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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
How can i protect my new digital vacuum gauge ? i typically have a vessel pulled down to high vacuum with epoxy, oil and other liquids which might volatize.
With the Bourdon tube type being robust but not being able to read low enough i need a more sensitive digital type but i've never used them with chemicals present, only physical tests on aircraft.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
Patrick, I want a pressure gauge that works down to 0.5 Pa or less - and is not expensive, any suggestions ?
Regarding your question - I have no experience, just an idea ... if you have a valve to isolate your gauge that you only open (slowly) after your vacuum has been established (monitor with your Bourdon gauge) then very little volatile content should reach it, and your valuable pressure gauge would be on a dead end line so probably very little contamination will reach it.
I'm also on a chemistry forum ( sciencemadness.org ) and there are some experienced folk there, I'll see if they have any suggestions ..... ____________________________________________
_________ Update ... the only reply so far :
The usual way its done is by an isolation valve and preferable a cold trap. The gauge being isolated until the roughing pump has exhausted most of the gases and closed before releasing the vacuum. He should consult the manual for the gauge if its an expensive one.
He should also note that the type of vacuum gauge determines how susceptible the gauge is to spurious vapours. Typically gauges with hot wire or filaments are more susceptible. He should also note that if he does have volatile material in his system he will not be able to reach a high vacuum (mean free path comparable to the dimension of his chamber).
An other option is to have a discharge tube connected to the system. The characteristics of the discharge indicate the vacuum level and the discharge color will give an indication of the contamination. When the discharge goes dark under vacuum the mean free path is about equal to the separation distance between the electrodes. ie high vacuum.
The discharge also helps to trap or destroy active gases. It can also help when trying to find leaks using a suitable gas or fluid sprayed on to the suspected leak point. ___________________________________________
___________ another reply : "There are chemical traps as well (charcoal, zeolite)...more for the protection of the vacuum pump post-cold trap...when valves are used they are often between the pump and gauge, for when the pump is to be turned off."
_________________________________________________
_____ ( FYI, at these pressures 'cold trap' normally means cryogenic, at least dry ice/acetone or commonly liquid nitrogen temperatures. )
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
very cool totally what I needed to know. Im going to buy a $130 one instead of the $256 or $433 which is over kill, but people complain about the sub-$180 ones leaking. Though many people blame instruments for leaks when really its other things.
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How to protect a vacuum guage ?
