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. Click here for a bigger version of the LOHAN graphic …
Have You Checked The Pump?
Since you are trying to obtain the maximum vacuum generated from the pump, why don't you attach the gauge directly to the pump? This would give you the maximum reference vacuum obtainable from the pump in the environment you are working in. Without this figure, you are trying to hit an imaginary target.
And be careful with that advice about PTFE tape - make sure you understand where you have a face-seal and where you have conical seals, and only use PTFE on the latter.
Do you have any facility to produce positive pressure? One earlier commentard suggested using Snoop or similar (washing-up liquid or childrens bubble mixture may be more economic) before everyone pointed out that it is useless on a vacuum system, but if you were able to pressurise your system, at least as far as your shut-off valve, then you can see what (and where) air is leaking out, which may help a little.
is a good idea, but it requires that every connection can withstand the pressure. You'd clearly have to attach the chamber cover somehow. Apart from that, a simple bike pump or ball pump will supply sufficient pressure to test your system.
Water in REHAB is a good idea
Here's a way to use water to monitor the pressure inside REHAB without hurting the pump.
Start by boiling the water and letting it cool in the glass you'll put in REHAB. Tap water contains dissolved air which, if left in it, will give the impression of boiling at a higher pressure as the air comes out of solution.
Now put the glass and water in REHAB, close it and suck away while watching the water. If it starts to boil, note the pressure and immediately close the stopcock before stopping the pump. This approach will minimize the amount of water vapor ingested by the pump. In fact., water vapor shouldn't hurt the pump because ambient air has a relative humidity of around 70% in the UK and the pump must be able to deal with this. Boiling water can only raise the relative humidity to 100%. Also, its unlikely that your REHAB design will allow liquid water to enter the vacuum tube if the water glass is sitting on the bottom of the chamber.
BTW, unless you're going to control the temperature inside REHAB to 20C, you should also invest in a reasonably good thermometer and/or install the REHAB internal thermometer now. Then find a table of water's boiling point vs. pressure - you'll need it to get a reasonable pressure reading from this method.
Water in REHAB is a good idea, Water in the Vac pump is not
When I worked in a lab, we always used a "cold finger" (http://en.wikipedia.org/wiki/Cold_finger) filled with a mixture of dry ice & acetone to condense and freeze any water vapour immediately before the pump to protect the vac pump from water. If not removed, this water can form an emulsion with the oil in the pump and this is not good news. Try removing “butter” from the pump as I had to as a new lab technician when I forgot to put the dry ice in the cold finger…
Because you are interested in the pressure around the motor, the point at which you measure that pressure should be as close to the motor/main chamber as possible.
Re: Water in REHAB is a good idea, Water in the Vac pump is not
Seconded, if for no other reason than I'm dying to read the article.
I'm sure Paris knows a thing or two about fingers and butter.
Have you given your gauge a calibration "tap" with your finger? Analogue type gauges are quite inaccurate, they're not bad to begin with but the higher the reading (lower the pressure) the more inaccurate they get, in my experience they can "stick" and a calibration tap on the dial can get a completely different reading.
I've just done an experiment here at work, I have an altitude chamber with both an analogue and a digital gauge attached, I vacced it down as far as it would go, the digital gauge read 30mbar and the analogue read 120mbar. Giving the analogue gauge a tap on the glass brought this to 100mbar - better but still a long way from the digital gauge. I'd get a digital gauge if you can. Also is your gauge "barometrically compensated" if so that might be affecting the reading.
Our chamber has 11 different connections/joints along the way from the pump for various gauges, air admittance valves, joints etc and yet still gets down to 30mbar (equivalent to around 75000ft) yours apparently has 8, you could try smearing silicone bathroom sealant over the joints and letting it cure overnight, we have done this to our setup and it helps a lot.
Our normal test chamber only has 6 joints and we regularly test at a simulated altitude of 130000ft, but maybe we have a more powerful pump than you.
Am I being thick?
If you have a pressure meter which is measuring relative to ambient rather than atmospheric, you need simply subtract the ambient pressure difference from nominal sea level to get a (probably approximate) correction factor.
At your research facility you're at what, a thousand metres - that's three inches of mercury to subtract from your measured figure.
Looming disaster ... take avoiding action now!
Is there any vague chance that the El Reg Space Agency can learn from the past and not repeat NASA's mistake of using a useless rag-bag of mediaeval measuring systems for a science project? I mean, you're continually converting from inches of mercury to pounds per square inch to equivalents in feet of altitude....
You're just BEGGING for trouble. Use sensible measurements from the get-go and be done with it please! Altitudes in metres, pressure in bar(*) (well millibar mostly in this case). Many of the suggestions from readers are coming in using those units - if you keep converting to and from your antiquated ones, sooner or later you'll have a cockup on your hands...
..which, considering the name of the project is probably appropriate! Arf, arf.
I'll get me coat.....
(*) Pressure measurements should be in Pa (Pascals) if you want to be *proper* rocket scientists, but 'bar' is easier to visualise (being pretty close to normal atmospheric pressure).
BTW: 1 bar = 100000 Pa, a.k.a 0.1 MPa.
Rocketry isn't science
You need two vane pumps
Similar to a turbine which provides pressure via two or three sets of vanes, you need a similar setup with this pump, but in the reverse. Two in series will bump up the vacuum....and change the PVC pipe to copper...the pipes will collapse. I discovered this attaching two pumps in series to drain a pond, and could hear the water boiling in the pipe as it collapsed, and the vanes having a hard time with vapour cavitation. All I got was a stream of spluttery water and vapour from the nozzle. In investigation of the cause I nearly lost my trousers at the inlet end.
If you don't get the vacuum the pump should achieve you still got leaks. Try an inclined tube gage
for the really low pressures
From the figures you give, it seems that when you allow for the drop in ambient pressure with altitude, there's nothing wrong with your apparatus. If the gauge reading can be believed, you have a negative pressure.
Either I have slipped in the calculation, or the gauge isn't accurate at low pressure.
Yup, it looks like the set-up is fine, and the gauge is to blame.
Try swapping round the position of the shut-off valve and the gauge. After pumping to minimum pressure, close the shut-off valve. A rising value on the gauge would indicate a leak in the plumbing.