Let me be the first to point out…
…that your vacuum pump sucks.
Mine’s the one with the vacuum line in the pocket.
We've been beavering away on the Rocketry Experimental High Altitude Barosimulator (REHAB) element of our Low Orbit Helium Assisted Navigator (LOHAN) mission, and last week enjoyed a lightbulb moment as to how to create a decent seal between the metal hypobaric chamber and the glass lid. Click here for a bigger version of the …
Out of interest, why did the first vacuum pump fail - was it perhaps one that relied on airflow to cool it, meaning that you will need to use a different type? Or possibly just run for too long? (normal fridge motors will expect a fairly short on/off duty cycle.)
a/c & refrigeration compressors use the colder refrigerant from the suction line for cooling, but they are expected to be able to pull in & contain the factory charge within the condenser. If the compressor wasn't burn-your-hand hot after running a short while, then it was probably too weak to begin with.
i like your ingenious use of silicone (inspired by the real LOHAN?) but wonder if a simple latex ring seal (as in the original diagram) would not have worked. in my mind's eye the lid is pulled down sufficiently tightly to prevent slippage, especially if it is made out of non-slippery rubber/latex and was wide and flat.
right, i'm going to stop thinking about LOHAN's ring now and get back to work.
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Have you considered the effect of a jet of rocket exhaust on the proposed glass-plate lid of your vacuum chamber, and the effect on the vacuum therein of all that exhaust gas that's being generated? It occurs to me that you may experience a loss of vacuum from either 1) the glass plate being cracked by the exhaust gas jet, or 2) the evolved gas quantity raising the internal pressure and possibly blowing the lid clean off.
Some possible solutions: a) Increase the chamber volume considerably; this will reduce the effect of the added gas volume on the chamber's internal pressure. It would be instructive to know just how much gas the motor generates throughout its firing, as this may require an improbably big chamber. b) Put the viewing window at 90 degrees to the exhaust jet rather than bang in its path. To quote Larry Niven, "A reaction drive's efficiency as a weapon is in direct proportion to its efficiency as a drive."
6mm glass won't crack and the idea is for it to blow off when the rocket starts.
Personally I would have used tried and true heavy duty laboratory gas jars but there's no reason this setup won't work for the intended purpose - however for pulling a sustained vacuum it'd be outgassing for months.
Using a flat latex ring with a lip over the outer edge would easily take care of slippage.Forces due to air pressure are easy to under (or over) estimate (Hint, 1: if something can hold 14-16 pounds of pressure there's no good reason it can't hold the inverse. The actual movement pressure on the sealing ring is likely to be trivial given the crossectional area around the jar's circumference.)
Leakage isn't a big issue, just use a higher capacity pump. Moisture is more of a problem - or lack of it in some cases after sustained exposure to low pressures.
It could be the seals have let go. Or, the values could be plugged up; that's especially a problem if sucking on something that may spew debris. Do you have a cold trap or molecular sieve in front of the pump (or, at least, some kind of debris filter?). It could also be low on vacuum oil (which is hideously expensive, much more so that your typical 10W4 motor oil). In any case, it very well may be cost effective (but time consuming) to recondition the vacuum pump.
Your pumps are from a 'fridge by the looks (apologies if that's been covered before) so you can use PAG oil from a car air con system, any garage that can recharge such a system will be able to supply some, it's not 'cheap' but then again it's not that expensive either for the amount you want (about 5 quid for 250mL last time I sold the stuff). Worth a punt on a bottle just in case that's the problem.
If you are intent on replacing them then it might be worth looking at aircon service equipment again, Robinair is good quality kit and a decent pump will cost you around £200-250 for a new one.
Have to agree with others here. That seal, whilst it looks lovely, is like buying an iPhone to make a phone call ... a complete waste of time and effort! A simple ring cut out of some thin latex and a finger-full of vacuum grease to stick it in place is more than good enough (assuming you machined the tube face to within a few thou' or so).
Since Katarina now sports a magnificent beard, I take it that she must be close to graduation (see "Boffinry" here: "http://forums.theregister.co.uk/forum/1/2012/03/14/lohan_truss_test/").
May I be the first to congratulate Katarina on successfully completing her apprenticeship?
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