Thread: SparkChambers
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Old 26th May 2019, 5:30 am   #26
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
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Default Re: SparkChambers

Originally Posted by ColinTheAmpMan1 View Post
Originally Posted by Philips210 View Post
I remember reading that helium is able to permeate through glass such as in valves and impair the vacuum. Is this a real possibility if the valve is exposed to helium?
I recall that when I worked on sector-field mass spectrometers, a way of finding where there was a leak in the vacuum system was to squirt a little helium around the suspect places while the mass spectrometer was tuned to 4 mass units. If a peak at 4 mass units was seen, that was where the leak was. This was pretty sensitive, as most of the joints would leak a little, anyway.

Isn't it true to imagine that if helium can diffuse through the glass envelope of a valve, then it can also leak out of the valve too? This will depend on the partial pressure of helium on either side of the glass, not the total pressure difference, surely?

There was a small (1cm radius or so) mass spectrometer called the 'minimass' I think made by AEI. It had a pre-set positon on the controls for 4 mass units, to be used as a leak detecctor as you suggest (go round the joints of the vacuum system with a jet of helium). I have such an instrument (the mass spectrometer head, big horseshoe magent and the control electronics) but no vacuum system to use it with.

As for helium diffusing out, this was certainly a problem with HeNe laser tubes when I was at school. They would lose the helium and stop working.
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