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Greetings

I have been a some time lurker for the simple fact of having not to much
to add but today hopefully that could be different.

As an employed welder I would suggest doing something in the opposite
direction. Apply about 10 pounds air pressure to the inside of the
system and then spray the weld areas with a soapy water solution and all
(they are very visible and its cheap too) the pinholes will produce neat
bubbles whereupon you mark the spots and haul out your trusty welder
(most likely grinder too) and have another go at making that puppy leak
free!

I thank you all for allowing the lurkers to absorb your processes - I
hope to one day travel the same path!!

Darald

On Fri, 2 Jan 2004, James Lerch wrote:

Greetings All,

Just thought I'd share the initial results on my latest project, a 24" mirror
coater :) Today I fabricated the Square O-Rings from stock 10mm O-Ring cord,
and ran the first leak test (which it failed, but not to terribly bad, I still
made it down to the 18 torr range on the rough pump..)

I note for ultrahigh vacuum copper seals are used. Just a thought. Also,
you cannot mean 18 Torr. I presume you mean something like 10e-3 Torr,
which is characteristic for a common roughing pump.

I would be interested in your solution for firing up the tungsten
filaments, presumably you used (green) TIG welding electrodes. I am
testing this now. I note on your web page you mention wanting to improve
your heating control. I have built a high power (light) dimmer
circuit that works pretty well that uses minimal components and is
dirt cheap (a couple $USD). The thinnest TIG electrodes around here are
1 mm diameter and about 150 mm long so there is not much resistance. On the
other hand they are so cheap that one can use 10 or 15 of them in series
at which point the circuit cold resistance actually increases to sanity
levels, comparable to a light bulb. My current plan is to get the whole
series working exactly right and then replace the linear dimmer potentiometer
(there is also a threshold pot so the dimmer pot changes TIG temperature
thoughout its entire range) with a switch between two resistance values that
give either low heat (melts aluminum and wets the electrode) or high
heat (vaporizes molten aluminum). In other words, the final control unit
will not have a range of temperatures. It will only have two; melt and
vaporize. Although, I will keep the dimmer pot as a third setting in case
I try other adventures (anyone that know me at all knows this is likely).

If you or some else want the circuit I can arrange this. Basically,
it uses a BT139 triac (600 V @ 16 Amp) and a diac tuned by a couple
of resistors and potentiometers that regulate the rate of charge/discharge
of a capacitor (600 V @ 33 nF) that triggers the triac. Circuit includes a
filter for radio interference, but this can be skipped because high power chokes
add signicantly to the price and might not even be needed. I think if you
get noise you can just as well try turning the potentiometer a tad. I have
now made a PCB layout and will post the final result on my website when I
get more time. It is built for 230 VAC mains, but can be adapted for 120
VAC. I of course welcome any criticism of such a circuit for this application.


Dominic-Luc Webb

Greetings All,

Just thought I'd share the initial results on my latest project, a 24" mirror
coater :) Today I fabricated the Square O-Rings from stock 10mm O-Ring cord,
and ran the first leak test (which it failed, but not to terribly bad, I still
made it down to the 18 torr range on the rough pump..)

The first interesting part of today was cutting and gluing the o-rings for the
square openings for the door and base-plate. Cutting the o-rings was simpler
than I had imagined, I used a wood jig, razor blade, and the trusty hammer to
make all the 45 degree cuts.

Next I threw a jig together to hold the two pieces together while I super-glued
the joints. BTW, super glue and N-Buna o-ring material is simply amazing! The
glued joint is just a flexible as the stock material and when destructively
tested, the glue doesn't fail, but instead the o-ring material eventually tears
away in "chunks".

Once the O-rings were installed, I hoisted the 400lb beast up and rolled my
portable vacuum station underneath the chamber, and gently lowered the chamber
down.

