Cooking a Rb lamp - Page 1
Forum section: electronics / projects

author: S57UUU
author_rank: Regular Contributor
timestamp: January 19, 2025, 09:38:00 am
content: I have a couple of Rb boxes (LPFRS) with low lamp voltage. I read here (and at other places), that heating the lamp can restore it, by removing the Rb settled on the glass. I would like to do it as "scientifically" as possible. So here are few things I do not know:1. To what temperature should I heat it? Obviously, to evaporate the Rb, but at the pressure inside the lamp, what is the evaporation temperature? I guess the glass being opaque for IR, I can measure the surface temperature with a thermal camera.2. How long must I keep it hot? Will there be a visible change on the glass?3. Once thr Rb has evaporated, how do I persuade it to condense on the tail of the lamp, and not the face? Should I try to make a thermal gradient, like blowing hot air from one side only? Or is the orientation important (which side up?)Marko Cebokli

author: Gyro
author_rank: Super Contributor
timestamp: January 19, 2025, 10:33:03 am
content: The last paragraph of this Efratom FRS Lamp Assembly repair document is what I've seen. It doesn't look as if it's a critical procedure, just heating the glass to persuade the spluttered metal to condense back in the pinch...https://bama.edebris.com/manuals/efratom/rfs

author: S57UUU
author_rank: Regular Contributor
timestamp: January 20, 2025, 08:36:49 am
content: OK. According to the document,"use a typical heat gun / paint stripper gun and uniformly rotate the lamp heating the glass..."I will have to find a way to rotate it - obviously, it will be too hot to hand hold.The document also implies that you can see when it's done. So, as for the temperature, the procedure is: just heat it, until a visible change occurs.My first idea was to put it into my mini reflow oven, and set a suitable temperature profile. But in there, I can't rotate it, nor see it.So, heat gun will it be, must just find/build some rotating support.Checked the boiling point of Rb, it is 688 degrees. I'm quite sure that temperature needs not be reached for this procedure (at 688 degrees things already glow red), so we are working with the liquid phase (it melts at about 40C). Therefore, gravity should be taken into account, keeping the lamp tail down?Normally, the lamp runs at about 110C, so what about just orienting the whole unit to point the lamp up, and letting it run some time?Or is it there some adhesion between rubidium and glass, which needs to be overcome by a higher temperature?

author: Gyro
author_rank: Super Contributor
timestamp: January 20, 2025, 10:38:09 am
content: Ok, applying some logic, as I see it just based on the photos in that document...- The envelope is almost certainly Quartz rather than glass, so it will take quite a lot of heat. Yes, the tail (pinch) of the bulb needs to be pointing downwards so that the metalic Rb will run down into it. It will also be slightly cooler, encouraging it to condense there. I suggest that you fabricate some sort of support, a hole in a scrap of wood or metal for the pinch to fit in so that you can orient and tilt it.- The interior of the bulb is pretty much at a vacuum, apart from a little inert gas. The Rb evaporation point should be significantly lower under those conditions.- Instead of rotating the bulb, you should be able to move the heat gun rather than the bulb. From the document, it looks as if the tiny drops of Rb might run together and then drain down into the pinch - or maybe they evaporate first. but yes you should see some clearing of the envelope. You may need to tilt the lamp with your 'holder' to assist this.- I suspect that the Rb droplets are deposited on the inside of the envelope over many heating and cooling cycles of the lamp (similar to what you might see in a Mercury lamp). They are probably so tiny that they just adhere to the (never perfectly smooth) surface of the Quartz rather than being affected by gravity as they condense.- As I understand it, the initial inert gas discharge is just enough to evaporate a tiny amount of Rb from the pinch to create the spectral line, but it might not all end up back there when the lamp cools, hence the need for the procedure. Running the lamp with pinch down might eventually work, but might take averylong time.- The quartz envelope needs to be scrupulously clean before heating, otherwise finger prints and contaminants will be burnt onto the Quartz and might even cause it to crystallize and shatter. Use Isopropyl alcohol or similar and don't handle with bare hands afterwards.- The author said he has done this procedure successfully 5 times. I have done it zero times. I am interpreting using general knowledge and experience. Maybe somebody here has actually done it, I don't know.EDIT: Looking again at the document, he says that the procedure is needed if the Rb is visibly "splattered" on the inside of the lamp. If that isn't the case, then the procedure isn't needed at all and you probably have a problem with the electronics. Do you see the "purple halo" that he talks about?

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: January 20, 2025, 04:29:20 pm
content: For those interested:https://www.eevblog.com/forum/repair/repair-sro-100-rubidium-lamp/https://www.eevblog.com/forum/repair/rubidium-lpro-101-fix/