Hot wire of anemometer working principle

Hot wire of anemometer working principle

Definition: 

A hot wire anemometer is a device used to measure the velocity and direction of a fluid. This can be done by measuring the heat loss of the wire being placed in the fluid stream. The wire is heated by electric current.

The hot wire when placed in the fluid stream, in this case, the heat is transmitted from the wire to the liquid, and then the temperature of the wire drops. The resistance of the wire measures the flow rate of the fluid.

Hot wire anemometers are used as a research tool in fluid mechanics. It works on the principle of heat transfer from high temperature to low temperature.

Building a hot wire anemometer

The hot wire anemometer consists of two main parts.

conduct wiring

Wheatstone Bridge.

The connecting wire is located inside the ceramic body. The wires come out of the ceramic body and connect to the Wheatstone bridge. The wheat stone bridge measures the diversity of resistance.

DC method

In the constant current method, an anemometer is placed in the fluid stream which needs to measure the flow rate. Constant volume current is passed through the wire. The Wheatstone bridge is also maintained at a constant voltage.

Constant current anemometer

When the wire is kept in the fluid stream, in this case, heat is transmitted from the wire to the fluid. Heat is directly proportional to the resistance of the wire. If the temperature decreases, this means that the resistance of the wire also decreases. The Wheatstone bridge measures the change in resistance equal to the fluid flow rate.

Constant temperature method

In this arrangement, the wire is heated by electric current. The hot wire when placed in the fluid stream, heat transfers from the wire to the liquid. Thus, the temperature of the wire changes which also changes its resistance. It works on the principle that the temperature of the wire remains constant. The total current requires that the wire in the initial state be equal to the gas flow rate.

DC anemometer

Liquid rate measurement with hot wire instrument

In a hot wire anemometer, heat is transmitted electrically to the wire which is placed in the fluid stream. The Wheatstone bridge is used to measure the temperature of a wire with respect to its resistance. The temperature of the wire is kept constant to measure the heating current. Thus, the bridge remains balanced.

ammeter

A standard resistor is connected in series to the heating wire. The current through the wire is determined by the voltage drop across the resistor. The value of the voltage drop is determined by a potentiometer.

The equation determines the heat loss from a hot anemometer

where, v - velocity of heat flow,

ρ - density of the liquid,

A and B are the constants. Its value depends on the dimensions and physical properties of the fluid and wires.

Suppose I is the wire's current and R is their resistance. in equilibrium,

Heat generated = heat lost

Equation 2 - Anemometer

The instrument's resistance and temperature are kept constant for liquid rate measurement by first current measurement.