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Q :Does pumping Noble gases require special considerations?

A : There are 6 noble gases found in group 18 of the periodic table: Helium, Neon, Argon, Krypton, Xenon and Radon. Their electronic structure is such that their valence outer shells are full and they are generally highly unreactive, a feature exploited for many applications.

There are several characteristics of these gases which affect the means and mechanisms for vacuum pumping. Here we discuss some examples.

We will make reference to some characteristics of these gases which are plotted below.

 

RMM

Viscosity

Thermal conductivity

Specific heat

g

gg

 

 

Pa.s x 10-6

W/mK x 10-2

Constant P (J/kg/K)

 

 

Nitrogen

28

18.2

2.56

1007

1.4

1.6

Argon

40

22.26

1.74

520

1.7

2.5

Helium

4

19.6

15.2

5175

1.7

2.2

Xenon

131

22.81

0.57

158

1.7

2.5

Krypton

84

25

0.98

248

1.7

2.5

Neon

20

31.29

4.81

1031

1.7

2.3

  • Their inert nature means that Ionisation based pumping mechanisms have reduced capacity. For example an Ion Getter Pump Argon speed is ~20% that for Nitrogen.
  • The low relative molecular mass (RMM) of Helium leads to a low turbo-molecular pump compression ratio (which can be typically less than 1/107 that for Nitrogen).
  • As monatomic gases they have a relatively higher gamma (g) value. As such this is significant heat of compression which in the case of Xenon and to a lesser extent (but still important for) Krypton and Argon, is compounded by relatively low thermal conductivity. This represents a significant thermal ‘load’ to the pump. This can be an especially important consideration for dry pump (claw, screw, scroll etc.) technologies where the heat load can affect running clearances.
  • Oil-sealed rotary pumps. Materials and especially the oil in these pumps have a significant capacity to absorb helium. This is especially an issue for leak detection instruments (direct or counter-flow) where the concentration of helium in the fore-pump can affect the leak detector performance - especially reaching ‘zero’ background reading. Historically OSRV pumps employed a ‘bubbler’ mechanism whereby air was introduced into the oil box to promote helium flow from the oil. Some other evaluation and techniques (air purges) have been discussed by your author: A D Chew,'The measurement of helium retention in fore-pumps' Vacuum, 5, 243 (1999).
  • Recirculation of Helium (He4 and He3 isotopes) has long been employed in cryogenic/dilution systems. In this case the ;cleanliness; of the pumping system is crucial. Hence pumps with no oil in the gas pathway are preferred since for example, an OSRV would need condenser/separator to remove oil carry over from the helium. Edwards have qualified and propose STP (maglev TMP) and scroll (XDS and nXDS series) or roots-claw combinations for this application whereby the gas is not exposed to any oil/lubricant.
  • More recently pricing of inert gases makes recovery/recirculation highly attractive The recently reported cost of Helium = 0.4p/standard-litre with Argon x 2, Neon x 20, Krypton x 100 and Xenon x 1000 this price. Xenon presents the most extreme heat challenge and STP/XDS (with optional booster stage) have been qualified for this.


Vacuum Measurement - the same considerations with regard to the characteristics of noble gases need to be considered when we measure pressures. For example, a typical Pirani gauge indicated Krypton pressure of 0.1 mbar has a true pressure of ~0.25 mbar: This reflects the difference in thermal conductivity of Krypton versus Nitrogen (0.98/2.56). Similarly if we consider a hot cathode ionisation gauge indicated Helium pressure of 1e-5 mbar has a true pressure of 5e-5 mbar reflecting the ionisation potentials.

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