Fusor Forums

Good enough to be in the Plasma club. I hope you will continue to imnprove your system.

Look at the name of the first guy ever admitted to the Plasma club back in 1998. Any relation?

Richard Hull

The gauges you point at are regular gauges for air conditioning engineers, however, they would actually perform a useful task, but what you really need is something that can measure sub-micron pressures so you know what's happening at that end of the vacuum scale. The ones you point at will simply read '0' when you get to the right point, but at least if they aren't reading '0' then you know it's too high!

I always recommend looking for active gauges. These can be Pirani (atm to fractions of microns), inverted magnetron (10s of microns down to 10^-9 mbar), or 'compact full range' that have both types in one device. The 'active' means you put power in and out comes an analogue voltage signal, no fuss. You have to keep you eyes peeled on ebay if you want the bargains, because these can go for Big Money [and seemingly higher and higher amounts these days - more amateur scientists than ever before, perhaps?], but you can still spot a bargain every so often.

With your system isolated, six hours from the bottom of your gauge reading (only 27 inch/Hg?) to atmosphere is rather poor performance for a sealed system. You still have leakage.

While even a fairly "tight" system will always rise due to outgassing (chamber walls/gaskets) but that should slow rapidly as it gets higher and should remain below atmosphere for a few days at worse (mine can stay below 50 torr for months but I have a high vac system that has been leakage tested in the low 10-6 torr. That is a very different requirement than what you need even for a neutron producing system.) Are all your threaded parts sealed properly - i.e. Teflon tape or a vacuum grade sealant? Did you use any type of grease on the gaskets (not a good idea in general. Gaskets at that pressure should never need any vacuum grease.) Your gauge is still far too high (should read 30, at least) or is it just a very poor quality gauge unable to read the real value? Possibly, the vacuum pump is poor and/or you still have a lot of leaking preventing your reaching 30 inch/Hg (Bottom of the gauge reading.)

From the picture your chamber looks very sound and if the gaskets are working properly they should easily bottom out that gauge.

Glade your system is doing far better but you have a ways to go if you decide to add (later) a high vac pump (turbo or diffusion.) If, however, that is not your goal, than your vacuum is fine for a demo.

All of my threaded parts are indeed sealed, however the gaskets have quite a lot of high vacuum grease on them... Do you think that is contributing to its poor performance? The gauge itself seems to be fairly well made, however, tomorrow I have a much better gauge coming in. I am hopeful the new gauge will allow me to diagnose the system better. What kinds of pressure should I be reaching in order to accomplish star mode? (in microns if you could)
Thanks once again,
-Blake

Hi Blake.
Don't be too critical of your relative-to-atmosphere vacuum gauge, even though it appears to read low by somewhere between 5% and 10% when close to full deflection.
It's marked with the brand of Holley, a famous maker of high-performance carburetors.
[edit] That was in an older post of yours -- I see you switched gauges. [\edit]
I think an accuracy spec of +/- 10% could be considered normal for inexpensive Bourdon-tube gauges, and is good enough for tuning engines.

Here's a video that shows what goes into one of those gauges.
I imagine the manual adjustment step is often skipped to save cost.
http://www.youtube.com/watch?v=-xF7d0nuFTw

p.s. an absolute accuracy of +/- 10% would be considered excellent in a high-vacuum gauge.

I use similar tubing and it works fine. Provided my fittings are snug, I have no problems getting below ~ 50-70 Microns or even better if my diffusion pump is turned on. There have been reports that teflon tape is not ideal for sealing threads. Some type of lock tight or epoxy serves me well.

The first gauge is the one I have. You can buy them on E-Bay, or if you are close to an air conditioning repair/ supply store, they usually have some gauges in stock.

Dan Tibbets

A suitable vacuum gauge looks more like this: http://www.pfeiffer-vacuum.com/products ... doId=12202
Please go through the manuals there. Pfeiffer also provides a very good knowledge base for vacuum newbies http://www.pfeiffer-vacuum.com/know-how ... ner.action You don't have to do the math which is provided there, but it gives you an idea about how vacuum behaves: Imagine - the better the vacuum becomes, the more it behaves like honey, rather than a gas or a liquid. And at about 10-8 mbar its more like honey out of the freezer.

