Pirani vacuum gauge
The heat conduction vacuum meter according to Marcello Pirani is a pressure measuring device for the measurement of fine vacuum .
Working principle
The thermal conductivity of gases is pressure-dependent within certain limits. This physical phenomenon is used by the Pirani heat conduction vacuum meter (Pirani measuring tube) to measure pressure.
The heat dissipation of the measuring wire occurs through three processes (see sketch, with increasing pressure from A to C):
- A - heat radiation of the wire and heat dissipation at the wire ends
- B - pressure-dependent heat conduction through the gas
- C - convection
In a rough vacuum above 1 mbar, the heat conduction through gas convection is almost independent of pressure, but by choosing the wire temperature and the dimensions of the tube jacket and wire, it has been possible to make the then predominant heat dissipation through convection pressure-dependent (measuring range approx. 5 · 10 −4 mbar up to 1000 mbar).
Another method for measuring rough and fine vacuum using Pirani vacuum gauges is the ramp pulse measuring method, in which the filament is no longer operated in a stationary manner, but is heated up cyclically by a ramp-shaped heating voltage up to a certain temperature threshold value. This means that a pure time measurement is sufficient for evaluation. This pulsed Pirani transmitter allows a high resolution, and the thermal influence on the gas pressure in the measuring object is reduced.
In addition to the measuring wire, combined Penning-Pirani vacuum gauges have a sleeve (anode, see schematic diagram above) around it, which forms an ionization vacuum gauge with this at very low pressures . This makes it possible to cover a measuring range from 5 · 10 −9 mbar to 1000 mbar.
construction
The thin measuring wire (mostly tungsten or nickel) in the sensor (blind flange with connector) is connected in the evaluation device as a branch of a Wheatstone measuring bridge . The heating voltage applied to the bridge is regulated in such a way that the measuring wire resistance and thus the temperature of the measuring wire is constant regardless of the heat output. In equilibrium, the electrical power supplied is equal to the thermal power dissipated, which is proportional to the pressure in the event that the mean free path is large compared to the wire diameter.
Since the heat transfer from the measuring wire to the gas increases with increasing pressure, the voltage across the bridge is a measure of the pressure. The S-shaped characteristic curve that is characteristic of the Pirani principle is unsuitable for standardized evaluation. The measurement signal plotted logarithmically for the pressure is the characteristic curve of the sensor. In the past and in some cases today, pointer instruments with a corresponding non-linear scale were used for display. Today the sensor signal is mostly linearized with the help of a microprocessor (μP) and converted into a numerical display.
Influence of the type of gas
Since the heat transport is not only dependent on the pressure, but also on the molar mass and the molecular structure, the pressure measurement provides different results for gases of different weights.
A general rule says: the greater the atomic or molecular mass of the gas present, the smaller the heat conduction.
The diagram opposite is used to correct the display value of an instrument calibrated for air or nitrogen when measuring in other types of gas.
Influence of the installation position
When installing the vacuum gauge, it is essential to pay attention to the installation position and to follow the manufacturer's specifications. The reason for this is that in the pressure range above approx. 10 mbar there is increased gas convection, the movement of which is influenced by the direction of gravity.
Areas of application
- Chemical industry
- Electron microscopy
- Physical vapor deposition
- Electron beam welding
- Vacuum metallurgy
Advantages and disadvantages
- advantages
- large measuring range from 10 −4 mbar to atmospheric pressure
- good reproducibility (<1%)
- inexpensive measuring cell
- short response time: approx. 20… 50 ms
- disadvantage
- The display depends on the type of gas (mostly calibrated to nitrogen or air)
- Measuring wire is sensitive to contamination, especially with organic outgassing and with Penning-Pirani combinations due to sputtering of the cathode and decomposition of organic vapors
Web links
- Construction accessed on July 24, 2018
- Measurement principle accessed on July 24, 2018