Bloke with spanners attacks LOHAN's dodgy plumbing
REHAB vacuum pump tubery-pokery
The epic task of getting our Rocketry Experimental High Altitude Barosimulator (REHAB) experiment ready to test solid rocket motors at low temperature and pressure continued over the weekend with the deployment of some proper garden shed toolage: spanners and pipe-cutters.
Last week, we took delivery of a proper vacuum pump and set about connecting it to the REHAB chamber.
We managed to draw a vacuum of of 27 inches of mercury, roughly equivalent to to an altitude of 55,000ft (16,760m), and a metric pressure of 76mmHg.
This didn't much impress anyone, and there was a suspicion among our reader experts that the dodgy plumbing might have something to do with it...
Henry Huang had the following suggestions:
1. Thin coat of high vacuum grease (no volatiles) to coat all joints.
2. Use metal T-tubes and straight-thru connectors. The ones used for plumbing (water supply line) work fine. The plastic gadgets you have may be leaking like a sieve! For the shut-off valve: try to use a metal ball-valve. Make sure its connectors are greased, and apply teflon tape to the hose-valve joints on the outside – any leaks will tend to pull the teflon inwards towards the leak, potentially sealing it. The worm-gear clamps you are using work fine.
3. Thick vacuum hose. You can get away with Tygon tubing, if it is thick enough (mostly so it doesn't collapse under the vacuum).
4. Use as short a length of hose as possible. More hose, more volatiles ...
5. Upon startup, do open the gas ballast, and let it pump for an hour or two to get rid of volatiles in the system (especially if the oil is old or of unknown provenance). And do check the oil level (via the idiotic little sight window).
He concluded: "Don't let the pressure get to you!"
Well, we didn't, and set about resolving the tubing issue. Here's the REHAB pump now connected to thicker PVC tubing, some shiny 10mm copper tube, and a matching ball valve, which we're sure you'll agree adds a nice touch of bling to the whole rig:
A quick test showed this set-up to be completely airtight, and we're now drawing an indicated vacuum of 27.5 inches – that's 60,000ft (18,290m) – equivalent to a pressure of 60mmHg.
Well, that's way below the pump's vacuum capability, so what's the issue here? The SPB's Low Orbit Helium Assisted Navigator (LOHAN) base is at 1,100m above sea level, and Hugo Tyson was among those readers who identified the probable cause:
Um, don't they mean that at altitude the vacuum you can get is going to be less *in comparison with ambient*? That is, if you're trying to test a thing to withstand vacuum inside and 1Bar outside, doing it at altitude is no good. Ie: if you start at 800mBar, you can only pump it down to 800mBar below ambient, ie: 80% of the relative pressure that you might want.
jubtastic 1 weighed in with:
Yes, you're not getting less performance, it's just that the gauge is calibrated against sea level atmospheric pressure and so gives an inaccurate measurement at altitude. You could work out the sea level reading using maths if you don't fancy dragging the whole thing down to the coast.
I would imagine the pump's performance actually slightly increases with altitude as leakage losses will drop due to the lower pressure differential.
Eddy Ito entered the fray with:
I agree that at altitude there is less to suck but the setup does beg the question: What is being measured? If you are measuring vacuum and getting 27 inHg that would indicate you are 27 inHg, or 13.26 psi, below ambient. If we consult the handy chart you so thoughtfully provided we see that nominal atmospheric pressure at 3,000ft is only 13.16 psi. Given that information I can conclude that you must have performed the test on a day with a high pressure weather system in place and it was likely quite a nice day. I say that because if a low pressure weather pattern was providing a dreary day then there wouldn't be 27 inHg available to suck.
Mike Manes wrote:
Hey guys, your new vane pump is working like gangbusters! The problem you cite lies more in how you're reading your vacuum gauge. It reads the pressure at its measurement port >relative< to the lab ambient pressure, which at 1,100m is in fact about 27 inHg. What you should do is replace that relative, or "gage", pressure instrument with one which reads >absolute< pressure, such as an aneroid gauge which reads against a built-in hard vacuum reference. This is how barometers work.
Fair enough, although we're going to have to run with the REHAB pressure gauge we already have. Eddy Ito suggested:
What you really want to know, as stated above, is absolute pressure which is a bit trickier without an absolute pressure sensor. Fortunately we have this handy material we can use as a substitute if we know the temperature: it's water. If you grab a chart, this one at wikipedia should do, you'll notice the vapor pressure of water at 20 degrees Celsius is 17.3 mmHg so a 20 ºC glass of water placed in REHAB will boil once the absolute pressure reaches that mark.
Interesting. We're not sure steam will do much for the vacuum pump, however. We'll continue to scratch our heads, and take on board your invaluable comments and suggestions and we creep steadily towards an explosive REHAB climax. Keep 'em coming ...®
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