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----- Original Message -----
From: "Ken Hunter" <atm_ken_hunter@...>
To: <VacuumX@...>
Sent: Wednesday, September 17, 2003 12:17
Subject: [VacuumX] plans for vacuum system

Hi Gang...

James Lerch has posted to another forum (atm@...) a
description of how and why he made his system. I recommend that you
read it and see if this information is applicable to your use.

James... I am requesting that you also post the info here or give me
permission to copy it to the VacuumX GROUP for our use here also. I
think it is very appropriate as a basic vacuum system description.

Thanks,

Hi Ken,

Here a copy of the post, I had meant to putting it over here as well, just
life's been a touch busy.

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

This is contrary to what most others on the list have warned me
about. Getting the very low pressures needs special pumps, not
just roughing pumps, and leak detectors. These are not cheap.
But if someone (maybe you Bob) has suggestions how to set up
one of these without spending 15-25 thousand dollars, I am
interested.

Hi Dominic,

I scrounged my 11" aluminizing system together for less than $800us in parts
(the labor and experience were / are priceless :)

You will need more than just a roughing pump, as a high vacuum pump is a must,
but a 'cheap' diffusion pump will do the job nicely. Towards the bottom of this
post I'll place some references to possible parts supplies, and a 'rough' guess
at what it would take for me to start-over again..

To avoid this being one of those "look at what did" posts (that are frowned
upon for some reason), I'll first include the list of things I'd do differently.

#1 Electrical Pass throughs / Glow discharge cleaning:
Automotive Spark Plugs make terrible electrical pass throughs. A simple
piece of 1/4" plate glass, siliconed to the tank, with a hole for a bolt and an
O-ring, work much better. Once you get the High Voltage (from neon sign
transformer) inside the tank, attach it to a simple aluminum wire and use it as
an antenna (or "glow bar" as its called in the trade) for the purpose of "glow
discharge cleaning" of the optic.

#2 Pressure measurement
While unconventional, a simple (and cheap) thermocouple vacuum gauge will
suffice as the *only* pressure measurement tool. While the thermocouple gauge
won't give you a value for your ultimate pressure, it will tell you when it is
NOT ok to evaporate. TC gauges are good to about 10-3 torr, so if the gauge
reads more than its lower limit, your pressure is too high.

BTW, my TC gauge is NOT calibrated (nor do I find this to be a problem!) I
have a mark on the gauge face that tells me where my mechanical rough pump
bottoms out at (about a 1/3 of the needle travel on the display gauge). Once
the mech pump is done, I switch over to the diffusion pump and the gauge rapidly
heads towards my "zero" mark (which is about an 1/8 of the needle travel on the
display gauge). Once the TC gauge gets to my "zero" mark and stays there, I've
never had a problem evaporating aluminium.

BTW, if you can get a TC sensor, you can build the display gauge for it!
see:

Another trick I use to tell when its time to evaporate (when my penning
gauge is acting up, and your mileage may vary) is when the "glow" from my glow
discharge cleaning, flickers out.

Finally, you can use the TC gauge and a bottle of automotive carburetor
spray cleaner to find leaks. Pump the chamber down as low as it will go. Spray
suspect areas with carburetor cleaner in small squirts. When you get close to a
leak, the TC gauge will JUMP. Large leaks need to be mechanically fixed, Medium
sized leaks can be plugged with Silicone, Small leaks can be plugged with
"Vacuum Leak Sealant" (looks like small bottle of clear finger nail polish) see:
If you have a REALLY small leak and its inside the
chamber somewhere, coat the outside of the chamber with pump oil, pull your best
vacuum, evaporate some aluminum, leaks will be revealed by black stains where
the aluminum reacted with the oil

#3 Vacuum Mythologies I'd like to dismiss:
First, let me start by saying the below mentioned myths are probably TRUE,
but only when a vacuum system is operated at pressures below what we need for
evaporating aluminum (IE less than equal to 10-6 torr). Your mileage may vary
if your doing Di-Electric coatings, or have a really large chamber and a small
diffusion pump which can't keep up with the out-gassing.

