LOHAN could suck a golf ball through a garden hose.
This week really sucked down at the Low Orbit Helium Assisted Navigator (LOHAN) headquarters as we put the vacuum pump for our Rocketry Experimental High Altitude Barosimulator (REHAB) experiment through its paces. Click here for a bigger version of the LOHAN graphic Those of you unacquainted with REHAB should have a shufti …
Friday 30th March 2012 10:44 GMT Mostly_Harmless
getting lower pressure
You could exploit Gay-Lussac's law*, thus....
1 - Open chamber
2 - Heat chamber
3 - Seal chamber
4 - Depressurise chamber using vacuum pumpy goodness
5 - Cool chamber...cooling with reduce pressure of any air remaining in chamber
* Over 25 years since I was first taught this is high school chemy, and I still snigger sometimes
Friday 30th March 2012 11:39 GMT Pete 2
Re: getting lower pressure
6 - fill a significant volume of the chamber with water (in one or more strong plastic bag). Use a water pump to empty the bag(s). The decrease in waterbag volume will further reduce air pressure inside the chamber. Heavy duty water pumps are more common than heavy duty vacuum pumps.
Friday 30th March 2012 11:14 GMT Peter Black
Friday 30th March 2012 11:17 GMT Inventor of the Marmite Laser
Friday 30th March 2012 11:33 GMT Pete 2
When you do apply the suck to REHAB, can we please have a video record of the outside of the garden shed? Once you start sucking the air out of it, the pressure on the shed's walls will increase a lot. We've all watched video of pumps evacuating oil cans and seeing them getting crushed by external air pressure. However, I've never seen a garden shed implode for that reason and it would be kinda interesting.
Friday 30th March 2012 11:44 GMT Anonymous Coward
Some definitions to avoid a vichyssoise of vapid verbiose
"Maximum Vacuum Capability" = Base Pressure
"Once switched off, the pump held the maximum vacuum, representing approximately 0.15 bar absolute pressure, for around five minutes. It took the gauge around forty minutes to return to "zero"." = Leak Up Rate
Also, guage pressure is most confusing!
Friday 30th March 2012 13:00 GMT Dodgy Geezer
I hope that....
...the old compressor used Freon, and you just vented it to the atmosphere.
Current science suggests that the Montreal Protocol ban on Freon was a complete waste of time, as the chemical is not now thought to cause any 'hole in the ozone layer'.
The ban worked out very nicely for Du Pont, however, as it was just about to lose it's patent protection on this chemical, and now people have to use other, less effective chemicals which Du Pont DOES have a patent on...
Friday 30th March 2012 13:23 GMT Dan Paul
Pressure is one of the MOST confusing units of measurement
Zero on Vacuum Gauges is considered to be atmospheric pressure or 14.7 PSIA, 0 inches of Hg, One Bar, 760 Torr, etc, etc, etc. This the same for Gauge Pressure (PSIG) but those don't usually go below zero. There are bidirectional gauges as well that show both positive and negative pressure on the same device.
PSIA is absolute pressure which is always referenced from Zero.
It seems every time some physicist made a pressure measurement, they had to create a new unit of pressure. Then someone had to come up with metric and the notion of vacuum gauges and "Gauge pressure" became very hard to grasp.
Inches of Mercury became a popular pressure unit because mercury is very dense and the primary pressure measurment device was a "U" shaped glass tube filled with liquid (Manometer). Mercury allowed measurement of very high and low pressures in a compact device that was only 3 foot long.
Friday 30th March 2012 13:53 GMT EddieD
Re: Pressure is one of the MOST confusing units of measurement
"Mercury allowed measurement of very high and low pressures in a compact device that was only 3 foot long"
And also led* to the development of florescent tubes when someone saw what happened when you polished one with a silk cloth and got a static charge through the torricelli vacuum at the top of his shiny new manometer.
*According to James Burke's Connections (first series) and if that's not an authority, then I don't know what is...
Friday 30th March 2012 13:37 GMT Peter Simpson 1
Friday 30th March 2012 16:38 GMT john bertelsen
Re: Low pressure only until ignintion, right?
Why not add a second chamber to the system to act as a vacuum accumulator? It will enable the pressure to drop more slowly after the motor ignites.
If the accumulator sits between the pump and the chamber, it will reduce the amount of hot gasses ingested by the pump while firing the motor.
Friday 30th March 2012 17:26 GMT Stoneshop
Re: Low pressure only until ignintion, right?
Yes. Keeping the pressure low once the motor burns is a) as good as impossible without some really hefty pump and b) totally unnecessary anyway, as the internal pressure inside the engine, where the actual burning is taking place, is now well over atmospheric with the pressure outside the case irrelevant
Friday 30th March 2012 15:00 GMT A. Coatsworth
Friday 30th March 2012 17:26 GMT Schultz
Get a water-jet pump
and you'll get a 10-fold lower pressure. Those things are not expensive (e.g., http://bit.ly/GY54dq, or http://bit.ly/H2dcZl) and they truly suck!
Also get some Epoxy-putty to seal off whatever holes you may discover in your setup.
If you need more expertise, try to find an old copy of 'Bulding scientific apparatus'; there you'll find all the practical tips for whatever you might consider building.
