Pirani gauge#Pulsed Pirani gauge

A pulsed Pirani Transmitter is a vacuum gauge according to the ramp pulse measurement method for measuring pressure in the rough and medium vacuum range.

Principle of operation

Pirani Marcello Pirani meters to be used for measuring absolute pressure in the fine vacuum range since 1906, for over 50 years in almost unchanged form as a stationary -powered vacuum gauge with a constant filament temperature. For controlling the temperature in this case is a Wheatstone bridge circuit

The measurement principle according to Pirani based on the fact that the thermal conductivity of a gas of low pressure is of a pressure-dependent. The heat loss of an electrically heated filament and the bridge voltage required to maintain a constant filament is thus a measure of the absolute pressure. Is the mean free path of the gas, however, is smaller than the filament diameter, which in commercially available sensors usually in excess of about 20 mbar is the case, the thermal conductivity gradually goes into saturation, and is independent of pressure. Therefore, the characteristics of classical Piranis for measuring pressure in the rough vacuum range are not or only very limited suitability.

At a transmitter of the Pirani ramp pulse principle, the filament is not operated stationary but cyclically heated by an upramping heating voltage to a certain temperature threshold. Upon reaching the temperature threshold, the heating voltage and turns off the filament cools down again. At sufficiently low pressures for the filament supplied electrical heating power and the filament temperature, the following relationship applies:

Wherein the specific heat capacity of the filament and the mass of the filament and are constants.

The electric power supplied thus equals the sum of the by heat conduction discharged from the gas power ( term 1 ), the heat losses at the Filamentaufhängung ( expression 2), the radiation losses ( expression 3) and the time required to heat the filament power ( expression 4).

Disregarding the heat loss to the filament ends and the radiation power results in a constant heat output approximately in an exponential heating the filament with respect to the initial temperature Ta in accordance with

With. The use of a ramped heating voltage allowed with the appropriate setting of ramp rate and temperature threshold high resolution. This is true for high pressures, in which the heating voltage limited, the increase in pulse durations to a tolerable level, but also at low pressures, where the ramped heating dispenses the power supply and provides sufficient time resolution.

Gas Type

Measurements with different test gases show that the pulse measurement method in its gas type dependence is not significantly different from the classical driven Pirani.

Combination of sensors

As with conventional Pirani vacuum gauges, a very high measuring range can be covered .. Due to the wide range of Pulspiranis it is possible to turn on the ionization sensors only at very low pressure through combination with other sensors, such as hot - or cold-cathode ionization vacuum gauges, so that their life is increased. This measuring ranges can be realized by, for example, 1000 mbar (atmospheric pressure ) to mbar.

Pros and Cons

  • Higher resolution
  • Large measuring range
  • Reduced need for operational performance
  • Thermal influence on gas pressure in the measuring object is reduced
  • Higher calibration effort
  • Longer warm-up times
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