Thermal anemometer

Hot wire Hot-wire anemometer Thermal flow sensors... 

Coriolis flow meter Hot-wire anemometer Thermal mass flow meter Piezo-electric element Magnetic transducer... 

Measurement by Thermal Effects. When a fine wire heated electrically is exposed to a flowing gas, it is cooled and its resistance is changed. The hot-wire anemometer makes use of this principle to measure both the average velocity and the turbulent fluctuations in the flowing stream. The fluid velocity, L, is related to the current, /, and the resistances, R, of the wire at wire, and gas, g, temperatures via... 

Because of its small size and portabiHty, the hot-wire anemometer is ideally suited to measure gas velocities either continuously or on a troubleshooting basis in systems where excess pressure drop cannot be tolerated. Furnaces, smokestacks, electrostatic precipitators, and air ducts are typical areas of appHcation. Its fast response to velocity or temperature fluctuations in the surrounding gas makes it particularly useful in studying the turbulence characteristics and rapidity of mixing in gas streams. The constant current mode of operation has a wide frequency response and relatively lower noise level, provided a sufficiently small wire can be used. Where a more mgged wire is required, the constant temperature mode is employed because of its insensitivity to sensor heat capacity. In Hquids, hot-film sensors are employed instead of wires. The sensor consists of a thin metallic film mounted on the surface of a thermally and electrically insulated probe. 

The sensor is the element of an instrument directly influenced by the measured quantity. In temperature measurement the thermal mass (capacity) of the sensor usually determines the meter s dynamics. The same applies to thermal anemometers. In IR analyzers used for concentration measurement, the volume of the flow cell and the sample flow rate are the critical factors. Some instruments, like sound-level meters, respond very fast, and follow the pressure changes up to several kHz. 

Usually this type of anemometer does not provide information on the flow direction. Vice versa, the. sensors are made as independent of the flow direction as possible—omnidirectional. This is an advantage for free-space ventilation measurements, as the flow direction varies constantly and a direction-sensitive anemometer would be difficult to use. Naturally, no sensor is fully omnidirectional, but satisfactory constructions are available. Due to the high sensor thermal inertia, this type of anemometer is unsuitable for high-frequency flow fluctuation measurement. They can be used to monitor low-frequency turbulence up to a given cut-off frequency, which depends on the dynamic properties of the instrument. 

The precision of a thermal anemometer is dependent on the instrument quality and the conditions of use. A general rule is, the lower the measured ve locity, the higher the inaccuracy and vice versa. When measuring very low indoor velocities, around 0.1 m s, the relative error can be as high as 100% and not much lower than 30%. Low velocities are extremely difficult to measure with accuracy. 

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. ... 

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. 

Thermal anemometer An anemometer that employs the principle that the quantity of heat removed by a gas stream passing a heated element has a direct relationship to the velocity of the gas stream. 

Air flow velocity electronic microanemometer with tube array thermal anemometer, vane-type anemometer, or equivalent... 

Among the renewable energy processes, it is the wind turbines that benefit the most from the measurement of wind direction and velocity. Doppler-type sensors are used to determine the wind velocity and to obtain three-dimensional air motion profiles and also in the balancing of HVAC systems and measuring of the velocity of wet and dirty gases in industry. For a more detailed discussion of Pitot tubes and thermal flowmeters, also refer to the Sections 3.9.7.2 and 3.9.10. Here, the focus is on mechanical- and Doppler-type anemometers. 

Thermal flowmeters also can directly detect low-mass flows without any laminar elements. In that case, they are installed directly into the pipeline as either thermal flowmeters or anemometers. They have a 100 1 rangeability, and can be provided with integral controllers. 

As opposed to inertial sensors, micromachined anemometers are often based on micro hot plates, that is, combinations of heaters and thermometers on thermally insulating membranes or multilayered thin films [4]. Functional sensor parameters are obviously determined by thermal conductivities and heat capacities, which can be monitored to reject faulty chips. Until recently, suitable wafer-level testing methods have not been available, and most sensor designs were based on literature values, despite the fact that these values depend strongly on fabrication processing parameters. 

Air flow measurement technique at different hood surfaces, volumetric flow rate, transport and capture velocity, hood static pressure, pitot tube, thermal anemometer, and magnehelic gauge. 

Figure 12.6 (a) An exploded view of a microchannel PA. The anemometer chip is secured in the wall of the channel with epoxy, (b) Experimental data comparing the response of the PA chip in the microchannel to flow with corresponding data taken with an MKS 100 thermal mass flow sensor. 

The instantaneous pressure and velocity distributions along the resonance tube are monitored by piezoelectric transducers and hot-wire thermal anemometer that are connected to the oscilloscope and photo film recorder. These make possible the tuning of the generator to the acoustic resonance by varying the rotational speed of the crankshaft. Also, there is a provision to attach resonance tubes of various lengths and diameters to obtain required amplitudes of pressure and velocity pulsations. 

Many of the well-established macroflow sensing methods such as the use of shear stress sensors, thermal anemometers, magnetohydrodynamic sensors, particle imaging, and fluorescent... 

Heat-Transfer-Detection-Based Flow Sensors These thermal-anemometer-based flow sensors can sense very low flows in microchannels. The measurement principle is based on the thermal time of flight. The length of the heating pulse and the time of flight used in the measurement are measured in milliseconds. An example of the structure of a flow sensor is shown in Fig. 5 [1]. The structure consists of a heater in the middle, with an upstream and a downstream temperature sensor integrated into the wall of the channel. When there is no flow in the channel, heat diffuses into the two temperature sensor regions and no differential temperature is detected. An increase in the flow rate in the channel favors convection of heated fluid in the direction of the flow, and the differential temperature detected by the sensors increases. 

Hot-wire anemometers ( Micro/Nano-Anemometers ) have been developed for a wide spectrum of applications from experimental fluid mechanics to aerospace engineering to measure physical parameters such as temperature, flow rate, and shear stress. The advent of microelec-tromechanical systems (MEMS) and nanoscale thermal sensors has provided an entry point to microfluidics, biomedical sciences, and microcirculation in cardiovascular medicine. These MEMS and nanoscale devices are fabricated... 

For a practical micro- or nanoscale anemometer, the sensor consists of a resistive element patterned on the surface of a supporting substrate (Fig. 3). The thermal element resides within the... 

A thermal anemometer typically consists of a heater and one or more surrounding temperature sensors relative to the flow direction. The heater usually works when its Joule self-heating is maintained constant. Any fluid flow through the heater will cause a shift in its temperature and thus its resistance. By measuring the temperature profile around the heater, the flow rate and direction of fluid flow can be identified. 

In terms of the calorimetric principle, a widely used microthermoresistive flow sensor is the thermal anemometer, which typically ccmsists of a middle heater with upstream and downstream temperature sensors, relative to the flow direction [2]. Such a calorimetric sensor is based on measuring the asymmetry temperature profile around the heater, modulated by the fluid flow [1,8]. The schematic representation of a calorimetric device and the temperature distribution in the flow direction are shown in Fig. 3. The MEMS flow sensor... 

As shown in Fig. 5 for the three-dimensional structure of a typical thermal anemometer, different types of heat transfer occur in and around the microchip structure [2]. Based on the model developed previously [2], a complete modeling of the thermal state of the flow sensors can be obtained as follows. 

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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