> When I have a large system to evacuate it takes rather long, granted. But if > I have leakages somewhere (for example at the cock that I use to feed the > penning mixture to the system) then the intruding air will distribute > homogeneously inside the vacuum.
That's not really true. Pressure flows like electricity does. If your vacuum system is a long thin hose, it acts like a resistor, and you can easily have a higher pressure in one end of the hose than the other. The higher the vacuum, the slower things tend to propagate, too. Even without a leak, you can have a pressure differential. You'd think that things would stabilize eventually, but all your surfaces are outgassing at various rates, and the pump is moving molecules from inside your system to outside, so the pressure will usually be lowest at the pump inlet and rise from there. The narrower the bore, the faster the rise. > When I have a small(er) system, it might not take so long to evacuate, but in > this case small leakages will ruin the system more rapidly. Not really - your pump's capacity and the rate of leakage will reach an equilibrium, and that would be similar regardless of system size. And, due to the flow resistance I mentioned above, the pressure will be highest near the leak and lowest near the pump. The situation you describe is more like if you'd pumped down to a low pressure and then a leak appeared for whatever reason (common when doing gas fills). In this case, the pressure will tend to rise more quickly in a small system, but again this depends on the geometry. If you have a small, long, thin system, with the leak at one end and the pump in the middle, the pressure will get pretty high pretty fast near the leak. If you have a large system that's a sphere, the pressure near the leak will rise more slowly. Vacuum systems take some getting used to, and require their own sorts of thinking. > So basically, the idea is to include a small volume in between to compensate > for that. Is that nonsense? That's like adding a capacitor to a circuit - it will give you some surge capacity, but will tend to give you slower pumpdown and it'll have more surface area to outgas. And if it's connected to the thing where you care about the vacuum (your tube) by a resistance (narrow passage), it won't help particularly. And this is the same reasoning that would apply to a low impedance power supply - you can put in a big filter capacitor, but if there's impedance between it and your load, the voltage is going to sag when your load draws more current (like a leak in a vacuum system). So it's not pure nonsense, but my guess is that it isn't going to do what you want. The winning strategy is generally to keep your pump capacity higher than your leak capacity, keep your leaks far from where you care about your vacuum, and provide a wide, short passage between your pump and the place you care about your vacuum. Why yes, I have worked on vacuum systems, why do you ask? We had a time with this on one project where we had this loud screaming 1200 l/s turbomolecular pump exhausting through a 6-inch port into our target area. But 3 meters away, through 1½ tubing, we had an electron gun with a 1cm diameter dispenser cathode that outgassed like mad when heated. We finally had to tack on some ion pumps at the gun end to keep the vacuum there clean. - John -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To post to this group, send an email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/neonixie-l?hl=en-GB.
