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  #16  
Old 10-02-2014, 12:21 PM
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  #17  
Old 10-02-2014, 01:31 PM
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That could be an issue. "Galvanized steel is suitable for high-temperature applications of up to 392 ░F (200 ░C). The use of galvanized steel at temperatures above this will result in peeling of the zinc at the inter metallic layer."
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  #18  
Old 10-02-2014, 03:52 PM
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Oops. I should have checked that. I assume that means that the steel is still structurally OK, just that the galvanized coating will separate. If that is true there isn't a problem. It will just look bad and maybe rust.
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Old 10-02-2014, 07:13 PM
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Zinc oxide - Iron oxide - not a problem ? Stuff flaking off into an environment
where extreme steps are being taken to keep from poisoning a new cathode....
Not a problem....?
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  #20  
Old 10-03-2014, 05:31 AM
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Remember that the CRT is completely sealed when it goes in the oven. The only opening is the stem, which goes into the diffusion pump and is sealed with an O ring. Any contaminants that are in the oven never get into the tube, where the gun is.

I don't think the nickel coating, or potential rust, will be a problem.
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  #21  
Old 10-03-2014, 08:28 AM
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OK,

I am sure it will smell lovely during the first few times of operation

I like that temperature controller, I would personally also opt for the 25A solid state relay since the internal relay is only a 3A.

Just my $0.02

Matt
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  #22  
Old 10-03-2014, 09:18 AM
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Quote:
Originally Posted by mpatoray View Post
OK,

I am sure it will smell lovely during the first few times of operation

I like that temperature controller, I would personally also opt for the 25A solid state relay since the internal relay is only a 3A.

Just my $0.02

Matt

Bob beat me to the punch on the Zinc issue...(Actually I went to bed after posting.) But he was thinking along the same lines.

Just a heads up, you do NOT want to inhale zinc fumes. https://en.wikipedia.org/wiki/Metal_fume_fever

Mild or Stainless. The choice depends on if you can live with rust and/or have the budget for stainless...

Your target temperature is just high enough that you wouldn't catch me in the same enclosed space without an apparatus for breathing.


Squirrel Boy also helpfully points out that anything hot and galvanized around virgin cathodes is a very bad idea. The vapor *will* travel, and produce fine granules of zinc, which *will* get into/onto everything. If you want a practical demo, just break open a bottle of laser toner in the rebuild lab. You'll be finding it everywhere for MONTHS afterwards no matter how carefully you think you managed to clean it up.

Last edited by NoPegs; 10-03-2014 at 01:05 PM. Reason: Augh, names are hard!
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  #23  
Old 10-03-2014, 10:04 AM
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I was not going to say this, but I do believe that at that temperature
zinc would be a bad idea. Its probably not a health risk, but would be at
100 degrees hotter.

I would break down and change to 300 series stainless steel. For
a system that small it does not have to be thick.

This is advice from a person who has done tons (literally!)
of vacuum stuff involving electronics and heat.
At that temperature even stainless will be strongly oxidized and
turn brown, but it will not flake.

In many cases of even slightly larger ovens than yours brick
is normally used, but since you already have it, go to stainless.


Doug McDonald
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  #24  
Old 10-03-2014, 04:05 PM
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I asked a friend of mine at MIT who works with high vacuum systems as part of his research on nuclear fusion. He strongly agrees with the prevailing opinion that zinc in the rebuilding room is a bad idea:

Quote:
Well, it looks like the zinc itself is not vacuum-facing, just in the oven surrounding the tube which is being pumped on. The bigger issue is what was already raised in the thread — the zinc melting off and potentially putting nasty vapor in the air and otherwise putting zinc flakes everywhere.

That being said, zinc inside the vacuum is such a bad idea even Wikipedia has a comment on it:
http://en.wikipedia.org/wiki/Materia..._use_in_vacuum
Notably, the zinc will almost certainly sublimate at your temperatures/pressures and can leave conductive deposits in places you probably don’t want them.

See also NASA’s regulation on zinc in vacuum for a more technical/authoritative source:
http://nepp.nasa.gov/npsl/Prohibited...rohibition.htm
Also prohibited are cadmium and (pure) tin:
http://nepp.nasa.gov/npsl/Prohibited/index.htm

This alone may be sufficient reason to keep the zinc flake from being produced in an environment where you are working on tubes — even if it weren’t direct threat in the oven itself, you probably don’t want it floating around your nominally clean workspace and getting into other tubes.
I am happy to chip in to help pay for the cost of stainless and strongly suggest that we should go ahead and replace the galvanized.