A) "A single finger print will ruin your vacuum" Bullocks! I have 6
tungsten filaments and 6 bits of aluminum I install with often SWEATY fingers
and I've never worn gloves. I do wash my hands before installation to remove
most of the oils, but sweat from my hands and arms bumping into the chamber
while loading it are all too common. Clean up is done by simply wiping the
obvious stuff up with a paper towel, blow drying with an electric heat gun, and
"dusting" with O2 from my oxy-acetylene torch.

B) "You must use expensive Viton O-Rings and Stainless Steel Chambers"
FALSE! I used Buna-N o-rings for the base-plate seal, and "Home-Depot Plumbing
Supply" o-rings in the electrical pass-troughs, and a rusty propane tank for a
chamber (yes it had rust on the inside, which I lightly sanded to get rid of the
really bad parts, but rust STILL exists on internal parts of my chamber and no
problems as it all gets overcoated in evaporated aluminum, and is now nice and
shiny rust :)

C) "You must use expensive synthetic diffusion pump oils, and purpose built
roughing pumps" NOT! If your wondering what expensive is, try $500us for 500ml
bottle of synthetic oil! (ouch!!!) I use 'cheap' $60us / gallon Invoil-20
diffusion pump oil in both my diffusion pump and my 'roughing pump' and I have a
LOT left over! See: for reference. Oh, my roughing
pump is a used HVAC service pump, that was locked up solid when I got it. I
took it apart, cleaned out the goo that used to be the pump oil, put it back
together and it works fine! For reference I'm using a Dayton model 4Z577 1/3hp
refrigeration vacuum pump, and it will pull my 12" diameter, 11" tall chamber
down in less than 10 minutes and has no problem backing the diffusion pump.

D) "A liquid nitrogen cold trap is a must" No Sir! First, the Mt. Wilson
observatory coated their 100" optic for many years with out a cold trap. They
now use refrigerated Glycol and a copper chevron baffle as their cold trap. See:
I use a loop of copper coil with R-22 freon at
about -50C and an old refrigeration pump. However I've coated a few mirrors
without using the freon based cold trap (freon leak, now fixed). I believe its
possible to build a simple coating system with out a cold trap, instead use
baffles to prevent a "line of sight" between the diffusion pump opening and the
optical surface of the mirror and the evaporative source. For instance the
mirrors I coated w/o the cold trap all had a slight coating of oil on the
backside of the mirror (VERY SLIGHT). The optical side of the mirror came out
just fine!

E) No myth here, but I found it interesting that PVC irrigation pipe can be
used for the rough pump plumbing lines. While I haven't done this, I would
consider it when its time to build a new coating system. See:
for my reference on the above suggestion. BTW the
reference is an EXCELLENT paper on how the monolithic MMT mirror was coated :)

Now, here's a parts list for a "no-frills" coating system. This system is going
to be on the slow side as it doesn't have a provision to "Rough Out" the chamber
and then switch over to the diffusion pump for the final vacuum. Instead, we
pull a rough vacuum in the chamber THROUGH the diffusion pump, once the chamber
is down to the rough vacuum, we turn on the heaters for the diffusion pump and
wait for it to warm up. Once the diff pump is up to operating temp, the
pressure will drop rapidly in the chamber and we evaporate our aluminum. Once
the aluminum is evaporated, we close the large valve between the tank and the
Diff pump, and vent the chamber revealing our coated optic.

Word of advice, "Never expose Hot Diffusion Pump Oil to atmospheric
pressure!!!!!!" It makes a terrible mess, been there done that during an ill
timed power outage!

Parts:

#1 Rough Pump: Ebay, $150.00

#2 Diff Pump with valves:
A) small system, Ebay, $200.00
B) large system, Ebay, $500.00

#3 Thermocouple Gauge w/ tube Ebay, $25

#4 Diff / Rough pump oil, quart $17

#5 High Voltage transformer Ebay, $10

#6 Tungsten Wire .032", smallparts.com, $14

#7 Ceramic Standoffs, smallparts.com $36

#8 Transformer filament power supply,

A $50 smart ups 1250 on ebay that's minus the batteries. Buy it,
salvage the
big old transformer out of it, and power it off a 600watt 120vac dimmer switch
from hardware store..

