Re: ATM Re: [VacuumX] 24" vacuum chamber is getting closer :)

["James Lerch" <jlerch1@tampabay.rr.com>]

----- Original Message ----- 
From: "Dominic-Luc Webb" <dlwebb@canit.se>


>
> 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..)

Hi Dominic,

> I note for ultrahigh vacuum copper seals are used. Just a thought.

Very true!  Unfortunately I can barely manage the fabrication of a simple O-Ring
implementation, the machining for copper seals is well beyond my grasp at this
time ;)

> Also,
> you cannot mean 18 Torr. I presume you mean something like 10e-3 Torr,
> which is characteristic for a common roughing pump.

No, it only made it to 18 Torr last night :(

However,that was with three "whistling", almost plugged with spray paint, pin
holes in the chamber weldments...

Perhaps I should have waited to post until I welded the pin holes shut and
achieved a proper vacuum, but honestly last night was pretty exciting and I felt
the need to share :)

> I would be interested in your solution for firing up the tungsten
> filaments, presumably you used (green) TIG welding electrodes.

For the 12" chamber I have been using 0.032" tungsten wire from smallparts.com.
I plan a similar arrangement for the 24" chamber, but with 3 times as many
filaments for a total of 18 in two concetric rings.

> 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).

Have you calculated the wattage used on your light dimmer, or is the dimmer just
used as a signal source?  I'm using 6 filaments, each 40 mm length, in a series
/ parallel arangment, and consuming about 500 watts of power at a few volts and
several hundred Amps.  The dimmer switch is rated at 600 watts so I am pushing
it a little...

The 24" chamber will use the same filament arrangment, but in three sets.  I
plan on firing off each of the three sets individually using the same transfomer
/ Dimmer switch power source I'm using on the 12" chamber..

> 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.

You do know that the Tungsten wire is only good for One Burn, yes?  I find that
after the first burn, the tungsten becomes so brittle it is impossible to load a
second charge on it with out breaking it into several small pieces.  In
addition, because of thermal exansion of my existing filament supports, most of
the filaments "pop" during cool down several seconds after the power is turned
off..

I've read that Stranded tunsten wire may help in this area, but I haven't any
experiance with it yet (although I do have three pieces of 0.01" wire waiting to
be wound into a single piece, that is once I figure out how to wind the
miserable stuff!)

>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 find yourself having "black" coatings, try implementing a third
"Pre-Heat" option.

I recently ran into a string of failed coatings.  While I had originally thought
the problem was contamination, I now believe the problem was a result of my
rapidly heating the filaments up due to a lack of humor and the resulting
impatience that followed.

The work around for the string of failed coatings has been a more controlled
ramping up of filament tempature instead of my "frustrated, full throttle, get
it done" approach I had during my string of failures... :)

My supposition is that while loading the filaments, some contamination is
inevitable.  A pre-heat session boils off the contaminents and gives the pumps a
chance to remove the material from the chamber enviroment.  Another possibility
is the pre-heat and slow ramp up allows for an opportunity for the removal of
residual H20 that clings to the surfaces inside the chamber.

In any event, while not an expert, I do know that gentle application of filament
power produces a nice coating.  A rapid "get it done" application of filament
power results in a non-reflective flat black coating :0

> 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.

If I understand, your basicly building a solid state switching  power supply to
go from mains power to low-voltage high current, yes?  If you have a schematic
handy, I'd love to see it!

James