Positive Pressure Leak Checks

[John Futter]

Jason
if you have a big leak / suspected leak positive pressure testing is valid
All of our ultra high fittings @ work are ultrasonically treated in hot water with detergent then rinsed and put in a hot air oven at above 100 degrees Celsius
and while still hot out of the oven are assembled and pumped on. And we expect high ten to the minus eight millibar after 24 hours of pumping.

strange fittings that are leaking at above our leak detectors minimum vacuum are positive pressure tested at several atmospheres with the aircon leak detector fluid it can take 2-10 hours to see the small tell tale bubbles ( they take days to burst)

[Dennis P Brown]

May I suggest that your "positive" pressure check with a soap solution isn't a bad idea and in fact, is a good idea that many people and professionals use! I am sorry I wasn't clear. Also, wasn't being critical of the "under water test" but didn't think it was a good idea for the average person to try (for many reasons.)
In any case, if a positive pressure/soap procedure fails to detect a significant leak, are you certain it is a leak? Have you tested just your vacuum system dead ended on the gauge?

[Jason C Wells]

Dust Off is pretty cool. Two questions on your technique.

- Do you hold the can upside down to wet the joint under investigation?
- How do you get at things that are upside down?

As it turns out, I was unable to find any leaks with Dust Off. I was unsure of my technique so I loosed a couple fittings and tried again. I was able to identify the leaky fittings. After I re-tightened the connections, I was again unable to find leaks.

I was unable to force a small leak in an Ultratorr fitting. It was all or nothing with the Ultratorrs. Does that jive with your experiences?

Thanks,
Jason

[Dennis P Brown]

The "Dust Off" gas is what one uses - the issue is that one needs to be in the low 10^-3 torr range to use this technique; that is a no go for you since you are in the micron range. A "massive" leak can easily be heard using a toy stethoscope or just getting close to the item. All other leaks - between 10-3 torr and overt "hissing" are the real tough ones. Here, a few methods can be done. First, do examine all 0-ring surfaces and o-rings with a magnifying glass and make certain that there are no scratches or cracks. Reassemble and be careful not to over tighten. Any hose clamps? Check these. Also, reduce number of parts - that is, starting at your fore pump, add just a gauge and see what vacuum you achieve. If in the tens of microns, stop and go no further. You found your problem. If under 10 microns (what ever your pump generally can do - 4 microns? 5 microns? etc), continue. Add components one at a time and check to see if one is the problem (remember, can be more than one so after finding a bad component, don't assume you are home free ... .) Of course, this requires you have proper caps for the components. old oil and similar grim could be an issue, too. If all components check, then consider possible contamination - water is a big one that just does not pump out and will keep even a tight system in the many of microns and act like a real leak. If that is suspected, use 70% alcohol, clean out each part (to remove organics like oil; this isn't necessary but might be worth the effort since everything is already apart), bake out (not the 0-rings!) at 100 C. Then reassemble, put under vacuum and see if that makes things better (one can using heating tape wrap on the system but most don't owe those.) Finally, one can use very high end vacuum grease, wipe on with a lint free cloth and wipe off with a lint free cloth. Reassemble and test. These are common methods.

[Richard Hull]

Don't forget, a good high current glow cleaning will heat every part of the Fusion chamber and rid the system of water very well. Just make sure the grid is of a healthy construction (heavy gauge wire.) This demands a supply capable of about 50ma though yet need not be more than 5kv. A separate supply can be fashioned to easily do this. Many ion pump supplies can serve here. when I glow clean or run the fusor for fusion, my fusor's shell exceeds 100 C. That is why it improves markedly after the first run or two as it is warmed up for the HEAS each year. Water is, indeed, the big nasty.

Richard Hull

[Jason C Wells]

We are running afield of the original topic but I'll follow up here to close this out.

Part of my linear accelerator is very much like an ion pump, though I am using it to produce protons. I went back to the person who made it for me. At his shop is a Varian V-1304 controller for a 16 cell ion pump that has the current and pressure scales shown on the meter. Using some rough order of magnitude comparison of my ion pump to the setup in the shop, we determined that my baratrons are reading incorrectly. (They were contaminated by my aforementioned error. Tried to clean them but now I think they are still dirty.) My ion pump is drawing 40 microamps which should represent a pressure just a bit less than 10e-5 region Torr. This doesn't give me a precise pressure number, but it does tell me that I should be skeptical of my baratrons until I can get a good cal/zero on them.

Here's the meter we used to characterize my ion pumps current/pressure performance.

[Richard Hull]

As I mentioned in another post, ion pump supplies are great for powering a fusor to get the water out as their high voltage and high current will do a great glow cleaning and carry the wall temp of a fusor up to 100C readily.

It is cool that you have used the ion pump to get an idea of your vacuum level. Unfortunately, ion pumps crud up fast at higher pressures than about 10e-4 torr and are usually only turned on in the 10e-5 to 10e-6 torr range to obtain 10e-7 or a little better, with a Ti sub pump finishing to a super vacuum level at or beyond 10e-8 0r -9. I do not even want to contemplate a struggle to obtain 10e-6 torr, much less those lofty super vaccum numbers. Once I drop into the 10e-4 torr range using my diff pump, I am as happy as a clam.

Richard Hull

[Jason C Wells]

I was warned about running my ion pump at high pressures. I don't turn it on until my no longer trustable baratron is reading microns.

My friend at the university glass shop found a calibration curve for a single cell ion pump in air. That curve puts me at 8e-6 Torr for an indicated current of 42 uA in air.

Today I am pulling 30 uA in hydrogen. Even if my pressure is incorrect by an order of magnitude, I am still go for launch. I'm waiting for darkness now. My alpha "detector" is zinc-sulfide with a thin foil screen to filter out keV protons.

An ion pump installed in a linear accelerator column. Also, a small cockroft-walton ladder multiplier, microammeter, and battery pack.

[Dennis P Brown]

Measuring your accelerator pressure via indirect methods just isn't a good idea; very inexpensive real ion gauges are available all the time on e-bay. Get one. These units are also great for detecting small leaks - when the "Dust Off" method is used, these gauges jump when a leaking section is sprayed. These units work from 10^-4 torr down to 10^-9 torr and are calibrated. Without a good handle on your real pressure you are not going to easily solve your issues. The reason I say this is that an accelerator just has major issues due to ion collisions/gas charging when operated in the 10^-5 torr range and you really need to know the actual tube pressure.