ROHINI COLLEGE OF ENGINEERING & TECHNOLOGY
3.5 HOT WIRE ANEMOMETER:
Hot wire anemometers are hot wire resistance transducer which are used for
measurement of flow rates of fluids or air. In hot wire anemometers resistive wire
is used as a basic sensor, which is heated initially by passing an electric current.
This heated resistive wire mounted on a probe is exposed to air flow or wind,
which is cooled because of fanning effect. The amount of cooling depends on the
velocity of air flow. The resistance of the probe when it is hot is different from
that when it is cooled. This difference in resistance, or this variation in resistance
is converted into a voltage variation. Broadly hot wire anemometers are
commonly used in two different modes.
1. Constant current type 2. Constant temperature type
Construction
Fig 3.5.1 Wire anemometer
[Source: Neubert H.K.P., Instrument Transducers – An Introduction to
their Performance and Design, Page: 602]
The main parts of the arrangement are as follows:
Conducting wires placed in a ceramic body.
OIC 751 TRANSDUCER ENGINEERING
, ROHINI COLLEGE OF ENGINEERING & TECHNOLOGY
Leads are taken from the conducting wires and they are connected to one of
the limbs of the wheat stone bridge to enable the measurement of change in
resistance of the wire is shown in Fig 3.5.1.
Principle of Operation
1. Constant current mode:
The fine resistance wire carrying a fixed current is exposed to the flow velocity.
The flow of current through the wire generates heat on account of i 2R loss. This
heat is dissipated from the surface of the wire by convection to the surroundings.
(The loss of heat due to conduction and radiation is negligible). The wire attains
equilibrium temperature when the heat, generated due to i 2R loss is equal to the
heat dissipated due to convective loss. The circuit is so designed that i2R heat is
essentially constant and therefore the wire temperature must adjust itself to
change the convective loss until equilibrium is reached.
2. Constant temperature mode:
The current required to maintain the resistance and hence temperature constant,
become a measure of flow velocity.
Heat generated = I2Rw
where I - current through the wire;
Rw – resistance of wire
Heat dissipated due to convection = hA (Θ w – Θf)
where
h -coefficient of heat transfer; W/ m2 -°C
A -heat transfer area; m2
Θw- temperature of wire; °C,
Θf - temperature of flowing fluid, °C,
For equilibrium conditions, we can write the energy balance for the hot wire as
OIC 751 TRANSDUCER ENGINEERING
3.5 HOT WIRE ANEMOMETER:
Hot wire anemometers are hot wire resistance transducer which are used for
measurement of flow rates of fluids or air. In hot wire anemometers resistive wire
is used as a basic sensor, which is heated initially by passing an electric current.
This heated resistive wire mounted on a probe is exposed to air flow or wind,
which is cooled because of fanning effect. The amount of cooling depends on the
velocity of air flow. The resistance of the probe when it is hot is different from
that when it is cooled. This difference in resistance, or this variation in resistance
is converted into a voltage variation. Broadly hot wire anemometers are
commonly used in two different modes.
1. Constant current type 2. Constant temperature type
Construction
Fig 3.5.1 Wire anemometer
[Source: Neubert H.K.P., Instrument Transducers – An Introduction to
their Performance and Design, Page: 602]
The main parts of the arrangement are as follows:
Conducting wires placed in a ceramic body.
OIC 751 TRANSDUCER ENGINEERING
, ROHINI COLLEGE OF ENGINEERING & TECHNOLOGY
Leads are taken from the conducting wires and they are connected to one of
the limbs of the wheat stone bridge to enable the measurement of change in
resistance of the wire is shown in Fig 3.5.1.
Principle of Operation
1. Constant current mode:
The fine resistance wire carrying a fixed current is exposed to the flow velocity.
The flow of current through the wire generates heat on account of i 2R loss. This
heat is dissipated from the surface of the wire by convection to the surroundings.
(The loss of heat due to conduction and radiation is negligible). The wire attains
equilibrium temperature when the heat, generated due to i 2R loss is equal to the
heat dissipated due to convective loss. The circuit is so designed that i2R heat is
essentially constant and therefore the wire temperature must adjust itself to
change the convective loss until equilibrium is reached.
2. Constant temperature mode:
The current required to maintain the resistance and hence temperature constant,
become a measure of flow velocity.
Heat generated = I2Rw
where I - current through the wire;
Rw – resistance of wire
Heat dissipated due to convection = hA (Θ w – Θf)
where
h -coefficient of heat transfer; W/ m2 -°C
A -heat transfer area; m2
Θw- temperature of wire; °C,
Θf - temperature of flowing fluid, °C,
For equilibrium conditions, we can write the energy balance for the hot wire as
OIC 751 TRANSDUCER ENGINEERING
