I think the real question we're all asking is
Can you stick your c*** in there?
The epic saga of our shed-built hypobaric chamber – the Rocketry Experimental High Altitude Barosimulator (REHAB) experiment – continues today with the news that we've just laid our hands on a proper vacuum pump which allows LOHAN to suck an impressive 27 inches. Click here for a bigger version of the LOHAN graphic REHAB …
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.
But for your application, you only care about absolute pressure in the vacuum vessel, so it doesn't matter at all that you're at altitude. IMHO. So your losses are coming from somewhere else.
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 1100 m is in fact about 27" Hg. What you should do is replace that relative, or "gage", pressure instrument with one which reads >absolute< pressure, such as an anaeroid guage which read against a built-in hard vacuum reference. This is how barometers work. I use a Honeywell 142PC15A and a panel digital voltmeter in my altitude chamber and have selectable gain to read out in either mbar or Torr (mmHg). Works great against a 0 - 50 Torr Stokes gauge.
Mike Manes, CTO EOSS
Yes, you're not getting less performance, its 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.
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 3,000 ft 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.
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.
Now then, you know what to do so off you go and see if you can suck her down to the point she boils over.
having worked in academia with pumps where 1mm Hg was considered 'okay', check:
1) your grease is high vacuum grease silicone
2) your tubing is reinforced stuff suitable for high vacuum (costs more, but wall thickness of 5mm+ is often needed
3) seal all tube joints with Jubilee clips to make sure no leakage is there
4) is the oil in the pump new/clean? Bad oil ==> bad vacuum
I used to have a vacuum pump maintaining 50-100 microns vacuum over a period of months, using a dedicated single-vane rotary pump (I forget what cfm capacity).
As already mentioned, some of the tricks are:
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).
Don't let the pressure get to you!
Seconded - especially 4.
One of my first projects in the academia 20 years ago was putting a lab setup in order to actually do some work and the difference between bad old wet oil and brand new was going down from 15mm Hg to 1 straight away.
As far as 1 even if you have proper vacuum grease use as little of it as possible - it still throws volatiles.
By the way do not be surprised if the pump start burning oil once past 5mm. The oil I chucked out from the one I fixed 20 years back looked like the oil you drain from a tractor at the end of the ploughing season.
The biggest problem for a solid rocket motor is not staying lit. Its getting it lit in the first place.
Once the propellant has started reacting, internal pressure is mostly dictated by the throat of the nozzle. The outside pressure is only a very small margin of the internal pressure and is thus of little influence and interest at that point. To get the propellant properly lit requires a build-up of internal heat and pressure to a level suitable for the sustained reaction of all the chemicals. Thus this is what the SPB team is trying to test.
>Once the propellant has started reacting, internal pressure is mostly dictated by the throat of the nozzle.
Actually that's a good point. Most off the shelf rocket motors will have a nozzle designed for normal atmospheric pressure and might not be suitable (optimal) for very low pressure environments.
I guess the SPB are in touch with rocket nutters who know this shit though...
Hey, what do think, what vacuum you can attain with such a long, small lumen tubing.
I suppose the pump has standard Leybold.Heraeus flanges. Use appropriate metal tubing. Such a pump can definitely reach a much lower vacuum than 15 mm Hg.
(in a former live a chemist)
(in a former live a chemist) - Yes me too.
I should have mentioned that it might be worth running with the gas ballast valve open for a few minutes - This will help get rid of any solvent (from glue, siloxane resin, gaskets, etc.) or water that may be entrained in the system/oil.
Yes, will be a huge problem? Do you know the amount and type of fuel and oxidizer?
Than it's simple to calculate, how much gas is produced
Anyway: A leat a battery of cooling traps (nicely cooled by much liquid nitrogen) is needed in the vacuum line.
having said this, I doubt that you can maintain any self-respecting inside your test chamber when the motor starts.
Let's assume,the motor produce 1 mol, i.e.44 g of CO2, this will have a volume of about 22 l
at 0°C and 1 bar. To maintain for simplicities sake 10 mBar, you've to pump away
22*( 1/0.01),= 2200 l = 2,2 m^3 gas.
(1) Pump for a while. Water vapor pressure at your current ambient temperature of 23 °C is 21.1 mm Hg, but the temperature and therefore the v.p. may be higher when you have everything in the sun. Just a little moisture somewhere (and it's everywhere) can keep your pump working for hours or days.
(2) Leak test. At your pressure regime, you can spray ethanol at the joints. A leak will suck in ethanol, the liquid will evaporate and give you a jump to a higher pressure. Later, the leak may freeze shut with ethanol (pumping a liquid cools them down), so the pressure may improve for a while. Best have one person do the spraying and another watch the vacuum gauge.
(3) Use vacuum connectors instead of pressure connectors. The quick-connects you use look to me like connectors for pressurized air. The seals are different, so the wrong kind may or may not seal.
OK, it was funny at first lin a 'Carry On' way but now this is all just crude.
Ask yourself - what happens if you want to a teacher wants to show school kids what engineering is about and what they can achieve?
PARIS was a gret example that is needed to inspire.
I expect comments slagging me off for this BTW.
You are getting some good and some indifferent advice further up the page here Lester. The pump is capable of 2Mbar, which according to my chart will give you 1.6million feet of altitude. That will require a damned good balloon!
If you are only getting 27" then you MUST have leaks, but looking at the photograph the gauge you are using is hardly a precision instrument!
As Pete and Kate have said, a pleasure to help, and any more advice, then we would be pleased to help.
John (big bad boss at AVE!)
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