What is Hot Wire Anemometer? Working, Advantages & Applications

Hot Wire Anemometer (or Thermal Flow Meter) makes use of temperature coefficient effect i.e. the resistance of conducting material changes with change in temperature.

Fig. 1 : Hot wire anemometer.

Construction & Working of Hot Wire Anemometer

A wire made up platinum is kept in the liquid whose flow is to be measured. A constant current is passed through this wire. The heating of wire takes place and the liquid flow removes the heat from this wire. The temperature of the wire will drop due to heat removal. This wire can attain steady-state temperature or equilibrium state if the heat generated (I²Rt) is equal to heat removed due to convection and radiation. The wire can be arranged in bridge circuit as shown in Fig. 1. R_p is resistance of thin platinum wire kept in the pipe. This R_p is connected in Wheatstone’s bridge circuit as shown in Fig. 1.

A suitable current is adjusted in the circuit by varying R_h. The bridge is first balanced by varying R_p. The resistance R_p gets heated due to current flow. Due to change in flow rate the resistance R_p changes and the bridge is unbalanced. The unbalanced voltage can be measured using galvanometer or millivoltmeter.

If flow rate is high, the resistance R_p decreases since flow causes cooling effect. The bridge unbalance is more i.e. potential difference between points (A) and (B) is greater. Thus flow can be measured in terms of voltage.

Advantages of Hot Wire Anemometer

1. Useful for measurement of unsteady flow of liquids/gases in the range of 0.5 cm³/min to 18000 kg/hr.
2. Useful in research work.
3. Uses thermal method for flow measurement.
4. Accuracy ≈ ± 2%.
5. Can work upto a pressure of 1200 psig (8.3 MPa).

Disadvantages of Hot Wire Anemometer

1. Limited physical strength of fine wire.
2. Calibration is affected due to dust deposition.
3. Temperature heating sensor and the heater is directly placed in flow path, can be easily damaged by corrosion and erosion.
4. Large input power is required to measure high flow rates.

Applications  of Hot Wire Anemometer

1. Usefully used in research work.
2. Measurement of flow rate of liquids and gases.
3. Unsteady flow rate can be measured.