Subject: How to find vacuum leaks?

From: [email-masked] (Dave Ide)
Date: 30 May 1996 00:51:45 GMT

Can any of you tell me how to find the leak in a
     vacuum chamber?   It could be the various fittings,
     or it could be my welding.....                    
                                         David Ide

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From: [email-masked] (John Dammeyer)
Date: Mon, 10 Jun 1996 16:50:38 GMT

>     or it could be my welding.....                    

Usually ultrasonics are generated when something leaks.  A company
called UE Systems makes an instrument that shifts the Ultrasonics down
to an audible level.  

Actually,  Most better automotive repair shops will have the same
thing for detecting a leak in the miriad of hoses under the hood.

Principle is just like the IF stage in a radio.  Microphone picks up
ultrasonic, mixed with another frequency, the result is audible which
is amplified and sent to headphones or even just detected  with a
diode and sent to a meter.

John.

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From: [email-masked] (Walter K. Stockwell)
Date: 10 Jun 1996 21:00:32 GMT

If it's a big leak, you can use "Snoop" (basically soapy water.)  The idea
is you *pressurize* your vacuum system to a few psi, then dab the snoop over
all the likely places, like your joints and welds.  Then look for bubbles
to form.  

-- 
That bird was my bird --	__
	or some vile copy.     {^^}              Walter Stockwell
				\ \              [email-masked]
				{  -}

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From: [email-masked] (michael jensen)
Date: 10 Jun 1996 20:49:01 GMT

John Dammeyer ([email-masked]) wrote:
: [email-masked] (Dave Ide) wrote:

: >     Can any of you tell me how to find the leak in a
: >     vacuum chamber?   It could be the various fittings,
: >     or it could be my welding.....                    
: >                                         David Ide

     The best leak testing that I know of for finding a leak is using
a helium leak detector.  You connect up the leak detector which is 
pumped down using a roughing pump and then a diffusion pump.  Attached
to these is a small mass spectrometer that is tuned for helium.  You 
then get a tank of helium and using a little hose test your fitting 
and welds.  Our leak detector make a humming sound when it detects 
helium.  This works very good for high and ultra-high systems.  For a 
system that is only going to 10^-3 Torr attach a roughing pump with a 
therocouple gauge and using acetone squirt a little on the fittings 
and see if the vacuum pressure rises.

     That is the problem. Is it the fittings or the welding that is 
leaking.  Some time both can be a real pain to find. Best of luck.

     Mike J. 

--
Mike Jensen ([email-masked]) 
Chemistry Instrument Shop              
University of Nebraska - Lincoln
(402) 472-3542

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From: "W. L. Smith" <[email-masked]>
Date: Mon, 10 Jun 1996 21:52:05 GMT

It depends a lot on what vacuum range you are working in.  If you are 
below 1 x 10E-07 torr range you can use a Residual Gas Analyzer (RGA) in 
the chamber which is set to detect helium gas which you spray on the 
outside of the chamber at joints and fittings (very expensive).  If you 
are in the 1 x 10E-06 torr to 10E-01 torr range and you are using a 
thermocouple gauge to read the pressure, you can squirt alcohol or 
acetone on the joints and fittings (one at a time) and watch for a rise 
on the gauge as the high vapor pressure expands through the leak.  If 
you cannot use either of these methods, you can sometimes put a slight 
(<2 psi) overpressure in the chamber and squirt a soapy water mixture 
around the joints and fittings to look for bubbles.  Last option - make 
the room quiet and listen very carefully.

Good luck ... Will

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From: [email-masked](Max ben-Aaron)
Date: 11 Jun 1996 03:46:44 GMT

High their:

How about turning the problem on its head: Try pressurising the system
with air and looking for leaks with the old soapy water trick (looking
for bubbles)?

Would this work?

Max ben-Aaron

====================================================================

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From: [email-masked] (Kevin AstirCS "1U" KO0B)
Date: Tue, 11 Jun 1996 11:14:58 -0700 (PDT)

"W. L. Smith" <[email-masked]> wrote:

>  Last option - make 

A piece of PVC tubing stuck in your ear (don't tell your mother you
heard it from me!) makes an effective stethiscope here.  Much easier
than getting your head down under the pump plate!  Beware of "seashell
effect" when probing any cavity.