After running the rough pump for a few minutes I heard that distinct "whistling"
sound of air leaks, and "Maybe" found an interesting way to pin-point the
failure. I sprayed white spray paint near the suspect areas. This did two
things, first the paint was sucked thru the leak leaving a tell tail white paint
mark inside the chamber for easy repairs, and secondly it actually sealed some
of the smaller leaks :)

Tomorrow I hope to weld shut the few leaks that were found and try again! :)
I'm also thinking that another method for finding the smallest of the leaks
might be to use some HVAC pump oil that includes fluorescent dye. The hope is
the smaller leaks will eventually pull the oil thru, and inspecting the inside
of the chamber with a UV light might reveal their location. Any thoughts on
this?

I also shot several pictures of the 'as-is' chamber and moved them over to my
web-server if anyone is interested. I put together three different file size
versions of each image, and organized them into their own web-page. Here's the
links:

(200Kb
Images)

(1Meg
Images)

(~2Meg Images)

Take Care,
James Lerch
(My telescope construction,testing, and coating site)

"Anything that can happen, will happen" -Stephen Pollock from:
"Particle Physics for Non-Physicists: A Tour of the Microcosmos"

On Friday, December 19, 2003, at 09:26 AM, Dominic-Luc Webb wrote:

My diff. pump uses a silicone fluid, specifically DC-704 (I think, I am
still waking up and my memory is less than stellar), so I assume (hah!
there
he goes again assuming!) that I don't have to worry about this...

This pricy fluid should be quite safe from my reading.

Yes, its high cost probably makes it impractical for use (or Fomblin, etc)
by most amateurs (including me!) but I was very fortunate to be given about
500cc's of it by a good friend.

On Friday, December 19, 2003, at 09:12 AM, arcstarter wrote:

Guys,
A question on how to seal NPT-type joints which will see vacuum
service (such as thermocouple gauge heads etc). Do you favor using
teflon tape, or some of the teflon paste?

Since we don't know what makes up the non-Teflon part of the paste,
I would use the tape, _if_ that's the only choice. I have read that
teflon is fairly porous and not recommended for high vacuum work - it
probably depends on what ultimate vacuum you need to achieve.

In ultra high vacuum work, threads are a real bugaboo - some people
hit them with heat after roughing down the chamber, to bake out gases,
water vapor, etc, that are hiding in the threads.

Others I know have used Teflon tape, and then put Glyptal or the like
on the outside of the threads (which does make an ultimate seal, but
you still have the "virtual leak" of all those thread channels on the
chamber side of the Glyptal.

Another guy I knew who serviced / managed all of our smaller space simulation
chambers when I worked at Martin Marietta insisted that the only way to go on
TC tube threads was some high vacuum sealing compound, probably Apiezon L or
Dow Corning High Vacuum Grease. However, I would think that a grease would
entail the risk of migration and ultimate failure of the seal.

For what its worth, my TC tubes are installed with virgin Teflon tape.
I'll post my results to the list once I get my system up. (It will be
another couple of months, there is a bunch of remodeling stuff sitting
in what is supposed to be my garage lab right now)

- Gomez

........................................................................ ....
en.gin.eer \en-ji-nir\ n 1: a mechanism for converting caffeine into designs

My diff. pump uses a silicone fluid, specifically DC-704 (I think, I am
still waking up and my memory is less than stellar), so I assume (hah!
there
he goes again assuming!) that I don't have to worry about this...

This pricy fluid should be quite safe from my reading.

Dominic

Guys,
A question on how to seal NPT-type joints which will see vacuum
service (such as thermocouple gauge heads etc). Do you favor using
teflon tape, or some of the teflon paste?

Thanks.
-Bill

On Friday, December 19, 2003, at 08:02 AM, arcstarter wrote:

Guys,
I own some 'random' diffusion pump - and as such I don't know the
proper oil charge. Inlet diameter is 3 inches, water cooled.

My question is - what will happen if the diffusion pump contains
too much oil?

I am using "Diffoil 20" from www.lesker.com - which is a low vapor
pressure hydrocarbon oil suitable for both diffusion pumps and
mechanical roughing pumps. As such I'm running the same oil in the
diffusion and the roughing pump. At $55/gal is isn't free but is
remarkable more affordable than the full synthetics!

