Cutting open tiny filament bulbs - Page 1
Forum section: electronics / projects

author: ChristofferB
author_rank: Frequent Contributor
timestamp: December 23, 2018, 04:35:45 pm
content: Hi! Figured it's been a while since I've made any headway in the "DIY-gas-chromatograph" department, but i've run into a very simple solution to a complex problem:I need to mount tungsten filaments in 1/8" OD compression fittings as sensor elements. Winding 4 identical filaments, making an insulating yet gas-tight and temperature resistant mount in 1/8" space is a terrifying prospect.But fortune struck! They make tiny cylindrical filament light bulbs with 1/8" envelope! cutting the top of one of those and mounting it in a PTFE ferrule would sole ALL the problems.Here's the one:https://uk.rs-online.com/web/p/filament-indicator-lamps/6559198/Issue is, cutting the top off. What's the best approach? I'll buy a whole bag of them and experiment, but I'd like to know if someone had tried similar.I've thought of two methods: 1) score around with carbide tool, tap against sharp steel edge.2) dremel with a diamond wheel or similar, cutting it off.with both methods, the question is if i should drill a tiny hole with a suitable tool in the round end of the bulb to relieve the vacuum.Also, i'm not certain the glass bulb with top removed will survive the compression of the ferrule, but that's impossible to know before it's tried. maybe i'll cut back the ferrule so it compresses on the solid glass base.Whatcha think?Thanks for the interest and merry xmas--Chris

author: duak
author_rank: Super Contributor
timestamp: December 23, 2018, 05:15:53 pm
content: I would try to mount the bulb first, score it and then attach a small handle to the piece to remove with cyanoacrylate adhesive.An old glass blowers trick is to put a drop of water on the crack before breaking. The water molecules force themselves into the crack and help make the fractured end square. My chemistry teacher taught me this and it works. He would use a drop of saliva as it holds itself in place.Good luck, Merry Xmas & Happy New Year.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: December 23, 2018, 06:10:57 pm
content: that is a good point, getting some leverage on the offcut end of the bulb would be good. being able to snap it over like a piece of tubing would be ideal.Scoring around and pulling the two pieces straight in either direction, then putting the cut on a hot NiCr wire might also do the trick.The spit trick is indeed very good, the difference between a wet and a dry snap is unbelievable.I'm not sure how snapping a glass tube under vacuum will differ though.Thanks!

author: SeanB
author_rank: Super Contributor
timestamp: December 23, 2018, 06:16:56 pm
content: Get an industrial carbide tool insert, or a mounted industrial diamond scribe. Mount the lamps in a tube with a drop of epoxy and long leads, and fill from the rear of the tube. Cure epoxy and then superglue a small length of bamboo skewer on the end to act as a lever, and score with the carbide, then break off. You will break a number of them doing so, but regard the first few as "alignment of the process line" instead.Tip for better sensitivity is use the 28V versions of those lamps, thinner filament, faster response and they make a very good replacement for RA53 thermistors in Wein oscillator bridges if you design it to run them at around 3v across the lamp.Got a whole little box of those lamps around, but sadly they are a mixed bag, 5V and 28v versions mixed, I was just a little careful in checking each one on 3V coin cells before installing them. glow and it is 5V, no glow but still is a resistor probably a 28V version. Front panels that would variously use a 5v one, and another string in the same panel would use 28V lamps, and they were both soldered in, and buried in acrylic resin.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: December 23, 2018, 06:33:35 pm
content: That would be ideal, yeah, unfortunately the idea is that I really need the OD to be 3.1mm to crimp the ferrule around, so glueing in a tube would make them too large. Making a jig to hold them steady while rotating and having a fixed carbide scribe would really help, maybe if i temporarily shrink wrap them and then mount carefully in a drill chuck on a (very high precision) drill press, mounting the scribe sorta like a lathe tool and rotating slowly by hand while pulling at the end.I fully anticipate to break more than I make, but considering commercial tungsten TCD filaments cost around 250 usd a piece, I think i can live with it.Would the 28V versions also run at 12V? I'm really aiming for a 12V filament supply.