So it takes large diameters to "conduct" vacuum. A pipe with 40mm diameter is much better than one with only 6 mm.
The pipes should more look like this: http://www.vacom-shop.de/epages/VacomSh ... cts/302512

And using such gauges you can find your leaks much easier: Just take some small amount of a gas with higher molecular weight (dust cleaner, propan/butan from a gas lighter) and bring it close to the leak. Very small amounts of this gas will rapidly change the vacuum values on your gauge.
I use a pirani-cold-cathode combi gauge and found every little leak within a minute.

To answer the question above..........Decent star mode stars are generally found to be best below 20 microns. (10 microns is better)

A superior mechanical pump, in a well sealed system, can easily hit under 30 microns regularly and after a small run time and gas ballsting, end up at or below 20 microns. A good diff pump added to this will easily drop 2-3 more orders of magnitude to 10e-5 torr or better. These levels are not generally to be attained in a gasketed bell jar system with small tubing, greases and large gasket areas exposed to the vacuum.

Richard Hull

Interesting... I just attached my new gauge to the vacuum chamber, and with some more tinkering it reached 325 microns. I also removed the vacuum grease from the gaskets, which made the whole setup much easier to manage. The pump that I am currently running claims that it can reach 22 microns in a perfectly sealed system. The only areas where I believe the leaks are occurring is through the connections from the vacuum tubing to the fittings. However, I do not know of a way to remedy that. Currently, I have my fittings wrapped in teflon tape and covered in a bit of high vacuum grease. I am using a hose clamp to secure everything together. One thing is for certain though, I am defiantly feeling the limitations of my vacuum chamber...
Thanks,
-Blake

Blake,
You are learning fast.
What does your new vacuum gauge show when connected directly to the pump inlet,
using a minimum number of adapters and fittings, with no chamber or long hose?
You may find that the reading continues to drop for a while,
as the plumbing and the gauge itself outgas after long exposure to air.

To get favorable values for ultimate pressure, I have heard that pump makers use a McLeod gauge. That measures the volume reduction as a gas sample is brought up to atmospheric pressure. Vapors such as H2O will condense, so the vacuum reading does not include their partial pressure.

325 microns in your chamber is plenty low for common vacuum demonstrations, such as what happens to marshmallows.

Thank you for the kind words. When the gauge is attached directly to the pump itself, its pressure equalizes at 40 microns. Quite the jump from the 325 of the chamber. I think that I will remove the valve in between the pump and reactor body to eliminate any more points of leakage. That should buy me a few more microns

40 microns at the pump and 325 in the chamber tell us a lot about how leaky your system is, be it from outgassing (virtual leaks) to real leaks. There should normally be virtually zero differential between the pump head and the chamber in a well designed system.

Richard Hull

Vacuum grease on Teflon tape? Never - that defeats the purpose of the tape - makes it more likely to leak. Friction between the tape and metal surfaces is critical for deforming and forming a good seal. I use teflon tape in 10-3 torr systems with great success BUT it must be used properly AND only for specific situations (i.e. threaded systems designed for tape use.)

In general, vacuum grease isn't a good idea for any vacuum application but does have a few, rare uses. I'll skip that subject (the uses) but will say that for most people it is worse than useless and can cause major problems if used incorrectly.

If your pump can only reach 22 microns (mfd limit) than it isn't going to reach star-mode. As for your chamber only reaching 325 microns, that is rather poor. Consider re- tighten the chamber bolts while being VERY careful to get a uniform torque all around the gasket (that is, tighten each nut a little at a time in a sequential manner following a standard tightening pattern - look up this subject in auto repair on now to tighten head bolts. Tightening bolts on a chamber isn't as simple as many think and can lead to terrible leaks if done incorrectly!) And don't over tighten the bolts! That can lead to leaks, too. Your chamber looks very sound so the issue of the bolt tightening may be an issue.

I agree to remove that valve - it does not look vacuum grade at all. A larger pumping port and hose would, as some have pointed out, be a good idea.