If I have too much oil in the diffusion pump and it just gets
sucked out the foreline into the roughing pump - it sure seems like
this wouldn't matter one way or the other.

I'm thinking I'll just put in some standard charge of oil - as well
as can be guesstimated from similar pumps - and then I'll see what
happens. Can anyone provide the volumes of oil required on their
diffusion pump?

I have a Veeco VE-300 lab vacuum system, with an air-cooled, 3" Veeco
diff. pump. I believe the specified oil fill is right around 100cc's,
but I'll have to double-check the manual when I get home.

On Friday, December 19, 2003, at 03:11 AM, Dominic-Luc Webb wrote:

I didn't catch the error until the chamber was down to ~2 torr, and I was
turning on the cold trap. While I may have been exceptionally lucky, no
explosions or fires! To my utter surprise the Diff Pump still pumped the
chamber down as usual, and I didn't notice any unusual smells, like burnt oil.
Since that experience I've coated several optics, so all is well.

I don't think you were exceptionally lucky. I think explosions are
exceptional. Filthy messes and burnt oil all over the place I have
seen and smelled first hand. This was warning enough for me, especially
since I work with chemists who can rapidly emit sane physical
chemistry thinking and they too warn me of the potential worst case
scenario.

If one is concerned about this, one can even select oils according
to the physical properties that do not allow this.

My diff. pump uses a silicone fluid, specifically DC-704 (I think, I am
still waking up and my memory is less than stellar), so I assume (hah! there
he goes again assuming!) that I don't have to worry about this...

...........................................................
The problem with using bleeding edge technology is that the
blood that winds up on the floor is usually your own.

Guys,
I own some 'random' diffusion pump - and as such I don't know the
proper oil charge. Inlet diameter is 3 inches, water cooled.

My question is - what will happen if the diffusion pump contains
too much oil?

I am using "Diffoil 20" from www.lesker.com - which is a low vapor
pressure hydrocarbon oil suitable for both diffusion pumps and
mechanical roughing pumps. As such I'm running the same oil in the
diffusion and the roughing pump. At $55/gal is isn't free but is
remarkable more affordable than the full synthetics!

If I have too much oil in the diffusion pump and it just gets
sucked out the foreline into the roughing pump - it sure seems like
this wouldn't matter one way or the other.

I'm thinking I'll just put in some standard charge of oil - as well
as can be guesstimated from similar pumps - and then I'll see what
happens. Can anyone provide the volumes of oil required on their
diffusion pump?

Just last night I machined up the final adapter I need to test out
some of my gauge controllers. If all is well I'll be able to try
the diffusion pump sometime soon.

If anyone is dying for the information I could extract dimensionals
off of the pump. It would be possible to build one from scratch if
you so chose. Another great source of information on these pumps is
a book called "Scientific Foundations of Vacuum Technique", by Saul
Dushman (of GE fame). I purchased a (used) copy via www.alibris.com
for $15 or such.

Thanks!
-Bill

diff pump explosion are something that is really exeptional these
day's due to the fact that in the early day's of the VD pumps the
pumping fluid where much more prone to oxidation less stable under
air at heat ( less flamability under oxidation ),today VD oils are
much safer from accidental back flash ( accident from atm
pressurisation of pump) especially the new high end types of oils
witch are stable even after an pressurisation mistake but in the more
expensive

please correct me if i am wrong

Not at all; you hit it right on the mark! Some of us are essentially
amateurs building our own systems and this usually means looking
for lower price solutions. I would expect such people to read older
literature and deduce that there are cheaper solutions learnt long
ago, but the older literature will not tell you that the oils used
at the time were dangerous or that many years in the future there
will be safer oils (synthetics) being sold for considerably more money.

Dominic

--- In VacuumX@..., Dominic-Luc Webb <dlwebb@c...> wrote:

I didn't catch the error until the chamber was down to ~2 torr,

and I was

turning on the cold trap. While I may have been exceptionally

lucky, no

explosions or fires! To my utter surprise the Diff Pump still

pumped the

chamber down as usual, and I didn't notice any unusual smells,

like burnt oil.

