Here's a picture of the gas-handling line leading to the discharge region seen in the plasma post:
How many valves can you count in that picture? If you said "seven," give yourself a pat on the back. Here's the same picture with the valves numbered for your convenience:
("Seven! Seven valves! Ha ha ha ha ha ha ha!!! thunder, lightning)
So why all the hardware?
Well, the gas bottle contains krypton gas at a pressure of several atmospheres. The discharge tube feeds into a vacuum system at 10-7 torr, thanks to some honkin' big pumps. We'd like both of them to stay that way, so some valves are needed to throttle down the gas flow to the necessary level. It just got a little out of hand...
In order, the numbered vales are:
- The valve on top of the gas bottle, which is usually closed, and only popped open occasionally when the gas starts to run low.
- A gas regulator valve, which limits the rate at which gas is allowed to flow out of the bottle into the gas handling line.
- A cut-off valve, to allow us to stop the flow of gas completely.
- A low-flow valves with a micrometer handle, to throttle down the gas flow a bt.
- Another cut-off valve, to allow gas-bottle swapping without having to shut down all the vacuum pumps.
- Another low-flow valve with a micrometer handle, to throttle the gas flow down.
- A final needle valve, again with a micrometer handle, to throttle the gas flow down and set the pressure in the dischrage region.
Why are there three metering valves in there instead of just one? Well, it's one of the perils of science at a small college-- the gas regulator we happened to have on hand doesn't go to a low enough flow rate to let us reach the pressure we want with just one of the valves we had on hand, and better leak valves are expensive, and take time to order. So, I stuck in a second low-flow metering valve, which restricts the gas flow more, and allows greater control at lower pressures. That still wasn't getting low enough, though, so I stuck in a third.
This arrangement is sub-optimal for a number of reasons, chief among them being that valve #4 is not functioning entirely properly. It cuts the flow down, all right, but it doesn't actually allow much fine control, and may in fact be leaking air into the line. The bigger problem is that it takes forever to pump this line down through all those valves and tiny little bits of copper tubing...
I'm going to need to re-engineer this a bit. That's going to have to wait, though, because I start teaching again on Monday, and I need to spend this week prepping lectures for E&M with an entirely new curriculum.
This is a good explanation of the system. You correctly predicted my question, which was, "why do you have multiple needle valves in series?" Thanks for the interesting post.
And note the ubiquitous roll of duct tape on the table...
Calibrated pinhole leak into the system at the high pressure end, bit it can clog. Membrane diffusion is slow. Compromise is a supported micropore filter (in bakeable alumina - Anopore,
If it only has to be slow you can get it clean, too. Only a barbarian uses a needle valve for shut-off.
I haven't spoken up on here for awhile, but I just wanted to say that the lab setup posts have been really fun and informative. I'm trying to land a professorship now that I've finished my PhD, and I like getting ideas on how to get things done without access to my advisor's large budget and the expertise of those at a major research university.
You could improve the pump down if you added a larger diameter pumping line connected at the high pressure end with ... yet another valve!
Love the photo (I'd been picturing the setup somewhat differently), hate the earworm . . .
How about we take up a collection and buy you a nice mass flow controller? :)