Hot wire anemometer | Hot Film Anemometer | Advantage | Disadvantages
Here we learn about hot wire anemometer, Hot Film anemometer, and it’s advantages and disadvantages.
~ A sensor of 5 micron diameter platinum tungsten wire welded between two prongs of the probe and heated electrically to form part of Wheatstone circuiit.
~ When the probe in introduced into the flowing fluid, it tends to be cooled by the velocity and there is decrease in its resistance.
~ The rate of cooling of wire depends upon the:
(i) Dimension and physical properties of the wire.
(ii) Difference of the temperature between the wire and fluid.
(ii) Physical properties of the fluid.
(iv) Stream velocity under measurement.
~ There are two methods of measuring fluid flow.
Fig. Hot wire bridge circuits.
1. Constant-current mode :
~ Fig 4.3.1 (a) shows Constant-current mode hot wire anemometer in which the voltage across the bridge is kept constant.
~ Initially the circuit adjusted zero galvanometer reading when the heated wire lies in still air.
~ When the fluid flows the hot wire cools the resistance changes and the galvanometer deflects.
~ The galvanometer deflections are amplified measured and correlated with fluid velocity by previous calibration.
(2) Constant-temperature mode :
Fig 4.3.1 (b) shows Constant-current mode hot wire anemometer in which resistance of the wire and its temperature is maintained constant.
~ The interior of the anemometer is an exposed hot wire maintained at a constant temperature. The heat lost to fluid convection is a function of the fluid velocity.
1. For measurement of steady velocities the constant temperature type is often used.
Main application of hot wire anemometer:
for measurement of rapidly fluctuating velocity.
2. Majority of application is in gas flow meters.
1. Fine wire has limited physical strength
2. Due to dirt accumulation calibration of instrument changes.
~ It is a thermal anemometer which has thin metallic plate sensing element having thickness less than 0.1 um. The hot film is a conducting film on a ceramic substrate.
~ Most of the films are of platinum due to its good oxidation resistance and the resulting long term stability.
~ The hot film sensor has lower temperature coefficient of resistance. Fig .4.3.2 shows typical hot film anemometer.
It overcomes the problem of hot wire anemometyer as they have great mechanical strength and can be used at high temperature by providing internal water cooling passages.
Hot wire Anemometer :
~ Hot wire anemometer is used for measurement of flow rate of fluctuating and unsteady flow.~ A sensor of 5 micron diameter platinum tungsten wire welded between two prongs of the probe and heated electrically to form part of Wheatstone circuiit.
~ When the probe in introduced into the flowing fluid, it tends to be cooled by the velocity and there is decrease in its resistance.
~ The rate of cooling of wire depends upon the:
(i) Dimension and physical properties of the wire.
(ii) Difference of the temperature between the wire and fluid.
(ii) Physical properties of the fluid.
(iv) Stream velocity under measurement.
~ There are two methods of measuring fluid flow.
Fig. Hot wire bridge circuits.
1. Constant-current mode :
~ Fig 4.3.1 (a) shows Constant-current mode hot wire anemometer in which the voltage across the bridge is kept constant.
~ Initially the circuit adjusted zero galvanometer reading when the heated wire lies in still air.
~ When the fluid flows the hot wire cools the resistance changes and the galvanometer deflects.
~ The galvanometer deflections are amplified measured and correlated with fluid velocity by previous calibration.
(2) Constant-temperature mode :
Fig 4.3.1 (b) shows Constant-current mode hot wire anemometer in which resistance of the wire and its temperature is maintained constant.
~ The interior of the anemometer is an exposed hot wire maintained at a constant temperature. The heat lost to fluid convection is a function of the fluid velocity.
Advantages of Hot wire anemometer :
1. For measurement of steady velocities the constant temperature type is often used.
Main application of hot wire anemometer:
for measurement of rapidly fluctuating velocity.
2. Majority of application is in gas flow meters.
Disadvantages of Hot wire anemometer:
1. Fine wire has limited physical strength
2. Due to dirt accumulation calibration of instrument changes.
Hot Film Anemometer :
~ It is a thermal anemometer which has thin metallic plate sensing element having thickness less than 0.1 um. The hot film is a conducting film on a ceramic substrate.
~ Most of the films are of platinum due to its good oxidation resistance and the resulting long term stability.
~ The hot film sensor has lower temperature coefficient of resistance. Fig .4.3.2 shows typical hot film anemometer.
Advantages Hot Film Anemometer :
It overcomes the problem of hot wire anemometyer as they have great mechanical strength and can be used at high temperature by providing internal water cooling passages.
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