Since that experience I've coated several optics, so all is well.

I don't think you were exceptionally lucky. I think explosions are
exceptional. Filthy messes and burnt oil all over the place I have
seen and smelled first hand. This was warning enough for me,

especially

since I work with chemists who can rapidly emit sane physical
chemistry thinking and they too warn me of the potential worst case
scenario.

If one is concerned about this, one can even select oils according
to the physical properties that do not allow this.

Dominic

diff pump explosion are something that is really exeptional these
day's due to the fact that in the early day's of the VD pumps the
pumping fluid where much more prone to oxidation less stable under
air at heat ( less flamability under oxidation ),today VD oils are
much safer from accidental back flash ( accident from atm
pressurisation of pump) especially the new high end types of oils
witch are stable even after an pressurisation mistake but in the more
expensive

please correct me if i am wrong

I didn't catch the error until the chamber was down to ~2 torr, and I was
turning on the cold trap. While I may have been exceptionally lucky, no
explosions or fires! To my utter surprise the Diff Pump still pumped the
chamber down as usual, and I didn't notice any unusual smells, like burnt oil.
Since that experience I've coated several optics, so all is well.

I don't think you were exceptionally lucky. I think explosions are
exceptional. Filthy messes and burnt oil all over the place I have
seen and smelled first hand. This was warning enough for me, especially
since I work with chemists who can rapidly emit sane physical
chemistry thinking and they too warn me of the potential worst case
scenario.

If one is concerned about this, one can even select oils according
to the physical properties that do not allow this.

Dominic

Greetings all,

I was recently working on my Ion vacuum gauge which entailed evacuating and
venting the chamber several times in a row. After about the 3rd evacuation, I
was getting a little too hasty and inadvertently left the Diff Pump foreline ->
rough pump valve open while at the same time opening the Chamber -> rough pump
valve, while the chamber was at room pressure!

I didn't catch the error until the chamber was down to ~2 torr, and I was
turning on the cold trap. While I may have been exceptionally lucky, no
explosions or fires! To my utter surprise the Diff Pump still pumped the
chamber down as usual, and I didn't notice any unusual smells, like burnt oil.
Since that experience I've coated several optics, so all is well.

On my to-do list is to add a feature to some how prevent opening the Chamber
rough out valve while the diff pump foreline valve is energized. One idea is
similar to how I quickly learned to turn off the high voltage plasma supply
before venting the chamber.

It only took one occasion to permanently educate me on checking the plasma
switch state. The education came about as the result of the chamber loosing
ground as the base-plate o-ring expands, which then leaves the chamber with a
~10KV potential. Since the chamber vent valve handle is attached to the
chamber, I was made rapidly aware of my mistake, and have since never repeated
it! :)

However, attaching a 110VAC wire to the chamber rough out valve handle may not
be a prudent idea, I am certain I would ALWAYS check the state of the diff pump
fore line valve before opening the chamber valve :)

BTW, the new 24" box coater made it to the shop last night, still have a lot of
work to do, but I hope to have initial leak testing done in the next few weeks!
;)

Take Care,
James Lerch
(My telescope construction,testing, and coating site)

"Anything that can happen, will happen" -Stephen Pollock from:
"Particle Physics for Non-Physicists: A Tour of the Microcosmos"

Varian states this explicitly in their general info about diffusion pumps.
I'd have to hunt for the web page, but I found it in a general search
for info about ultra-high vacuum. Let me know if you have trouble to
find this or more detailed references.

Alternatively, basic physical chemistry leads the way. Trust this! In
absence of knowledge of errors, believe the handbooks that tell you at
such and such temp+pressure this is point of spontaneous combustion. If
you want to prove them wrong, be forewarned; more than just a few others
who have tried are now dead, so careful what you wish for.... IMHO.

:0

You can more easily contact manufacturers of pump oils and ask the
technical representatives. They generally know this. You will find
they have a selection of oils and explosions are key in their thinking
process as to specific choice of oil(s). This is in fact one reason
some manufacturers are hesitant to get involved with amateurs building
their own systems.

