While putting my initial vacuum system together with some centring rings made up with unknown composition rubber o-rings and centres made made of plastic. Bought off ebay with the clamps and elbows I wanted. I've been getting strange pressure behaviour which I can now only put down to the rubber o-rings and the plastic centres. This includes sudden increases in pressure ( 6 to 60 millitorr in less than a second). followed by sudden decreases, with a millitorr per second rise as soon the pump is valved off which reaches atmospheric overnight.
Needless to say I have the new viton o-rings plus the Aluminium rod and the lathe ready to create the replacements... its either this or the pirani gauge that is leaking...
beware of unknown o-rings?..
-
Steven Sesselmann
- Posts: 2128
- Joined: Wed Aug 10, 2005 9:50 pm
- Real name: Steven Sesselmann
- Location: Sydney - Australia
- Contact:
Re: beware of unknown o-rings?..
Derek,
Vacuum leaks can be incredibly frustrating, at micron pressures, air seems to get in just about anywhere. It could be anywhere, so I suggest to look everywhere.
Most likely it is not the Pirani gauge itself..
Check and double check every joint, seal and fitting, if you are loosing all your vacuum overnight, you have a huge leak, not just out gassing.
When you get the leak down to 1 micron (millitorr) every couple of minutes, then you can try running the high vacuum pump and squirting a bit of acetone on the joins, and if there is a leak, the acetone will make its way into the chamber and show up as a sudden pressure increase.
Be patient, you will find it...
Steven
Vacuum leaks can be incredibly frustrating, at micron pressures, air seems to get in just about anywhere. It could be anywhere, so I suggest to look everywhere.
Most likely it is not the Pirani gauge itself..
Check and double check every joint, seal and fitting, if you are loosing all your vacuum overnight, you have a huge leak, not just out gassing.
When you get the leak down to 1 micron (millitorr) every couple of minutes, then you can try running the high vacuum pump and squirting a bit of acetone on the joins, and if there is a leak, the acetone will make its way into the chamber and show up as a sudden pressure increase.
Be patient, you will find it...
Steven
http://www.gammaspectacular.com - Gamma Spectrometry Systems
https://www.researchgate.net/profile/Steven_Sesselmann - Various papers and patents on RG
https://www.researchgate.net/profile/Steven_Sesselmann - Various papers and patents on RG
Re: beware of unknown o-rings?..
an amusing story:
I once purchased one of these o-ring kits from ebay for vacuum work. it turned out that the things smelt horrible (indicating something volatile in them) and this smell not surprisingly got all over the vacuum system. being determined not to be beaten however, i dumped some in acetone overnight. Shour enough when i came back the next day the acetone was a nasty dark brown colour so i fished out the o-rings, rinced them in a little more acetone and let them dry. when they had dryed completely, i found they had lost all of there elasticity and become extremely brittle and crumbly. guess that nasty smelly brown gunk was doing something important!
Theres just no useing cheep o-rings, get some nitrile or viton ones from a vacuum supply place, its well worth the extra money.
Oliver
I once purchased one of these o-ring kits from ebay for vacuum work. it turned out that the things smelt horrible (indicating something volatile in them) and this smell not surprisingly got all over the vacuum system. being determined not to be beaten however, i dumped some in acetone overnight. Shour enough when i came back the next day the acetone was a nasty dark brown colour so i fished out the o-rings, rinced them in a little more acetone and let them dry. when they had dryed completely, i found they had lost all of there elasticity and become extremely brittle and crumbly. guess that nasty smelly brown gunk was doing something important!
Theres just no useing cheep o-rings, get some nitrile or viton ones from a vacuum supply place, its well worth the extra money.
Oliver
Re: beware of unknown o-rings?..
Well I swapped the unknown with a homemade aluminium centring ring (carefully cleaned) and a viton o-ring purchased from our local industrial rubber supplier - within 1 hour better than 20 seconds per millitorr- the rest of those Unknown o-rings and their centring rings are going in the bin.