Kiki,
I've put some notes into your
text.
Roger
Processing neon
tubing
After the glass is bent,
and electrodes are attached, you want to fill the tube with the gas
that lights up. How is that done?
We are filling the tube with
very pure gas. Obviously we want the tube to be very clean.
The tube walls have dust, some contaminates, and moisture.
Actually everything has a thin layer of moisture on it. The best
way to get rid of all of these contaminates, and to force the air out,
is to heat the tube. This can be done in an oven, but someone
realized they could do it faster by using a very large transformer and
overpowering the tube, which causes it to heat up. This is
called "bombarding" because you are bombarding the tube with a lot
of electricity to clean it.
The steps in processing a
neon tube:
First, the tube is hooked up
to the manifold. "Manifold" is just a fancy word for
"a bunch of tubes connected together." Once a vacuum is drawn,
the tube must be kept sealed, therefore everything you need to process
the tube - gasses, vacuum pump, all gauges - must be sealed
together with valves to open and close them as needed.
A vacuum is now drawn.
You need a vacuum pressure of
less than 5 microns for proper
processing of the tubes. Some manifolds with types of high vacuum pumps can get down to 1 micron and below.
It is impossible to draw a perfect vacuum, still, the longer you pump
the tube down, the better the vacuum. (No- pumps of a given type have an ultimate pressure;
they reach a limit, and time will not improve this) Since no arc can strike in a
vacuum (not really;
vacuum inhibits, but does not prevent arcing; arcing can occur because
the walls themselves break down in high voltage) , a spark tester will not show any arc, or
a very dim purple if there's a vacuum, or pink if there's air from
a leak. The spark will also be attracted to leaks because air conducts
better than glass. Be careful - the arc is hot enough to melt
a tiny hole in the glass!
Air is
introduced (actually, it
need not be introduced; it is just the residual air that was in there
all along) for
bombarding. Electricity cannot flow in a vacuum, so you must
use some sort of gas to strike the arc. A tube contaminated with
air will run hot. But this is what we want in bombarding!
So air is used during bombarding.
The tube is bombarded until it
reaches about 450F. A slip of paper is commonly used to
indicate when the tube gets to this temperature; it begins to char when laid on the glass at
thiis temperature. A
pyrometer,
thermocouple, or IR
sensor can also be used. All have their downfalls. The bombarder
is typically run at 200-250mA for this stage - keep in mind that
this is deadly!!
The electrodes are
processed. The electrodes come shipped with a protective
coating. This coating is converted to barium oxide,
(actually, it is generally a
mixed oxide, with calcium, strontium, and barium oxide
components) which is a good
electron emitter. The electrodes must be heated to a strong
cherry red to convert the coating - this is called "cherrying"
the electrodes. Cherrying is typically done with the bombarder,
which is turned up to 300mA-350mA, as part of the bombarding step, but
in oven pumping (when the tube is heated in an oven instead of
bombarding) the electrodes must be heated electrically with an
induction heater.
Vacuum is drawn again to
remove the hot impurities. Now that the tube is hot, we draw
off all of the contaminates, including contamination from the
electrode processing. It is necessary that this is done quickly
while the tube is hot.
The gas can be added when the
tube is cool to the touch. The pressure is measured in
millimeters of mercury, called "torr". Atmosphere is about
760torr. Neon tubes are filled at around 9-15torr, depending on
tube diameter. "High pressure" neon is filled around
40torr. One type
of gauge used to
measure the gas pressure is called a "manometer". It is a U
filled with oil that is calibrated to the different weight of the oil
compared to mercury. One side is vacuum, and the other is
connected to the pressure to be read. We use our vacuum from the
manifold for the vacuum side by closing one side of the manometer when
the gas pressure is about to be metered out. As the gas raises
the pressure in the manifold, it pushes down on the oil on the
manifold side, and the vacuum side moves up. The pressure is
measured by how much the oil moves up.
The tube is "tipped
off". The tabulation connection is heated so the glass melts
and seals close because the
external air pressure is greater than the vacuum pressure as the tube is removed from the
manifold.
The tube is "burned
in". There may be some trace amounts of contaminates still
in the tube. The electrodes have what's called a "getter"
that reacts and contains contaminants. A larger transformer
accelerates this process. Burning in cleans up problems
"snaking", the gas glowing the wrong color, or the tube getting
hot.
