Pirani gauge

The Pirani Marcello Pirani meters to a pressure gauge.

Principle of operation

Within certain limits, the thermal conductivity of gases is pressure dependent. This physical phenomenon is used by the heat conduction vacuum gauge according to Pirani ( Pirani tube) to measure pressure.

The heat output of the measuring wire is made by three processes ( see sketch in pressure increases from A to C ):

In rough vacuum above 1 mbar, the heat conduction is almost independent of pressure by gas convection, but by a suitable choice of the wire temperature and the sizing of tube sheath and wire has been able to make the then vast heat pressure-dependent convection (measuring range approximately 5 · 10-4 mbar to 1000 mbar).

Another method for the measurement of the coarse and fine vacuum by the Pirani vacuum gauge, the ramp pulse measurement method, in which the filament is no longer operated stationary but is cyclically heated by an upramping heating voltage to a certain temperature threshold. This satisfies a waste of time measurement for evaluation. This pulsed Pirani transmitter allows a high resolution, In addition, the thermal effect is reduced to the gas pressure in the measurement object.

Combined Penning - Pirani vacuum gauge possess in addition to measuring wire around it a sleeve ( anode, see schematic diagram above), which forms with it at very low pressures, an ionization gauge. In order to cover a measurement range from 5 x 10-9 mbar to 1000 mbar succeed.

Construction

The intensity at the sensor ( blind flange with connector ) located measuring thin wire (usually tungsten or nickel) is connected as a branch of a Wheatstone bridge in the evaluation. The voltage across the bridge, the heating voltage is controlled so that the measuring resistance wire and thereby the temperature of the measuring wire, irrespective of the heat dissipation constant.

Since the heat transfer from the measuring wire to the gas increases with increasing pressure, the voltage applied to the bridge voltage is a measure of the pressure. The characteristic of the S-shaped characteristic Piraniprinzip is not suitable for standardized analysis. The logarithmic pressure applied to the measured signal is the characteristic of the sensor. Previously and partially today is used for the display pointer instruments with corresponding non-linear scale. Today, the sensor signal with the aid of most of a microprocessor ( uP ) is linearized and converted into a numerical display.

Influence of the type of gas

Since the heat transfer is not only dependent on pressure, but on the molar mass, and the molecular structure, the pressure measurement at different heavy gases provides different results.

Is a general rule, the greater the atomic or molecular weight of the gas present, the lower the thermal conductivity.

The diagram serves to correct the display value of a calibrated instrument air and nitrogen, respectively, when measured in other types of gas.

Areas of application

• Chemical Industry
• Electrical / Electronics
• Vapor deposition
• Electron beam welding
• Vacuum metallurgy

Pros and Cons

• Large measuring range
• Good reproducibility ( <1%)
• Affordable cell
• Low response time: about 20 ... 50 ms

• Display is the type of gas (usually nitrogen or calibrated to air)
• Measuring wire is sensitive to contamination, especially in organic gas emissions and Penning Pirani combinations due to sputtering of the cathode and decomposition of organic vapors