Pirani gauge – A Thermal conductivity Gauge

The Pirani Gauge is a type of Thermal Conductivity Gauges.

Basic principle of Pirani gauge

A conducting wire gets heated when electric current flows through it. The rate at which heat is dissipated from this wire depends on the conductivity of the surrounding media. The conductivity of the surrounding media inturn depends on the densisty of the surrounding media (that is, lower pressure of the surrounding media, lower will be its density). If the density of the surrounding media is low, its conductivity also will be low causing the wire to become hotter for a given current flow, and vice versa.

Description of Pirani gauge

Pirani gaugeThe main parts of the arrangement are:
  1. A pirani gauge chamber which encloses a platinum filament.
  2. A compensating cell to minimize variation caused due to ambient temperature changes.
  3. The pirani gauge chamber and the compensating cell is housed on a wheat stone bridge circuit as shown in diagram.

Operation of Pirani gauge

  1. A constant current is passed through the filament in the pirani gauge chamber. Due to this current, the filament gets heated and assumes a resistance which is measured using the bridge.
  2. Now the pressure to be measured (applied pressure) is connected to the pirani gauge chamber. Due to the applied pressure the density of the surrounding of the pirani gauge filament changes. Due to this change in density of the surrounding of the filament its conductivity changes causing the temperature of the filament to change.
  3. When the temperature of the filament changes, the resistance of the filament also changes.
  4. Now the change in resistance of the filament is determined using the bridge.
  5. This change in resistance of the pirani gauge filament becomes a measure of the applied pressure when calibrated.

Note: [higher pressure – higher density – higher conductivity – reduced filament temperature – less resistance of filament] and vice versa.

Applications of Pirani gauge

Used to measure low vacuum and ultra high vacuum pressures.

Advantages of Pirani gauge

  1. They are rugged and inexpensive
  2. Give accurate results
  3. Good response to pressure changes.
  4. Relation between pressure and resistance is linear for the range of use.
  5. Readings can be taken from a distance.

Limitations of Pirani gauge

  1. Pirani gauge must be checked frequently.
  2. Pirani gauge must be calibrated from different gases.
  3. Electric power is a must for its operation.

Read more...

Hot Wire Anemometer (Thermal Method)

Basic Principle of Hot wire Anemometer

When an electrically heated wire is placed in a flowing gas stream, heat is transferred from the wire to the gas and hence the temperature of the wire reduces, and due to this, the resistance of the wire also changes. This change in resistance of the wire becomes a measure of flow rate.



Description of Hot wire Anemometer

The main parts of the arrangement are as follows:

  • Conducting wires placed in a ceramic body.
  • 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.

Operation of Hot wire Anemometer

There are two methods of measuring flow rate using a anemometer bridge combination namely:

  • Constant current method
  • Constant temperature method

Constant current method


  • The bridge arrangement along with the anemometer has been shown in diagram. The anemometer is kept in the flowing gas stream to measure flow rate.
  • A constant current is passed through the sensing wire. That is, the voltage across the bridge circuit is kept constant, that is, not varied.
  • Due to the gas flow, heat transfer takes place from the sensing wire to the flowing gas and hence the temperature of the sensing wire reduces causing a change in the resistance of the sensing wire. (this change in resistance becomes a measure of flow rate).
  • Due to this, the galvanometer which was initially at zero position deflects and this deflection of the galvanometer becomes a measure of flow rate of the gas when calibrated.

Constant temperature method


  • The bridge arrangement along with the anemometer has been shown in diagram. The anemometer is kept in the flowing gas stream to measure flow rate.
  • A current is initially passed through the wire.
  • Due to the gas flow, heat transfer takes place from the sensing wire to the flowing gas and this tends to change the temperature and hence the resistance of the wire.
  • The principle in this method is to maintain the temperature and resistance of the sensing wire at a constant level. Therefore, the current through the sensing wire is increased to bring the sensing wire to have its initial resistance and temperature.
  • The electrical current required in bringing back the resistance and hence the temperature eof the wire to its initial condition becomes a measure of flow rate of the gas when calibrated.

Application of Hot wire Anemometer

In research applications, they are extensively used to study varying flow conditions.

Read more...

About This Blog

Lorem Ipsum

  © Free Blogger Templates Columnus by Ourblogtemplates.com 2008

Back to TOP