulence), dynamic characteristics testing should be carried out as well. ... 

Manufacturers of thermal anemometers provide small rigs for their calibration. They typically consist of a nozzle, an air supply unit, and a regulating valve. The probe is placed into the nozzle jet. The reference velocity is calculated from the nozzle upstream pressure and nozzle characteristics. Due to its small size, this type of rig can be used only for hot-wire or other thermal anemometers. ... 

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-... 

Full use can be made for the on-site and off-site calibration of traditional anemometers such as thermal probes and propeller anemometers. 

A. Melikov, ed. Calibration and requirements for accuracy of Thermal Anemometers for Indoor Velocity Measurements. Report ET-1E9701. Technical University of Denmatk, Laboratory of Indoor Environment and Energy, 1997. 

The hot-wire anemometer can, with suitable cahbration, accurately measure velocities from about 0.15 m/s (0.5 fl/s) to supersonic velocities and detect velocity fluctuations with frequencies up to 200,000 Hz. Eairly rugged, inexpensive units can be built for the measurement of mean velocities in the range of 0.15 to 30 m/s (about 0.5 to 100 ft/s). More elaborate, compensated units are commercially available for use in unsteady flow and turbulence measurements. In cahbrating a hotwire anemometer, it is preferable to use the same gas, temperature, and pressure as will be encountered in the intended apphcation. In this case the quantity I RJAt can be plotted against /v, where I = hot-wire current, = hot-wire resistance. At = difference between the wire temperature and the gas bulk temperature, and V = mean local velocity. A procedure is given by Wasan and Raid [Am. Inst. Chem. Eng. J., 17, 729-731 (1971)] for use when it is impractical to calibrate with the same gas composition or conditions of temperature and pressure. Andrews, Rradley, and Hundy [Int. J. Heat Mass Transfer, 15, 1765-1786 (1972)] give a cahbration correlation for measurement... 

The velocity of the air flow is measured most frequently by thermoanemometers. A sensitive semiconductor or resistance thermometric sensor is situated in the flow of the air sampled. The resistance of the sensor depends on the velocity of the air flow and the calibration is provided at a constant temperature with the help of a suitable flow meter. The disadvantage of the thermo-anemometers is the necessity for constant temperature for the period of particular sampling procedures and also the need for a time consuming calibration. Connection to a recorder is advantageous. 

Based on the Proportion Law of hydro-mobility of hydrodynamics, with the standard wind tunnel, measured with the standard calibration of the Department, the relationship equation between the actual ventilator speed and the actual air velocity inside the wind tunnel was set up. It can be corrected traditionally with the anemometer readings and the actually tested air velocities for comparison, which has been improved to be the correction method of anemometer readings compared with the ventilator speeds, which is the equivalent wind speed law. 

Inferential meters include instruments in which the volumetric flow rate is inferred by the movement of a turbine, propeller, or impeller. The fluid impinging on a blade causes it to rotate at an angular velocity that is proportional to the flow rate. The early anemometers made with plates and cups are examples. These types of meters are becoming less and less common due to the need to calibrate and compensate for effects like viscosity. According to Crabtree (2009), the Coriolis and ultrasonic meters are replacing the turbine meters in most industrial applications. 

Airflows are usually measured with thermal anemometers or velometers. These instruments are available from safety supply companies or laboratory supply houses. The proper calibration and use of these instruments and the evaluation of the data are a separate discipline. An industrial hygienist or a ventilation engineer should be consulted whenever serious ventilation problems are suspected or when decisions on appropriate changes to a ventilation system are needed to achieve a proper balance of supply and exhaust air. 

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