Hot wire anemometer | Hot Film Anemometer | Advantage | Disadvantages

Here we learn about hot wire anemometer, Hot Film anemometer, and it’s advantages and disadvantages.

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.