#9 20lb propane tank, new, *empty* $25 hardware store

#10 1/2" sheet steel, 14" x 48", salvage, ~$50???

#11 O-ring, 0.25" thick, 12" ID, $9

#12 Two part silicone adhesive / sealant, $38,

#13 Misc: wood to build cart, glass for windows and electrical pass thrus,
bolts, Filament holders, Tank vent valve (Hose Bib, brass), electrical switches
and housings.

Tools:

Oxy/Acetylene cutting - brazing torch and supplies
Angle Grinder
Hand Tools

Assembly,

#1 Cut 14" square piece of the 1/2" sheet steel to use as base-plate. Cut
center hole to match diameter of Diff Pump opening. Drill holes to match diff
pump bolt pattern. Sand off rust, and coat with diff pump oil to prevent rust.
Place O-ring on Diff pump opening, use silicone sealant on diff pump to base
plate bolts. Bolt base plate to diff pump.

#2 Build wood cart to support base plate and diff pump assembly, leaving room
for a work platform, and rough pump support. Also good time to run water
cooling lines to diff pump, if water cooled.

#3 Install rough pump, and run a Copper / PVC plumbing line between rough pump
and Diff pump outlet.

#4 Remove valve from propane tank. Cut bottom off the propane tank with angle
grinder or cutting torch (it was a new empty, never been filled tank, yes?).
Precision not needed, but try to make a straight cut...

#5 Cut out a 12" ID, 13" OD steel ring from remaining sheet steel with cutting
torch. Braze this ring onto bottom of propane tank, leaving 1/4", or slightly
more, of the tank wall sticking down past steel ring.

#6 Grind the propane tank portion that sticks past steel ring down to ~0.2"
relative to bottom of the steel ring. This is where base-plate to tank O-Ring
goes..

#7 Install brass T where the old valve of the propane tank went. One side of
the T goes to Thermocouple Gauge tube, other side goes to tank vent valve (brass
hose bib)

#8 Cut 2x 1" holes in top of tank, braze 1" ID steel fender washers to opening
as re-enforcement. Cut 3" square piece of 1/4" (or thicker) plate glass. Drill
1/4" ID hole in center with ceramic tile drill bit. Silicone these to the holes
in the top of the tank. After silicone cures, slip a 1/4-20 bolt, with on o-ring
installed, thru hole, and hand tighten nut.

#9 Build 6 point filament system using ceramic stand-offs around perimeter of
tank near the top. Wire one side of filaments to tank ground, other side to 1
of the two electrical feed-thrus at the top of the tank.

#10 Install aluminum 'antenna' for glow discharge cleaning. Do Not Ground this!
Connect antenna to the remaining electrical pass thru.

#11 Install HV neon sign transformer, use only one side of the HV output and
connect to Glow Discharge antenna.

#12 Install low voltage / high current transformer, connect one side of output
to tank ground, other side to filament pass-thru.

#13 Install thermo couple gauge display

#14 Assemble and leak test, play with glow discharge system, evaporate some
aluminum onto a test piece of glass.

BTW, in above styled system, the optic will be coated face up (the back of the
optic used as a baffle for the diff pump) Every now and then (if you put too
much Al on the filaments) you will get a drip of molten AL. Its important to
make sure your filaments are as close to the tank walls as possible, and the
optic is centered in the chamber. If a drip should happen it will safely land
on the base plate BESIDE the optic, not ON the optic!! (really, really
important!!!)

So, there ya go, an 11" diameter coating system for ~$800us and a ton of labor.
If you want a larger system, buy the $500 diff pump, and build a large BOX
chamber out of heavy sheet steel. (I have one in progress that will coat upto
24" diameter mirrors. The box is being built from 1/2" sheet steel, and has
inside dimensions of 25"x25"x11", I'm pretty sure it'll work, but the first time
I pull a vacuum, I'll be standing back a fairly good distance!)

Best of luck
James Lerch