-Matthew
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  #25  
Old 10-03-2014, 04:39 PM
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Thanks for your comments. I think I'll fire it up and run it for several hours (It is in a large warehouse - I'll do it when there are no people in the room and have good ventilation) and see what happens. If there appears to be an issue I'll go the stainless route.

Matt, I've ordered a 40 amp solid state relay and heat sink. The heating element is 8400 watts.
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  #26  
Old 10-03-2014, 06:47 PM
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You know the zinc doesn't just flake off nice and neat, it powders off and it does make
extra stinky nasty gases..... Why risk getting that crap all over the building....?
That little fan in there, you gunna run that on yer test....?

And no laser printers in that building either......
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  #27  
Old 10-03-2014, 10:09 PM
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Quote:
Originally Posted by Steve McVoy View Post
Thanks for your comments. I think I'll fire it up and run it for several hours (It is in a large warehouse - I'll do it when there are no people in the room and have good ventilation) and see what happens. If there appears to be an issue I'll go the stainless route.

Matt, I've ordered a 40 amp solid state relay and heat sink. The heating element is 8400 watts.
Please, think about things before proceeding. Getting zinc everywhere is not a good idea. It could damage things in the museum. (Do you want metal powder settling on open-bobbin flybacks?) Not to mention the whole "Well we have a 30% failure rate on rebuilds after 100 hours of use and we can't figure out why?" issue of poisoned cathodes that awaits.

If you're not willing to take 7jp4-guy's offer of financial assistance to change up to stainless, at least consider buying a $16 pail of refractory cement at the hardware store and 'spackling' the entire exposed galvanized inside with a thin coating to minimize the zinc exodus. As a bonus this will add a bit more thermal insulation to the whole apparatus, plus because it is white it will reflect more heat back into the oven than metal would and thus increase systemic efficiency, which lowers your cost per tube.


Yes, I'm sure there were many one-man rebuild operations run out of terrifyingly dirty garages and basements. I'm also sure that they produced working rebuilds. What nobody is sure of is what the lifespan of these mom-n-pop back-alley rebuilds was compared to say an NVC rebuild. If people are going to pay you to rebuild a tube, I think they're hoping (like me) to get a rebuild that exceeds all original specs, because we have 60+ years of knowledge, tech, and tools to do the job.


I've refrained from posting the following observation/thought because I wanted to wait for more news from ATF on which way they were leaning toward, but it kind of goes hand-in-hand with the current topic of discussion.

Regarding re-phosphoring or just re-gunning.

If I'm budgeting to have a rare or significant tube rebuilt, much like a car there's things that you just do while you have it torn down because it is cheaper and easier to replace parts that you have easy access to. So if I'm going to have a tube refreshed, I'd feel better about stripping the jug and re-phosphoring and aluminizing. There will come a point where bent-guns are no longer an option, and if you can't aluminize a tube, then all it will be good for once re-gunned is short demos in a dark room or you're just going to wind up with a big goose-egg ion burn up front, and then it's really pooched because there's nobody to re-phosphor it, even if the gun is good for another 8k hours.


The solution to all of this is to determine the haz-mat disposal fee related to the wet-process of re-phosphoring a tube, and declare that up-front to the customers in the same way that you have to pay a haz-mat fee to dispose of tires and automotive fluids. The actual bench process can (and should!) be heavily revised to significantly reduce the volume of haz-mat produced at the increase of labor and/or in non-hazardous alternative consumables.


Specifically, I'm looking at the HFA based stripping process. You can strip the phosphors off with a weaker acid and some mechanical scouring action. (Put the tube on a rotary-shaker table with some water+acid/base and soda-lime marbles or even just a cup of lead-free birdshot for a few hours.) This way you don't produce many many gallons of water+HFA+barium/lanthanum/etc haz-mat like RACS did. You will still produce, say 1.5 gallons of non HFA haz-mat containing just the phosphors and weak acid/base, and then you do the final rinse/clean of the inside with the HFA process only once for the desired surface prep, using significantly less than a quart of acid solution, and probably being able to re-use the same acid two to five more times before it becomes exhausted since it isn't eating up ALL the phosphor and aluminum, it's just putting the final polish on.

We have now potentially reduced the haz-mat disposal requirements by an order of magnitude or more in volume, but we've added some extra processing time via the shaker-table. I'd wager the extra time is cheaper than the bulk haz-mat disposal rate, though.


