Pulsed Pirani transmitter

A pulsed Pirani transmitter is a vacuum meter based on the ramp pulse measuring method for measuring pressure in the rough and fine vacuum range.

Working principle

Thermal conduction vacuum gauges according to Marcello Pirani have been used for absolute pressure measurement in the fine vacuum range since 1906, and have been used in almost unchanged form as stationary vacuum gauges with constant filament temperature for over 50 years. A Wheatstone bridge circuit is used to regulate the temperature

The Pirani measuring principle is based on the fact that the thermal conductivity of a gas for low pressures is itself pressure-dependent. The heat loss of an electrically heated filament or the bridge voltage required to maintain a constant filament temperature is therefore a measure of the absolute pressure. However, if the mean free path of the gas particles is smaller than the filament diameter, which is usually the case with commercially available sensors above about 20 mbar, the thermal conductivity gradually saturates and becomes independent of pressure. Therefore, the characteristic curves of classic Piranis for pressure measurement in the rough vacuum range are not or only to a very limited extent suitable.

With a Pirani transmitter based on the ramp-pulse principle, the filament is not operated in a stationary manner, but is heated up cyclically by a ramp-shaped heating voltage up to a certain temperature threshold value. When the temperature threshold is reached, the heating voltage switches off and the filament cools down again. At sufficiently low pressures, the following relationship applies to the electrical heating power supplied to the filament and the filament temperature : ${\displaystyle T(t)}$

${\displaystyle P_{el}=C_{1}\lambda _{gaz}(T(t)-T_{a})+C_{2}\lambda _{fil}(T(t)-T_{a})+A_{fil}\epsilon \sigma (T(t)^{4}-T_{a}^{4})+c_{fil}m_{fil}{\frac {\mathrm {d} T}{\mathrm {d} t}},}$

where the specific heat capacity of the filament, the mass of the filament and and are constants. ${\displaystyle c_{fil}}$ ${\displaystyle m_{fil}}$ ${\displaystyle C_{1}}$ ${\displaystyle C_{2}}$

The electrical power supplied thus corresponds to the sum of the power dissipated by the gas via thermal conduction (term 1), the heat losses at the filament suspension (term 2), the radiation losses (term 3) and the power required to heat the filament (term 4).

Neglecting the heat losses at the filament ends and the radiation output, a constant heating output results approximately in an exponential heating of the filament compared to the initial temperature Ta according to

${\displaystyle T(t)=T_{a}+{\frac {P_{el}}{C_{1}\lambda _{gaz}}}\left(1-e^{-{\frac {t}{\tau }}}\right)}$

with . The use of a ramp-shaped heating voltage allows a high resolution with the appropriate setting of the ramp steepness and temperature threshold value. This applies both to high pressures, at which the rising heating voltage limits the pulse times to a tolerable level, and to low pressures, where the ramp-shaped heating doses the power supply and ensures sufficient time resolution. ${\displaystyle \tau ={\tfrac {C_{fil}m_{fil}}{C_{1}\lambda _{gaz}}}}$

Gas type dependency

Measurements with different test gases show that the pulse measurement method does not differ significantly from the traditionally operated Pirani in terms of its gas type dependency.

Combination sensors

As with conventional Pirani vacuum gauges , a very large measuring range can be covered by combining them with other sensors such as hot or cold cathode ionization vacuum gauges . Due to the wide measuring range of the pulse piranis, it is possible to switch on the ionization sensors only at very low pressure, so that their service life is increased. Measuring ranges from 1000 mbar ( atmospheric pressure ) to mbar, for example, can be implemented. ${\displaystyle 10^{-9}}$

Advantages and disadvantages

Advantages over classic Pirani transmitters

• Higher resolution
• Large measuring range
• Reduced operating power requirement
• The thermal influence on gas pressure in the measuring object is reduced

Disadvantages compared to classic Pirani transmitters

• Higher calibration effort
• Longer warm-up times

Individual evidence

1. ↑ M. v. Pirani, Negotiations of the German Physical Society 4, 686-694 (1906).
2. ↑ H. v. Ubisch, Nature 161, 927-928 (1948).
3. ↑ H. v. Ubisch, Vakuum-Technik 6, 175-181 (1957).
4. ↑ H. Plöchinger, patent DE 10115715 A (March 30, 2001).
5. ↑ ^{a b} W. Jitschin and S. Ludwig, vacuum in Research and Practice 16, 23-29 (2004).
6. ↑ [1]  ( Page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. .