Sciencemadness Discussion Board

"Draining" a fluorescent bulb (tube)

CRUSTY - 15-12-2017 at 10:25

Ugh, its been a while since I was last on here. I feel like I've missed so much. Anyways. I'm looking to build, or at least experiment with CO2 laser tubes. I recently pulled apart an old CRT TV, desoldered everything, and most importantly, got my hands on that juicy 30 kV flyback transformer. I've read about CO2 lasers being built from flyback transformers, so I figure I might as well try.

For the laser tube, I plan to (or would at least like to) use the tube from a fluorescent light with the phosphor scraped off and later chemically cleaned. That's not my concern. I have a plan to safely deal with the mercury vapor in the tube (I also have a full-face respirator with filters rated for mercury), but killing the low pressure in the tube without shattering it is the real dilemma. There isn't much data on them, but I read that fluorescent light tubes are generally at around 0.003 atm. Do any of you know a way to slowly bring a low pressure environment up to atmospheric without any access to the inside of the tube beforehand?

I was thinking maybe to epoxy some low-density neoprene onto the plastic end caps, and then drill a 1/16 inch hole through it into the tube. My reasoning here was that, in my experience, neoprene does not drill a clean hole, and tends to get pulled into the center of the hole, so any air entering would be effectively travelling through a hole smaller than what I drilled. I know this question doesn't pertain very well to this board, but it's the best I've got. Any ideas on how to do this without introducing air at a rate sufficient to shatter the tube?

Bert - 15-12-2017 at 10:51

How would you replace the ends of the tube, and with what material?

A long time ago, I tried to re purpose fluorescent tube glass. They always broke, even using a water cooled glass cutting saw in a well equipped glass shop. Just too thin.

CRUSTY - 15-12-2017 at 11:08

I found some plans online that used, to my surprise, JB Weld to adapt the tube (an empty neon sign tube in this case) to a copper pipe on either end for the electrodes. I would've thought the epoxy would degas for a while afterward, but I guess it's negligible. The creator of this laser tube had gotten at least 3 of them working. I can't remember what site I found this on at the moment.

weilawei - 15-12-2017 at 15:06

Cut inside a vacuum chamber and admit atmosphere gradually?

Apply heat to increase pressure?

Use ammonium bifluoride/alkaline solution/etc. to etch a small hole?

Use a torch and tweezers to pull a small hole?

Use various acids to dissolve the electrode metal out, leaving a small hole?

Use an electrolytic bath to dissolve out the electrodes, leaving a small hole?

[Edited on 15-12-2017 by weilawei]

wg48 - 15-12-2017 at 17:53

Under one of the caps at the end pf the florescent tube there is a small glass tube with its end sealed. It was used to evacuate the florescent tube. You can very carefully and gently grind off the seal at the end of the evacuation tube with a Dremel tool and a diamond disk or drill. Its best to stop as soon as you hear hissing and let the pressure equalise slowly.

On some tubes the florescent coating can be removed or washed off with dilute hydrochloric acid. Though on one compact florescent tube it required several hours.

I agree with Bert its difficult to cut the glass without it cracking. I assumed it was due to my wobbly diamond blade or perhaps strain from how it was manufactured.

The tubes made with leaded glass can be worked with a propane air torch but they discolour easily in even a slightly reducing flame that most Bunsen burner type torches have.

vmelkon - 19-12-2017 at 09:25

Quote: Originally posted by wg48  
Under one of the caps at the end pf the florescent tube there is a small glass tube with its end sealed. It was used to evacuate the fluorescent tube. You can very carefully and gently grind off the seal at the end of the evacuation tube with a Dremel tool and a diamond disk or drill. Its best to stop as soon as you hear hissing and let the pressure equalise slowly.


What I used to do is get my pliers and pop that tiny glass tube and air would suck in for 3 sec, because the hole is very small.
Quote: Originally posted by wg48  

On some tubes the florescent coating can be removed or washed off with dilute hydrochloric acid. Though on one compact florescent tube it required several hours.


I have never done it that way. After popping the tiny tube. I would cut the ends. Then use tissue paper and a long iron rod to push the paper through and it mechanical removes the powder coating.
Quote: Originally posted by wg48  
I agree with Bert its difficult to cut the glass without it cracking. I assumed it was due to my wobbly diamond blade or perhaps strain from how it was manufactured.


I use to melt it with a propane torch and quickly cut it with scissor to make a descent looking edge.
Yes, just using ordinary home scissors.

Sulaiman - 19-12-2017 at 10:45

I too think that the glass tube from a fluorescent lamp is too fragile,
there are many types of glass tube available, but these seem very suitable
(although a much smaller bore would give higher current density hence more intense beam,
and make reflectors much smaller/cheaper)
http://www.ebay.co.uk/itm/TMC-UV-PRO-CLEAR-QUARTZ-SLEEVE-15W...

nitro-genes - 21-12-2017 at 04:38

Something of help maybe, glass can be cut (even with siscors) when underwater. So maybe performing any cutting/grinding actions under water may help. :)

https://www.youtube.com/watch?v=usZ8MQzbglk

aga - 21-12-2017 at 06:27

Use glass measuring cylinder instead.

One of mine had a chunk broken off the top and it was quite easy to score/heat/cool then file to get a straight edge.

vmelkon - 21-12-2017 at 17:00

Quote: Originally posted by nitro-genes  
Something of help maybe, glass can be cut (even with siscors) when underwater. So maybe performing any cutting/grinding actions under water may help. :)

https://www.youtube.com/watch?v=usZ8MQzbglk


That's amazing. I can't believe it.

Reboot - 22-12-2017 at 12:08

Quote: Originally posted by vmelkon  

That's amazing. I can't believe it.


I believe it. The scissors create a fracture in the edge of the glass, then the fracture 'runs' forward as the scissors act as a wedge pushing the two sides apart. It isn't cutting the glass the way you'd cut paper; it's just cracking chunks off in a controlled enough way to get a rough shape. It wouldn't work for many shapes. The water has no effect on how it works; it's just there to keep splinters of glass from getting away.

To cut thin glass like a fluorescent tube, there's very little chance of success with any sort of saw; the vibration will shatter it. Your best chance would likely be to carefully score the glass (say by carefully etching a line all the way around the tube with a small metal file) and then cracking off one end by tapping it with something hard like a screwdriver. If done correctly, the fractures will follow the line of weakness created by the scoring.