Even water flowing through a cold trap can make enogh noise to mask
sound of a leak.  Hopefully you can pull enough vacuum to find leak
without turning on diffusion pump.

-KF-

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From: [email-masked] (Ladge Kviz)
Date: Wed, 12 Jun 96 04:43:11 GMT

>all the likely places, like your joints and welds.  Then look for bubbles
If its a small leak and you have a vacuum guage connected, you can squirt 
metho (alcohol) onto the suspect joints and welds while watching the vacuum
guage, as you spray onto the leak the alcohol will get into the vacuum and
rapidly evaporate causing  sudden drop on the vacuum guage. of course your
leak has to be small enough to allow the vacuum pump to pull enough vacuum to 
get a
reading on the vacuum guage for this to work.

Happy hunting.

Ladge

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From: [email-masked]
Date: 12 Jun 1996 16:05:22 GMT

>leaking.  Some time both can be a real pain to find. Best of luck.

Trust Mike.  The *only* way to find leaks (large or small) in a real
vacuum system is using a mass spec helium leak checker.  

You can fool around with squirting acetone on the system, and 
watching the pressure on a TC gage, or watching for a color
change in glass tube in the forepump line excited with a sparker
coil, but that is all mostly useless in the end.

If it is a real piece of vacuum aparatus (final pressure to be less
than 100 microns or so) then you will need to be able to leak
check with a helium leak checker, so try to borrow one, and learn
to use it.  

Veeco, Leybold, Varian, &tc all make some types which have 
differences, but are basically the same - a U-tube magnetic
analyzer to measure helium concentration and some sort of 
vacuum system to pump with.

If it is a UHV system, then the RGA (residual gas analyzer) is the
way to go - a dedicated mass spec unit that bolts onto your chamber.
Way expensive.

Best bet - find somebody with a leak checker, ask them to show 
you how to run it.  

Best of luck!

===========================================================

 1959 BMW R50                                           Jim Rozen              
 1969 BMW R60US                                (JRR0 @ watson.ibm.com)                 
 1975 BMW R75/6    (The "new" one)        

===========================================================

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From: [email-masked] (Clhuprich)
Date: 12 Jun 1996 18:33:54 -0400

If you want to find them so you can fix them why not just paint all
suspect
areas with a two component epoxy paint? This may be a bad idea if
you are using really low preasures (possible outgassing) or high temps.  
  There used to be an "arts & recipies" section in the old (30 years ago)
Handbook of Chemistry and Physics that covered such things.

Cliff Huprich

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From: [email-masked] (Peter Brooks)
Date: 13 Jun 1996 18:23:01 GMT

: Usually ultrasonics are generated when something leaks.  A company
: called UE Systems makes an instrument that shifts the Ultrasonics down
: to an audible level.  

Might work pretty well near atmospheric pressure.  if you are measuring
your vacuum in inches of mercury, go for it.

The following is for high vacuum systems:

The usual way in the electronics industry is to hang a gas sensor in
the vacuum lines (generally optimized for helium) and then to rig up a
tank/regulator of He to a wand.  Set the He flow to 'low' and put the
business end near any suspect locations.  When your gas sensor goes 
*boing!* you probably found one of the leaks.

In this application, He is preferred since it'll tend to make its way through
the smallest leaks with ease.

Note that this is best for a *high vacuum* system, running at 10e-3 torr or
less.  I'd doubt that a He sensor would work at pressures much above
that.  (If dim memories of vacuum work still hold, you can use an ion gauge
as a so-so gas detector.  Since He will get through, your pressure will start
going up when you're near the leak.)

Pete Brooks

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From: Mike Rehmus <[email-masked]>
Date: 13 Jun 1996 22:24:23 GMT

Perhaps you could use the ultrasonic
leak detector that is used in Air Conditioning
service.  Basically it is just a sensitive
microphone with frequency-specific amplification.

I have no idea how expensive one might be but
if you have an AC repair shop around, they might
have one they'd rent or loan.

Best regards,

Mike Rehmus