hot wire anemometer

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HOT WIRE ANEMOMETER HOT WIRE ANEMOMETER

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about hot wire anemometer used for temperature measurement

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Page 1: Hot Wire Anemometer

HOT WIRE ANEMOMETERHOT WIRE ANEMOMETER

Page 2: Hot Wire Anemometer

HOT WIRE ANEMOMETERHOT WIRE ANEMOMETER The term anemometer was derived from the The term anemometer was derived from the

Greek words anemos, "wind," and metron, Greek words anemos, "wind," and metron, "measure." "measure."

The hot-wire anemometer is principally used The hot-wire anemometer is principally used in gas flow measurement.in gas flow measurement.

It consists of an electrically heated, fine It consists of an electrically heated, fine platinum wire which is immersed into the flow.platinum wire which is immersed into the flow.

As the fluid velocity increases, the rate of heat As the fluid velocity increases, the rate of heat flow from the heated wire to the flow stream flow from the heated wire to the flow stream increases. increases.

Thus, a cooling effect on the wire electrode Thus, a cooling effect on the wire electrode occurs, causing its electrical resistance to occurs, causing its electrical resistance to change.change.

Page 3: Hot Wire Anemometer

ConstructionConstruction Figure 1 shows a hot-wire anemometer probe. Figure 1 shows a hot-wire anemometer probe. Typical dimensions of the wire sensor are 0.00015 to Typical dimensions of the wire sensor are 0.00015 to

0.0002 inches (0.0038 to 0.005 mm) in diameter and 0.0002 inches (0.0038 to 0.005 mm) in diameter and 0.040 to 0.080 inches (1.0 to 2.0mm) long. 0.040 to 0.080 inches (1.0 to 2.0mm) long.

This is the type of hot wire that has been used for such This is the type of hot wire that has been used for such measurements as turbulence levels in wind tunnels, flow measurements as turbulence levels in wind tunnels, flow patterns around models and blade wakes in radial patterns around models and blade wakes in radial compressors. compressors.

The film type of sensor is shown in Figure 2. The film type of sensor is shown in Figure 2. The hot film is used in regions where a hot wire probe The hot film is used in regions where a hot wire probe

would quickly break such as in water flow would quickly break such as in water flow measurements.measurements.

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Page 5: Hot Wire Anemometer

PROBE SHAPES In addition to the cylindrical shape, hot films have been made

on cones, wedges, parabolas, hemispheres, and flat surfaces. Cylindrical film sensors that are cantilever mounted are also made.

This is done by making the cylindrical film sensor from a quartz tube and running one of the electrical leads through the inside of the tube.

Figure 4 shows an example of a single ended sensor. This is an important modification for fluidic applications since they can be made very small and inserted into very small channels.

Also, for omni-directional measurements (e.g., meteorology applications when the vertical flow can be ignored), it permits unobstructed flow from all directions.

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WORKING PRINCIPLEWORKING PRINCIPLE

In hot wire anemometers, resistive wire is In hot wire anemometers, resistive wire is used as a basic sensor, which is heated used as a basic sensor, which is heated initially by passing an electric current. initially by passing an electric current.

This heated resistive wire mounted on a This heated resistive wire mounted on a probe is exposed to airflow or wind, which probe is exposed to airflow or wind, which is cooled because of fanning effect,is cooled because of fanning effect,

The amount of cooling depends on the The amount of cooling depends on the velocity of airflow.velocity of airflow.

This difference in resistance is converted This difference in resistance is converted into a voltage variation and thereby the into a voltage variation and thereby the flow velocity is transduced into a voltage flow velocity is transduced into a voltage variation.variation.

Page 9: Hot Wire Anemometer
Page 10: Hot Wire Anemometer

Types of AnemometerTypes of Anemometer

Constant Current TypeConstant Current Type The fluid velocity is determined from a measurement of The fluid velocity is determined from a measurement of

the resulting change in wire resistance. the resulting change in wire resistance.

Constant Temperature TypeConstant Temperature Type The fluid velocity is determined from the current The fluid velocity is determined from the current

needed to maintain a constant wire temperature and, needed to maintain a constant wire temperature and, thus, the resistance constant.thus, the resistance constant.