author: Tom45
author_rank: Frequent Contributor
timestamp: December 23, 2018, 06:55:04 pm
content: It's been over 50 years since high school, and the bulbs we used were somewhat larger, but from memory we scored around the glass with a small very fine metal file and then a sharp tap to break the glass envelope off. Probably was a triangular file.

author: james_s
author_rank: Super Contributor
timestamp: December 23, 2018, 07:05:25 pm
content: I opened up a C9 christmas light bulb by grinding the tip off with a belt sander, it was actually really easy.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 23, 2018, 07:10:11 pm
content: IMHO, scoring is a bit iffy. I'd use a thin diamond blade in a Dremel tool. You might also rethink the compression arrangement. There are good high temperature and vacuum epoxies. Make an entirely new fitting? Exposed filament bulbs can also be used to make airflow sensors.I've got a couple instruments that need replacement RA53 vacuum thermistors, so need to try the bulb trick.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: December 23, 2018, 08:57:59 pm
content: Quote from: james_s on December 23, 2018, 07:05:25 pmthat is not a bad idea either, simply grinding off the tip with some very fine grit tungsten emory paper. Sure is the slowest approach possible, though, and i'd worry about grinding debris getting stuck in the gas flow, but might be worthwhile.Quote from: Conrad Hoffman on December 23, 2018, 07:10:11 pmI agree, scoring becomes incresingly unreliable with thinner wall thickness. The glass tubing normally cut in that manner has a wall thickness of about 1mm.Dremeling off might be good too, but it induces a lot of vibrations which could harm the filament. High temp epoxies might be doable, but it's not ideal, and teflon crimp rings can be re-used with different sensors so it'd just be an order of magnitude more elegant.Thanks for the ideas! I think i'll try all of them, see what gets me furthest.

author: richard.cs
author_rank: Super Contributor
timestamp: December 23, 2018, 09:12:22 pm
content: For slightly larger lamps (mes indicators) I have found heating a spot on the envelope with a small gas flame melts a hole, doing it with a small enough flame gets just a hole with thickened edges, without the envelope collapsing onto the filament.Worth a try on these.

author: jpanhalt
author_rank: Super Contributor
timestamp: December 23, 2018, 09:25:46 pm
content: Water has been mentioned as an aid to spreading a scratch to become a crack. A little spit works better. Another trick is to make your fine scratch. The take a glass rod (2 mm or less) and heat it to make a little red blob on the end. Then touch it to the crack. Again, spit helps. That is a safe was to open evacuated ampules. When you make a scratch, don't saw back and forth. That decreases its effectiveness.Back to your basic problem, glass works entirely differently under water than dry. I can even demonstrate cutting it with scissors under water. Grinding is great for cutting glass bulbs under water.

author: richnormand
author_rank: Unknown
timestamp: December 23, 2018, 09:34:08 pm
content: I never got reproducible results with scoring and cleave or the Dremel approach (has too thick a blade and not that good at low speed). Never mind hand holding the motor either.If you are going to do several specimens I would be looking at an old low speed diamond wafering blade saw from Buehler Isomet.It has a micrometer adjustable arm and variable pressure that would be ideal for what you are trying to do.Precise adjustment of the depth, distance and angle of your cut.The blade is water cooled.They are sometime available on fleabay for a fairly low cost, even including the diamond blade.Another suggestion is a diamond wire saw. I made my own saw from a small low speed 12V motor driving a gear to give a see-saw movement to the diamond wire attached to a pulley with a weight for tension and the sample in between.Again the diamond wire is cooled and lubricated with drips of water. Also you can buy small length (2m or so) on ebay and do it by hand or motorize the process.In both cases the quality of the sawed surface depends on the speed. In both setups it is easy to wait an hour but you get excellent results.I can post photos should you wish to explore further.Good luck with it.