Electricity and what it's
doing in neon
Here's a quick and dirty
explanation of electricity. A good analogy for electricity is
the flow of water - much can be explained with this analogy (except
for things like magnetism, but we don't need that
here.)
Voltage is like water
pressure. High pressure requires strong hoses, and high voltage
wire has strong insulation - if the hose is too weak, it can spring
a leak, similarly, high voltage can arc out of the wire. If your
garden hose is too long or too small, you will get no pressure at the
other end. If your high voltage wires are too long, you will see
a voltage drop and your tubes may have trouble lighting. Voltage
alone cannot kill you, but can burn you and the higher the voltage the
more it hurts!
Current is like the volume of water moving. Current is
measured in amperes. High voltage with low amps is like a spray
bottle. Low voltage with high amps is like a slow river.
Current and voltage are inversely related much like sticking your
finger over a garden hose to make it squirt - lowering the flow, but
increasing the pressure. The amount of current is what will kill
you. Only about 75
milli-amperes of one amp can
stop your heart. Your
heart is most likely to stop because of a shock at about 600-1000
volts, which is enough to drive this amount of current. This
voltage is likely to cause your heart to go into fibrillations, and
you need a de-fibrillator to recover from this. Above about 1000
volts, even though you may have had more than 75 milli-amps, your
heart is most likely to stop, and then it may re-start spontaneously,
or it can be restarted by someone thumping sharply on your chest.
This is actually much better than going into fibrillations. Do
not try this at home!
Frequency is the rate at which
voltage and current reverse. Household alternating current reverses 60 times/sec,
or 60 Hz. Electronic power supplies for neon reverse at 30-60
kilo-Hz. Higher frequency can result in 'coupling' of the
electricity through the air; it is just like radio transmission.
You can even run neon with one electrode, or no directly connected
electrodes at all, at high frequency.
Alternating current is usually
used to power neon. The
transformer charges one electrode negative and one electrode positive,
then switches them, going back and forth many times a second.
Electrons stream out of the negative electrode towards the positive,
stripping the gas molecules of some electrons on the way. The
gas molecule, missing an electron or two, becomes positively charged,
and is attracted to the negative electrode, bouncing off the glass and
other molecules, and maybe running into another electron, where they
combine and give off light. Then the charge is swapped, and the
whole thing happens again in the other direction. Back and
forth, the neon tube stays lit.
This is why only core &
coil transformers can be used for burning in. They slosh the
molecules around 60 times a second. Electronic transformers are
high frequency - thousands of times a second! They switch
polarity so quickly, the molecules only vibrate in one place!
The contaminants don't make it to the electrode
getters. I doubt if this is true; electrons might
travel at some fair fraction of the speed of light, so 60 Hz and 60
kHz is not that different. I think that burning in is because a
core and coil transformer delivers much more current than the
typical high frequency units.
Neon transformers are high
voltage, low amps. You need the "pressure" to "push"
the electrons from the negatively charged electrode, through the gas,
towards the positively charged electrode, but once they go through,
they charge the gas up by dragging electrons in the gas along with
them, and this charged gas is more conductive and acts like a wire and
the electricity flows easier. Now, not much energy is needed, so
transformers are low amps - 30miliAmps is most common, but some are
60mA, making the tube brighter, and some even go down to 8mA which is
dimmer and best for indoors.
Bombarding transformers are
high voltage and high current!!
Bombarders are 7V-25V, and bombarding happens around
200mA-350mA!! Actually,
bombarders go higher than this; Christian's go to 1 amp or
so. So if you get struck,
you will be in excruciating pain for the short time you remain
alive!! STAY AWAY FROM THE BOMBARDER WHILE BOMBARDING IS GOING
ON!!! YOU COULD DIE!!!
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Roger Carr
Stanford Synchrotron Radiation Laboratory
Stanford Linear Accelerator Center
MailStop 69, Building 137, room 208
2575 Sand Hill Road
Menlo Park, CA 94025
Tel: 650-926-3965
Fax: 650-926-4100
Email: Carr@SLAC.Stanford.edu
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Notre imagination vole. Nous sommes son ombre sur la terre -
Vlad Nabokov
What are the 5 most important things you believe to be true that are
not?