Monday, December 7, 2009

Pumping at altitude

Been writing this all day in spare moments :-)

I got the basic bioaerosol numbers (I.e 1cfu/L at ground level) from the person (Fred) who builds the MB2 samplier, and I knew him through a science charity, so it was an interesting coincidence!

I think we need to work out some better way of getting the air through the sampler - according to my sums you'd have to pump a 50ml syringe for 60 years!!

Hmmm let's think that through
500ml - 6 years
5L - 0.6 years (219 days)
50L - 22 days, getting interesting
500L - 2.2 days (so a 1 day flight is worth it, the chances of finding something would be 50:50)

That's 500L per return syringe stroke - in say 10s, eqv to a continuous pump pumping 50L/s

The MB2, straight out of the box, is useless b.t.w, because its fan could never pull a vacuum Fred says - it is designed for ground level use whereas we need a pump that pump essentially to vacuum (I.e from 0.01bar, 10mbar, to something less than that, which to all intents and purposes is a lab vacuum).
Fred is quite interested in building us a hotrod version if we can think of a way round it though.

The problem with an axial-based fan design for a pump - what would first spring to mind - is that you can't really pump to a vacuum this way, which is what we are asking (see above)
In fact, electron microscopes etc use plunger-piston type pumps to pull a high vacuum - so not far off a syringe pump after all. You can't spin a fan that fast or that efficiently, but if you withdraw a given volume into a syringe or piston etc then you really have caught something in the barrel, even if it is very tenuous (ie.g 50ml at 0.01bar is eqv to only 0.5ml of sea level air)
Hmm can we cannibalise an EM pump? A surplus one perhaps? But likely to be heavy and high voltage. After all, all we need to pull is 10mBar to 1mBar, not from 1000mBar to zero (which is what the EM pump has to do)

How big a plunger would you need to shift 500L?

500L is 0.5m3

Let us assume a piston with a working stroke of 17cm - 0.17m (this is because this is the useful return stroke of the SMA actuators I have in the lab)

0.5/0.17 = 2.94m2
I.e, the piston has to have a surface area of 2.94m2 for a stroke of 0.17m to have a pumping volume of 0.5m3 (500L)

So say 0.5m radius, I.e a drum 1m across - just about doable, especially if you have a bigger stroke (multiples of 17cm perhaps or a different mechanism altogether) for a smaller radius - OR multiple pistons

Who makes really airtight, sterile pistons? Life support machines perhaps?

The other alternative is to pull a chamber to vacuum on the ground, put it on the balloon, and then suck from ambient air pressure at altitude (e.g about 0.01bar) into the chamber. I have no idea how much a such a chamber would weigh though. Remember it needs to resist 1 bar of ground level air pressure whilst holding vacuum at ground level.

Could you pump it to vacuum in early flight, once the air pressure is less but still "pumpable", say 100mBar?
But is the pump heavier than the chamber?
And so on!!

Any ideas/maths to add my friends?
Can I pump you for ideas? :-)


1 comment:

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