Saturday 31st March 2012 15:50 GMT Alan Brown
It's not just done on the cheap as a way of getting low pressure.
It's the _only_ way to get space-grade levels of vacuum in a test chamber...
I don't work on said chambers, but I work with people who do. Pump cascading is the norm (think of pumps rated not just to hold high vacuum, but to run like that for _months_ on end without burning out - getting water vapour out of spacecraft is a time consuming process and if it's not done they'll fly for years with their own resident contaminant cloud screwing up the instruments.)
WRT freon: the biggest offender for venting to atmosphere was the dry cleaning industry. One year's discharge in the USA alone was more than the accumulated amount in all fridges and freezers ever made worldwide.
The problem was that Freon couldn't be banned for just one use as that would make illegitimate supplies available to those wishing to bypass restrictions.
On top of that, CFCs were commonly used for plastic moulding and blowing insulation in fridge casings, etc. Such uses were all one-shot (and have mostly been replaced by nitrogen, which is fairly carbon intensive to produce).
Paradoxically the global agreements which banned CFCs allowed developing countries to keep using the stuff in fridges/freezers. Because of the potential size of those markets (China and India in particular), that meant that twice as much freon as had been used in freezer/fridge manufacture up to that point could be installed into new appliances. This was the source of some controversy when it became known but without the compromise those agreements would never have been signed.
Beware: Newer fridges often use flammable gases such as propane for the refrigerant.
Saturday 31st March 2012 15:59 GMT K. Adams
Temperature vs. Pump Lubricant Viscosity
If the projected internal temperature of the barosimulator is -60 Celsius, then we could run into a problem where the excessively low temperature of the evacuated gases moving through the pump causes the pump's lubricant to become too viscous to allow it to operate properly. Thermal contraction of the pump's housing or impeller vanes could cause things to bind to a halt, as well.
Also, have you thought about laterally and vertically bracing the interior walls of the 'sim, so that outside air pressure doesn't cause the structure to collapse inward? I seem to recall a high-school physics experiment involving steam, cold water, and an old-style (resealable) tin jug that was once used to hold mineral oil. We cleaned out the tin jug, filled it with a bit of water, heated it until it boiled and filled with steam. We then sealed it with the cap, and cooled the thing with a stream of ice-cold water. Collapsed like a poorly-rigged tent in a mild breeze...
Monday 2nd April 2012 06:58 GMT Don H
Re: Temperature vs. Pump Lubricant Viscosity
Avoid the problem by not cooling the chamber until the required pressure (vacuum) is achieved. Also consider that the compressor and its lubricant was designed for low-temperature use. Finally, it's a compressor. Chickens go in, pies come out. Or something like that. bv
Monday 2nd April 2012 07:00 GMT Don H
Re: Temperature vs. Pump Lubricant Viscosity
Avoid the problem by not cooling the chamber until the required pressure (vacuum) is achieved. Also consider that the compressor and its lubricant was designed for low-temperature use. Finally, it's a compressor. Chickens go in, pies come out. Or something like that.
Monday 2nd April 2012 00:48 GMT Tim Starling
Sounds like a risk to me
You want to conduct your experiment at a pressure which is 7.5 times the intended operating pressure? I don't know much about rocket ignition, but whatever effect pressure has on it, I would expect it to be proportional to pressure. A factor of 7.5 sounds like a lot to me.
Yes, it sounds less if you take advantage of the exponential relationship between altitude and pressure, and express it as a height difference instead of pressure difference. But the rocket motor is not going to know how high it is, only what pressure it's at.
To put it another way, sea-level pressure is only another factor of 7 above the pressure you're proposing for the experiment, so in terms of pressure ratios, the experiment will be further from reality than it will be from sea level.
Here's my advice: head down to your local 3rd-year physics lab, and say to whoever runs the place: "hey, that's a mighty fine rotary vane pump you've got there, mind if we borrow it over a weekend? It will be used for the glory of Britain. Of course we will pay for any damage." Then be really careful with it.
Monday 2nd April 2012 07:31 GMT Acadene
Even better way to get a vacuum
There is a simpler way of attaining a vacuum of 10-15mmHg. Source a 100mm dia 15m long steel pipe. Seal both ends and attach 2 valve taps to the top sealed end and one at the bottom. Attach one top valve tap to REHAB and fill the pipe with chilled water (2 degrees C will do) with the other. Close off the water tap and open the bottom tap and watch that air pressure drop baby. The chilled water will reach its vapor pressure somewhere close to the bottom of the pipe. This does mean though you will have to conduct this experiment in some suitable stairwell.
Wednesday 4th April 2012 00:13 GMT Mike Manes
Get a better vacuum pump
So this surplus fridge compressor only gets down to 0.15 bar or 150 mbar? That's about 15 times greater absolute pressure than one will see at 100,000 ft (33 km) MSL, and assuming that this won't affect rocket ignition is, may I say, whistling past the graveyard. I'd advise using a more suitable pump, such as what are used by auto shops to evacuate air conditioning systems, or simply buy one sold by Harbor Freight for under US$100 (60 quid) - the latter has been used by several amateur high altitude balloon groups down to less than 5 mbar (110K'), Or just beg or borrow one from an A/C service shop for the duration of this test.
73 de Mike W5VSI, CTO EOSS (www.eoss.org)