I would have to think a little on the total explosive force of a
completely filled pump, but typically it is not so terribly much.
On the other hand, if it happens to you, the fun is over and you'll
have a rather expensive junk heap on your hands.

A more likely scenario is that the oil burns up all over the place
and you will have a very unpleasant cleanup job. This also disrupts
the fun.

Both explosions and burns are readily preventable and this was actually
my point.

Dominic-Luc Webb

On Thursday, December 18, 2003, at 09:10 AM, Dominic-Luc Webb wrote:

My only caution is to be aware that diffusion pumps operate by
heating oil in a vacuum. If suddenly exposed to air, many of
these oils hit the spontaneous ignition point and explode. Some
degree of understanding and planning will be required to make
sure the diffusion oil is protected from higher pressures when
hot, as for instance in the event of an unexpected leak.

I've read and heard much about damaging the oil itself (oxidation)
due to exposing it at elevated temp to air, but I've never heard of
one exploding, although I'm not saying it couldn't happen. Do
you have any citations or references that mention this?

My only caution is to be aware that diffusion pumps operate by
heating oil in a vacuum. If suddenly exposed to air, many of
these oils hit the spontaneous ignition point and explode. Some
degree of understanding and planning will be required to make
sure the diffusion oil is protected from higher pressures when
hot, as for instance in the event of an unexpected leak.

Dominic-Luc Webb

I'm trying to build some vacuum stuff for physics demonstrations at
our little college, and for my own amusement/research. We have a
collection of superannuated mechanical pumps that have been donated
over the years: Welch 1400, 1402, 8905, and a chemistry lab full of
water aspiration pumps. Turns out that we also have a McLeod gauge,
which I don't quite know how to use, plus a big mercury manometer
which I rebuilt.

We also have a splendid collection of chemistry glassware and have
recently discovered in the rubble a small, ancient diffusion pump
that's suffered a bit of misfortune but which might be useable.

My one vacuum trick thus far has involved electrical discharges
through a not-so-swell vacuum in a three-necked round-bottom flask.
To make electrodes, I pushed nichrome wires through through rubber
stoppers and let the wires dangle into the flask. Hooked a 5kV
spark transformer across said electrodes, turned it on, and started
pumping down the flask with one of the Welch mechanical pumps. The
discharge spreads out on the wires and gradually turns into a purple
glow between them.

This impresses the heck out of the tourists, but it's about as far
as I've been able to go. I have grave doubts about vacuum
measurement. I've apparently been reading the mercury manometer
incorrectly: the difference in height between the two columns is
about 20mm, which indicates a fairly lousy vacuum. Yet one fellow
says that if I'm getting the sort of glow that I'm getting, my
vacuum must be down in the 1mm range.

I'd like to be able to evacuate the flask better so as to get some
more interesting effects; i.e., 'blackout.' It would also be nice to
demonstrate the 'flashing' of metal vapor over surfaces.

I suppose that the ultimate goal is to combine the vacuum work with
some glassblowing work and produce, say, a primitive cathode-ray
tube. Maybe a radiometer.

So I clearly need help.

When I did a search for Yahoo groups that dealt with 'vacuum,' I
found that vacuum pumps are sometimes used for unusual, non-
industrial purposes.

Mark Kinsler
512 E Mulberry St
Lancaster, OH 43130

On Tuesday, December 16, 2003, at 11:22 AM, Dominic-Luc Webb wrote:

On the continuing saga of the electrode-through-chamber story...

I now secured some green TIG welding rods and see that this is
the material for my first aluminizing attempt.


For feed-throughs I plan to use glass as insulator with a hole drilled
through it for a conductive rod to pass through, using o-rings to
seal the vacuum. There is a decent assortment of sizes, but I have
no idea about best material. The shops here have nitrile rubber. Is
this OK material, or someone know this will not work (outgassing, etc)?

I seem to recall reading that Viton was the ultimate, and not too expensive
from places like Rocket Seals et al.