Hell, I'm pretty sure that 24 ounces of bismuth bird-shot, an inch of warm water, and a pint or maybe 20 fl oz of regular old lye on a shaker-table will strip the face of all aluminum and the phosphor in 60-90 minutes. You'll have to put it on a tilt lathe or roller table to de-aluminize the rest of the bell, but still entirely feasible. Even if someone had to go in and 'bottle-brush' the neck and bell, lye will eat that aluminum right up. (Yes, I know, fumes. I'm pretty sure you're going to have to install a fume hood at some point to comply with OSHA, but that's an infrastructure cost over time, not a consumables cost.) On the up-side, the effluent produced is low-grade hazmat, only because of the phosphor compounds. The lye is just "caustic" and you can neutralize that on-site rather cheaply. I'm also fairly sure that there's a non-hazardous way to strip the dag if that somehow survives the caustic assault.

Again, instead of washing it all off with copious HFA and H2O, you can use something slightly more tame than HFA with a bit of mechanical assistance to do the real dirty work, and only unleash the HFA in a smaller dose to achieve the final ultra-clean surface that is required in re-phosphoring the face.


Go find an undergrad student majoring in inorganic-chemistry at your local edifice of higher education and ask how he(or she!) would solve the problem of cleaning glassware that has phosphor and a few microns of aluminum stuck to it. (CRTs are just very large erlenmeyers when it comes to "I need to clean this filthy thing extremely thoroughly.") Actually, even an org-chem major can provide workable solutions. They have all kinds of fun gloop stuck to their apparatus that they need removed entirely, too! They also probably know more about exactly what waste products are and are not classified as haz-mat these days and can advise the best way to minimize reportable waste while still achieving the necessary final result. They're also happy to be paid in beer, food, or fuel, if not for free just because someone finally asked them to help solve a real world problem with their knowledge even though they don't yet have that shiny degree on the wall. Grads want a budget and a notch on their resume, while post-grad students can only afford to consider problems that have grant-money attached to them. Undergrads aren't fully jaded like that, yet...


You folks at the ETF have come a long way towards the goal of rebuilding CRTs again, but don't scuttle the whole project with an avoidable mistake or by limiting your potential at only providing palliative care with a new gun shining onto old phosphor. If I have a tube that went to air in 1968 because the socket came loose, I really do not trust the phosphor coating to behave properly no matter how much you bake it out. If it was aluminized then it will certainly require the full service process to obtain an acceptable result. In 44 years with access to the atmosphere, those few microns of aluminum have become mostly aluminum oxide with just a tiny bit of actual aluminum underneath. Remember that AlO2 is an electrical insulator, and that alone is enough to result in an inferior finished product. I'd also put good odds on the AlO2 flaking off the face/bell and falling into the gun assembly during pump-down and bake-out, much to the detriment of the finished product from that side of things.
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  #28  
Old 10-04-2014, 05:55 AM
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Thanks everyone for your comments. I will replaced the galvanized metal to avoid the zinc issue.

As for re-phospohoring and aluminzing, we have the equipment to do that coming from RACS later this year. We also have a person who visited RACS for two weeks and learned the process. At some point we will deal with the issues of getting this equipment converted to operate on US power standards and with the environmental issues.

The vast majority of the dud tubes out there are still under vacuum, so the existing phosphor is most likely OK.

Bent guns are not available now - we plan to replace the heater and cathode in existing guns.

Last edited by Steve McVoy; 10-04-2014 at 06:03 AM.
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  #29  
Old 10-04-2014, 11:17 AM
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Well, I think that Steve is working very hard to get a really terrific thing off the ground.
I'm beginning to think that each step, and every little item may need to be looked at by
more people just so something can be avoided like short life tubes after a rebuild. Like
NoPegs said above. Not that Steve and the people at the museum are not doing the best.
But more along the lines, that an overlooked item may mean a hundred tubes will
possibly have a short life due to something no one realized.

I also think that as for the bent guns not being available, the best solution I think is to
rebuild the bent gun, and not mess with a good screen. Re-coating the screen should
only be done after rebuilding bent guns is tried and failed repeatedly.

It's just my thoughts that disturbing something that you are not sure you can replace
with long term reliability should not be jumped on quickly and without long term testing
first.
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  #30  
Old 10-04-2014, 11:39 AM
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Username1, thanks for you comment. We are not doing this project without a lot of expert help. Several of our members have participated in rebuilding tubes at Hawkeye and RACS. One of our members spent two weeks in France at RACS learning the process. Scotty at Hawkeye and Phillipe at RACS are available to us to help when we need them. Nothing we are doing is new - guns have been rebuilt before, and the process of rebuilding tubes is well understood and documented. I don't think we will have a problem with tubes having short lives.
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