To all you smart ones..

Define a "scan chamber". Is that the chamber leaking? Why are you using an RGA instead of a dedicated He mass spectrometer? What is ionized O2 gonna getchya? Why do you need "ionized air"? What IS "ionized air"? Simple production TC pressure gauges will detect N2 for larger leaks. What is the chamber under test? Is it cast or solid metal/plastic/etc? Can the leak area be localized?

Sorry, but I don't understand what you mean. I specialized in He leak detection for 25 yrs, so have more than a little experience. I'd be glad to help if you can clarify the problem a bit better.

nb

Ionizing causes air currents, and the air will try to go to some neutral discharge mass.

Greg

That's more or less what the RGA does, minus the charge/mass ratio selectivity. How about a webcam watching the RGA while you fiddle with stuff?

Cheers

Phil Hobbs

That's more or less the principle a penning gauge uses for pressure measurement. It will however only work in vaccum conditions, as otherwise you just get arc flashes. In principle you can make an array of small penning gauges and try to measure pressure gradients, that way you can calculate back to the flow, but I have a feeling you'll be spending a lot of time on something like that and I'm unsure if you'd be pleased with the result.

Cheers Adrian

Sounds like an ionization smoke detector has most of what you need.

"Most RGA software packages allow for audible indication of changes in the = helium level, so that users don=92t have to continually look at the monitor= ."

Oxygen has larger molecular diameter than helium. There will therefore be less oxygen leaking through a hole than helium. There will be less oxygen to detect than helium.

Helium tends to diffuse through some materials, porous welds, and allegedly air tight joints. Oxygen is too large to do that. Again, you will have less oxygen to detect than helium.

The last thing you want is a charged particle trying to squeeze through a small hole. If the chamber is slightly positively charged, the oxygen ions will be attracted to the walls of the chamber. If negative, it will be repelled. What you want is something with no charge and doesn't react with whatever you're cooking inside the chamber. Helium is the easiest.

Take a smoke detector, find the lump of Am243 in it, and ground it. Place six electrodes (N, S, E, W, up and down) around it in a octahedral array. Measure the leakage to each one and calculate the lateral difference.

Faulty assumption: the alpha source has to be isotropic. Average smoke detector bob looks like it's a grain of active stuff inside a metal case. not so great for a full 3 dimensions. Still, 2 would be achievable if it has a rotational symmetry axis.

Tim

-- Deep Friar: a very philosophical monk. Website:

"Jamie" wrote in message news:FOCVr.15837$ snipped-for-privacy@newsfe14.iad...

Ionization Vacuum gauges work on that principle.

BTW, have you tried Acetone for locating vacuum leaks?

Cheers

Have you ever seen a helium leak detector? We are on our 3rd unit, now. It can detect leaks down to 10^-10 cc/sec on a good day, so it is probably 1000 times more sensitive than what you are proposing. Trying to find a leak by the air moving around outside the chamber is never going to work due to the constant stirring of the air. He leak detectors work, and are used in industry all the time, especially working with vacuum systems, high-integrity welding and such things where leakproof status must be verified. Yes, they ARE expensive, but you can also get used ones on eBay for less, although they are not garage-shop price.

Jon

nd a

Ambient currents will drive you nuts, but you could try inexpensive laser holography of the air around a possible leak. Do some esoteric image processing and you might see air currents, but then again ambient air currents would have to be huge compared.

But at least we're talking less than $100 to do the experiment, and who knows might have sensitivity enough for something.

We did a lot of brainstorming about PCB-mounted air flow sensors, and had a thread here not long ago.

One idea was to ionize air (high voltage, small needles or brushes or whatever) with AC, and pick up (something) some small distance away, and measure phase shift associated with air propagation. I don't think that idea won the sweepstakes. There's probably a thermal spin on that.

We're designing a VME crate controller, which is all digital grunt work, and the air flow sensor - which we don't actually need - was the only really interesting part. And a couple of home-made LDOs, like one from 1.2 to 1.1 volts. I still think we could run both chips from 1.15 and nobody would know the difference.

Permanent instrument or just for checking out a vacuum plumbing job after assembly?

If its the latter, can you use a candle?

we need to be able to use a long light weight PVC pipe for example with the sensor attached to the end and hand held in hand. Also this PVC should be able to break down to a short pipe. All of that is not the issue..

THe problem is being able to detect a small movement of air current being sucked into the gasket.

THis is not a permanent installation but a hand tool which a tech can use to probe around the seams(welds) and gasket area's with out the need of others to assist and no need to carry bottles of helium, while up in the scaffolding.

Jamie

One of many thoughts I had was an acoustic device in the ultrasonic range where we would project an impulse into a parabolic reflector, a reflector that is suspended with a screen so that ambient air flow can pass through it. I was going to put 3 miniature traducers just around the return area of the incident point and form a balanced circuit. THe idea follows that if air current is flowing through the suspended area it'll disrupt incident angles and thus generate an unbalanced reference, one that I can measure.

But the more I got thinking about that, ambient noise around me could disrupt that type of device.

Jamie

Yes, we have a RGA system that has a tap in the vacuum system. It works but is hard to work with. It has to be calibrated like every hour. It is very unhappy many times but that isn't really the real issue.

It is possible for us to have a small canister of gas that can be ionized so to allow that to pass over a sensor if it is getting drafted into the chamber. That is something that is still on the back burner.

I also was doing some steady on acoustic resonance effects that could occur across the plane of a gasket surface while being drawn in. The amplitude would be low but at least it could be filtered so that only that spectrum is being analyzed.

More on that later.

Jamie

yes, we have all of that.. we use alcohol many times. All that works fine.

The aim is to have a hand held tool that can report a possible leak while you're crawling around the scaffolding and done by one person..

Jamie

I was playing around today with a dissected smoke detector.. I found it to be random in nature some times but I did get some interesting test results from it..

Thanks.

I wasn't worried about what goes on inside. When we do these test,. there is no beam, no flow of anything.. all there is vacuum and a ion gauge probe with a thermocouple. I can see the ion gauge being sensitive to this but that is fine.

I was aiming to have an external sensor detect a charge sample of some gas being pulled across the sensor plane as the sensor is held along a suspected leak..

You are correct about the oxygen over helium but you must remember, our system has to have a roughing pump and a 50k RPM high vac pump operating at all times just to keep that level of vacuum..

If all comes to worse, I can attach a wireless transceiver to the ion gauge and the tech can have a small bottle of HE with him.

Jamie