Page 11: Hot Wire Anemometer

Constant Current Type AnemometerConstant Current Type Anemometer

In this type a fine resistance wire carrying a fixed In this type a fine resistance wire carrying a fixed current is exposed to the flow velocity.current is exposed to the flow velocity.

The wire attains an equilibrium temperature when IThe wire attains an equilibrium temperature when I22R R heat generated in it just balanced by the convective heat generated in it just balanced by the convective heat loss from its surface so that Iheat loss from its surface so that I22R heat is essentially R heat is essentially constant.constant.

Thus the wire temperature must adjust itself to Thus the wire temperature must adjust itself to change the convective heat until equilibrium is change the convective heat until equilibrium is reached.reached.

This wire temperature is a measure of the flow This wire temperature is a measure of the flow velocity, which can be measured in terms of electrical velocity, which can be measured in terms of electrical resistance.resistance.

Fluctuating flow with a frequency of 160Hz and above Fluctuating flow with a frequency of 160Hz and above cannot be measured using this.cannot be measured using this.

Page 12: Hot Wire Anemometer

Constant Temperature Type AnemometerConstant Temperature Type Anemometer

Here the current through the wire is adjusted to keep the wire Here the current through the wire is adjusted to keep the wire temperature constant. The velocity is measured by its cooling temperature constant. The velocity is measured by its cooling effect on a heated sensor. effect on a heated sensor.

A feedback loop in the electronics keeps the sensor A feedback loop in the electronics keeps the sensor temperature constant under all flow conditions. temperature constant under all flow conditions.

The voltage drop across the sensor thus becomes a direct The voltage drop across the sensor thus becomes a direct measure of the power dissipated by the sensor. The measure of the power dissipated by the sensor. The anemometer output therefore represents the instantaneous anemometer output therefore represents the instantaneous velocity in the flow.velocity in the flow.

Sensors are normally thin wires with diameters down to a few Sensors are normally thin wires with diameters down to a few micrometers. The small thermal inertia of the sensor in micrometers. The small thermal inertia of the sensor in combination with very high servo-loop amplification makes it combination with very high servo-loop amplification makes it possible for the CTA to follow flow fluctuations up to several possible for the CTA to follow flow fluctuations up to several hundred kHz. hundred kHz.

For measurement of average velocity the constant temperature For measurement of average velocity the constant temperature mode of operation is used.mode of operation is used.

As far as steady flow is concerned a setup shown in fig. is As far as steady flow is concerned a setup shown in fig. is alright.alright.

But when there is large fluctuation in the velocity of flow, this But when there is large fluctuation in the velocity of flow, this circuit cannot be used.circuit cannot be used.

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Modern Constant Temperature Anemometer Modern Constant Temperature Anemometer systems are typically comprised of three elementssystems are typically comprised of three elements

A computer with an Analog to Digital converter and data A computer with an Analog to Digital converter and data acquisition software for sampling the analog signal from the acquisition software for sampling the analog signal from the Constant Temperature Anemometer and performing some Constant Temperature Anemometer and performing some degree of digital signal processing. degree of digital signal processing.

The Constant Temperature Anemometer, provides the user with The Constant Temperature Anemometer, provides the user with a range of configuration options. It should be possible to modify a range of configuration options. It should be possible to modify resistor configuration for the bridge top , since this resistor resistor configuration for the bridge top , since this resistor configuration will influence the maximum heat transfer from a configuration will influence the maximum heat transfer from a probe for a given application and moreover, the frequency probe for a given application and moreover, the frequency response of the system. Signal conditioning electronics should response of the system. Signal conditioning electronics should be present. High and low pass filters to prevent aliasing when be present. High and low pass filters to prevent aliasing when digitally sampling the analog CTA signal and to filter out DC digitally sampling the analog CTA signal and to filter out DC signal components to allow high gain turbulence measurements. signal components to allow high gain turbulence measurements. Input offset to eliminate V0.Input offset to eliminate V0.

Page 16: Hot Wire Anemometer