author: james_s
author_rank: Super Contributor
timestamp: December 23, 2018, 10:11:36 pm
content: The belt sander method is really quick and easy, I think it took about 5 seconds to sand the end off leaving a roughly 1/4" hole. At the time I was experimenting with a vacuum pump I had repaired and wanted to see if it would pull a good enough vacuum for an incandescent lamp.Incidentally it worked, the lamp ran for several hours on the pump and I suspect the main reason it did fail was a combination of being a tired old bulb and the vibration from the pump.

author: Globe Collector
author_rank: Contributor
timestamp: December 23, 2018, 10:24:34 pm
content: Hmmm, TCD for GC....interesting project.I'd take the chemical approach. Cover the lamp in wax, scrape away the wax where you want the bulb removed and dunk it into HF and etch the glass away. I'd monitor the filament resistance and detect the change once the HF gets through and use this end point to sound an alarm or activate a solenoid to add a lot of sodium hydroxide or lift the lamp to halt the process to prevent the HF etching through the fine filament. I reckon you would only loose a few lamps in trials to be able to refine the process to be reproducible and reliable.All the mechanical methods mentioned above are quite valid...but amazingly difficult to implement on such a small lamp where just holding onto it to take all the reactive forces of dremel blades or WC scratching tools will be an issue in itself not to mention the sheer size of the cutting edges compared to the bulb diameter itself.When I saw this post it bought back memories of Chemisrty at Uni. I remember learning about FID's (Flame Ionization Detectors) in an undergraduate lecture and at the end if the lecture I told everybody that I could probably build one out of bits from the junk box...they all "poo-hooed" it as "impossible", which was, of course like a "red rag to a bull". So I built it, i used a nozzle from a disposable cigarette lighter and cut the ends off...and pushed the "jellyroll" out of an axial electrolytic capacitor to get the cylindrical sensing electrode around the flame, I used a high voltage corona wire unit from a photocopier and I recall that the amplifier used a 12AU7 valve as the gain stage because of its high input impedance and immunity to high voltages. I took it to Uni a few days later and convinced them to let me connect it to the output port of the column in one of the GC's there...it worked just like the "real" thing and they all had to "eat their words".

author: james_s
author_rank: Super Contributor
timestamp: December 23, 2018, 10:35:11 pm
content: I just went and tried this right now since I realized I still had a burned out miniature lamp in my pocket. This took about 10 seconds on my belt sander, it was trivially easy just holding the lamp in my fingers and buzzed the end off. I wouldn't even consider messing around with HF when there's such a simple and easy way of doing it without any nasty chemicals.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: December 23, 2018, 11:09:02 pm
content: Quote from: richard.cs on December 23, 2018, 09:12:22 pmThat's a great idea! Letting the vacuum suck in a tiny bubble, definately worth a try.Quote from: jpanhalt on December 23, 2018, 09:25:46 pmAlso worth a try. Only issue is that hot glass fuses to cold glass VERY quickly, and the 1/8" tolerances are so tight I can't risk any deformation of the OD.Quote from: Globe Collector on December 23, 2018, 10:24:34 pmThat's how I'd do it in my research lab too, but I'm doing this at home. I'm not convinced it would work, as soon as the HF makes a pinhole through the envelope the vacuum would suck the solution in, spraying all internal surfaces, and damaging the filament. HF is pretty expensive too, not to mention unpleasant to deal with.your FID sounds intrigueing! I was strongly considering doing something similar before settling on the TCD. The GC runs fine with the current detector, though, you should check out my earlier thread on it if you're interested!Quote from: james_s on December 23, 2018, 10:35:11 pmGotta love christmas decorations, I assume? Thanks for the hands-on, that looks excellent! The finish of the edge isn't very critical, as the seal is made around the diameter of the envelope.

author: Globe Collector
author_rank: Contributor
timestamp: December 23, 2018, 11:40:13 pm
content: I moved house recently and found that FID while doing so. I had not seen it for 30 years and had forgotten about it, but now, its gone again, all packed away. I really should go and get it out again and post some pictures here.Yes, I agree that HF is very nasty, so if sanding works, without removing your fingernails or flicking the lamp across the room, then, by all means, use it.