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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Xray wrote ...
Patrick - I've been degassing dielectric oil for X-ray tube heads for about 12 years, and I'd be happy to tell you about my vacuum system and process that I use. I use a very large glass bell-jar (with safty cage) and a Welch model 1397 2-stage vacuum pump. A diffusion pump is not necessay.
Later today I'll post a picture of my vacuum system, and I'll give you some details about te process that I use for degassing. Right now I can't spend any time on 4hv because of my huge backlog of work that's been piling up. But I will get back here later this evening (Minnesota time).
Later.....
Yes, Yes !!! please do!!!
And if you could clarify carl Pugh's point too...and its really leakage current from 70kV to ground across 2 inches of oil i really worry about, id like less thaN 8-10uA of leakage. is such a number plasuible? is vacuum degassing, dehydrating nessacary?
I also dont have real diala/univolt oil, so I use food oil, if that matters.
Registered Member #3429
Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288
Patrick wrote ...
And if you could clarify carl Pugh's point too...and its really leakage current from 70kV to ground across 2 inches of oil i really worry about, id like less thaN 8-10uA of leakage. is such a number plasuible? is vacuum degassing, dehydrating nessacary?
I also dont have real diala/univolt oil, so I use food oil, if that matters.
Hello again.... First off, as far as clarifying Carl Pugh's point, I assume that you are referring to his comments: "Oil that has all the dissolved air removed has a high breakdown voltage. Oil that does not have any of the air removed has a high breakdown voltage. Oil that has some of the air removed has a much lower breakdown voltage."
Well, the words "high" and "low" are of course relative (and ambiguous as used here) but in general I can agree with only his first statement. It is my understanding, and through my own experience, that the less dissolved gas in insulating oil, the better it will perform as an insulator. I would like to know where he got that information from.
Here are a couple of pictures of my vacuum system: and
On a "good day" my vacuum system will get down to about 20 microns. With a large enclosure (such as my large belljar) you need a relatively fast pump if you want to degas your oil in a reasonable time period. I generally degas Diala oil overnight (about 16 to 18 hours) and I initially heat the oil to about 130F prior to pouring it into the X-ray head. After the pressure gets down to below 1 torr (1mm Hg), the oil begins to "boil" and foam up. Over a period of an hour or two, the boiling reduces to a random bubbling, which indicates that most gas and water vapor has been removed. Once the degas process is complete, I seal the X-ray head as quickly as possible to avoid gas and water vapor from re-dissolving into the oil.
Here are some things about dielectric oil to keep in mind: 1. Oil is like a sponge. That is, when a sponge is dry, and the ambient air is moist, the sponge will start to soak up the water from the air until there is a balance. If the ambient air is drier than the moisture in the sponge, then the water will evaporate from the sponge to the ambient air until there is a balance. Oil is exactly like that sponge. Therefore, once you go through the process of degassing and desicating it, you should SEAL it in a container to keep it that way. 2. When deciding how long to degas oil, you must consider a few variables such as its temperature during degassing, how has it been stored (how gassy or moist is it?), your vacuum system capable pressure, how dry you need it to be (based on working voltage and interelectrode spacing). The two most important variables are TIME and PRESSURE. If you have little time available, then you can expedite the degassing process by lowering the vacuum pressure. If you have lots of time but a crummy vacuum pump (such as a single stage one) then you might need to run it for a week! As mentioned earlier, heating the oil will expedite the process, but be carefull to not get it so hot that you chemically change it, or even worse, cause it to burst into flames! I typically do not heat Diala oil above 130F, although it could get a lot hotter without problems.
3. For serious high voltage applications, I would not use any oil other than Shell Diala-AX or Exxon Univolt N61B. But for most hobbyists, I realize that those industrial oils are not easily available, and so other types of mineral oils may work well, especially when thoroughly degassed. I cannot make recommendations for "drugstore" type oils, but there is plenty of good infromation on the hobbyist websites that deal with Tesla Coils and Fusors, etc. (most likely there is good information right here on 4hv).
