Pirani Gauge – Construction, Working, Advantages, Disadvantages, Applications, and its special form.

Pirani Gauge is a device that is used to measure pressure, especially in vacuum systems. It was invented by a German physicist Marcello Stefano Pirani in the year 1906.
It is based on a hot metal wire suspended in a tube and exposed to gas pressure media. This hot metal wire is made up of thin Tungsten, Nickel, or Platinum wire.
It works by measuring the change in thermal conductivity and use this measurement to get the pressure of the system.
This gauge is suitable for measuring pressure in very low pressure in the range from 0.1 to 100 Pascal.

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Construction:

• The Pirani Gauge consists of Wheatstone Bridge in which two of the legs are gauge tube and reference tube. Both the tubes have filaments present inside it.
• The gauge tube is open to the system whose pressure is to be measured and the reference tube is closed. The gauge tube filament is used to measure the pressure of the environment.
• The reference tube is present on the opposite side of the gauge tube which is used to get the temperature to which the filament of the gauge tube is to be heated when the temperature of the filament goes down due to contact with the gas molecules.
• The filament material in both the tubes is made of either Tungsten, Nickel, or Platinum. This filament is very thin and is always less than 25 µm.
• A power supply is used to pass the current in the Wheatstone Bridge.
• A sensor wire is used to convert the voltage reading into a pressure reading. This sensor wire is connected to the electrical circuit so that the pressure reading can be taken.

Working:

• At first, the power supply is given to the Pirani Wheatstone Bridge. The filaments in the gauge tube and reference tube start getting heated.
• The filament in both the tubes will get heated up to nearly 50 °C.
• The gauge tube is open to the system and the reference tube is closed.
• This Pirani Gauge is placed in the system of which the pressure is to be measured.
• As the gauge tube is open, the gas molecules from the system will enter the gauge tube and will collide with the heated filament. Then the gas molecules which collide with the filament will receive the heat from the filament and will escape from the gauge tube.
• The heat carried by the gas molecules will depend upon the pressure of the system. It depends on the system pressure because when the pressure of the environment is high, the density of the gas will be high and will carry more amount of heat from the filament. As it will carry more heat from the filament, it will cool the filament faster.
• When the pressure of the the system is low, the density of the gas will also be low. Due to low density and long mean free path between the gas molecules, the thermal conductivity of the gas will be low and will carry less heat from the filament. So, the filament will cool down slowly as compared to high pressure. So, we can say that the thermal conductivity of the gas depends upon its pressure.
• The filament wire is maintained at a constant temperature which is generally 50 °C. When the pressure of gas changes, its thermal conductivity changes, and hence the voltage required to maintain the temperature of the wire changes.
• The required voltage changes because the rate of cooling of filament changes with change in thermal conductivity of gas which changes due to change in pressure.
• Now, the voltage of the Wheatstone bridge is set to keep the filament at a constant temperature and this required voltage value is converted to a pressure value. The electric circuit is connected to a sensor wire from which a pressure reading is taken.
• In this way, the pressure of the system is measured using the Pirani Gauge.
• The Pirani Gauge is needed to be calibrated when the pressure of different gas is to be obtained as thermal conductivity and heat capacity varies from one gas to another.

Advantages Of Pirani Gauge:

1) They are rugged and inexpensive.
2) It gives accurate results at low pressure.
3) Pressure measurement can be read from a distance.
4) This gauge is preferred over Thermo Couple gauge which is another type of thermal conductivity gauge. It is preferred because it is 10 times faster and a wider range of pressure can be measured.

Limitations: Error in measurement using Pirani Gauge:

1) Pirani Gauge is not accurate at high pressure:-
In high pressure, the change in thermal conductivity is very less. The change in thermal conductivity is the key thing for measuring pressure using Pirani gauge.
At high pressure, the heat loss from the filament is not so significant such that the rise in temperature of the filament that will be measured by the bridge circuit is not appreciable and this measurement cannot be used to measure the pressure of the system accurately.
2) Transient response of this device is very poor and attaining equilibrium needs several minutes at low pressure.
It means the correct reading can not be measured instantly when the pressure changes because this device needs several minutes to attain equilibriumor steady-state after the change in pressure. For several minutes the pressure reading will fluctuate before attaining a steady state.

Applications:

1) Used to measure pressure at low pressure and high vacuum pressures.
2) It is also used plot graph on basis of data obtained.

Special Form Of Pirani Gauge:

A special form of Pirani Gauge is Pulsed Pirani Vacuum Gauge. In this special form of Pirani gauge, the filament wire is not operated at constant temperature instead it is cyclically heated to a certain temperature threshold.
The filament wire is heated using an increasing voltage ramp. The heating voltage is switched off when the threshold temperature is reached after which the filament wire cools down again.
The time required to heat the filament wire to a certain threshold temperature is used as a measure which is then converted to a pressure value.