Well that's about all I can think of right now concerning degassing oil. Please feel free to ask questions and I'll try my best to answer them.
Registered Member #3610
Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506
Holy cow! You weren't kidding about that being a BIG bell jar! Imagine the amount of energy that thing would release if it failed under vacuum. I wonder if someone here with access to fancy industrial transformer oils could sell small quantities to hobbyists? I've had to resort to hydraulic jack fluid and drugstore oil myself, but I'm not doing anything really heavy duty.
Patrick wrote ...
Whats a "thermocouple" guage thingy ? how does it work?
Everything you need to know about TC gauges is in Sam's FAQ that I linked to earlier. I have a Hastings DV-6 that I use with my vacuum setup. It's good down to a couple of microns, which is as far down as most hobbyists need to measure anyway.
Registered Member #3429
Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288
Patrick wrote ...
Does the oil have too remain visible?
Yes, for me it's essential to keep an eye on the process.
wrote ...
How much leakage in uA is a resonable number for 70 kV across 2 inches? Even a wild estimate would be useful.
Sorry, I have no idea. For my application, it doesn't matter to me what the leakage is so long as there is no arcing during an X-ray exposure.
wrote ...
Where do you get your oil from?
I get it in 55 gallon drums from a local oil distributor in St. Paul. I use about one drum every 12 to 18 months. In case anyone is wondering, no I will not sell small quantities to people. Sorry, but it's just too big of a hassle for me (been there, done that!)
wrote ...
Can I get 5 Gallon buckets from a supplier? How much $ for the amount you buy? (per Gallon)
I could go through a gallon every month or so.
You can try calling around to your local oil supply companies, but I seriously doubt that anyone will sell less than a 55 gallon drum. I used to see it for sale on Ebay, so that might be a possible source. Currently I pay around $11 per gallon for Diala-AX. The price jumps up and down (mostly up) like a roller coaster.
wrote ...
Whats a "thermocouple" guage thingy ? how does it work?
It's a type of guage that measures vacuum pressure in the range of atmosphere to about 1 micron. It's the type of guage you need to monitor the pressure of a mechanical pump vacuum system. As James mentioned, you can learn more about it and other types of vacuum guages on Sam's FAQ website and others. Like Jame's guage, my guage is also a Hastings model DV-6.
NOTE: I believe I've been spelling "gauge" incorrectly.
Registered Member #3429
Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288
There's a seller on Ebay currently selling 55 gallon drums of Diala-AX oil for $350. That's a great price, but he won't ship. Local pickup only in Crestline, Ohio. Search for "Diala Oil" on Ebay if interested.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Xray wrote ...
Patrick wrote ...
Does the oil have too remain visible?
Yes, for me it's essential to keep an eye on the process.
I once designed a steel 'bell jar' with a perspex/acrylic 'window' for coil winding company who used it for vacuum impregnation.
They had it built at a local fabrication company so I'm not sure exactly what plastic they used in the end, but the window was around 1/2 inch thick.
The window served the same purpose as here, so they could see when the bubbles stopped.
For your application, I assume you have the pipework on one end of the 40 shedule tube. you could fit a 'window' to the other end without much difficulty.
A two inch diameter window probably wouldn't need to be 1/2 inch thick. .
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Ash the pipe has been changed to steel pipe with 1.4" ID, I will use 1/2 " acrylic, or maybe i can snag some poly-carb, but Carl Pugh suggests some plastics are unstable at vacuum.
I think 15 PSI across 1.4 dia. is like 1.53in^2 so its seeing 23lbs of force, not that much, of course SA increrases as a square, so force can build quickly on larger windows.
I would fabricate this as a 3 or 4 inch diameter plastic circle on a pipe flange so only 1.4" dia is actually unsupported at vacuum.
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100 micron Vacuum drying dielectric oil...
