# Vacuum Pump Selection

I'm doing work in microfluidics, and need help selecting a vacuum pump as I have no experience in creating or controlling a vacuum. Let me simplify my device: I have water confined between two plates. One of the plates has a hole, covered with a small patch of material that lets gas through, but not water. I want to remove the gas from the water using a vacuum, with tubing going directly from the patch to the pump. This system is small, at 5 cm x 2 cm x 0.02 cm and therefore delicate. My tubing will be between 1/16" and 1/4" inner diameter.

I'm designing my system based on the following quote: "The air trap was first connected to a vacuum source, which was the house vacuum at -97 kPa relative to the atmospheric pressure.

Knowing little of gage pressures and so forth, I assume this means that I need to maintain a pressure of 101.3 - 97 = 4.3 kPa? For convenience, this is about 30 Torr, or 30 mmHg. Here is a chart of vacuum pressures I found on PF.com:

Atmospheric pressure 760 Torr 101.3 kPa
Low vacuum 760 to 25 Torr 100 to 3 kPa
Medium vacuum 25 to 1×10-3 Torr 3 kPa to 100 mPa
High vacuum 1×10-3 to 1×10-9 Torr 100 mPa to 100 nPa
Ultra high vacuum 1×10-9 to 1×10-12 Torr 100 nPa to 100 pPa
Extremely high vacuum <1×10-12 Torr <100 pPa

This puts me on the border between a low and a medium vacuum. I need something small, inexpensive (< $300, the cheaper the better), and not loud because other people work near me. I want to use a hand operated pump (http://www.crscientific.com/vacuumpumps.html) but it only goes down to 50 mmHg or 50 Torr. Can you please offer some advice? Would a hobby pump work for a fish tank, for example? ## Answers and Replies Just to clarify, I'm looking to maintain a constant vacuum pressure over many days of operation. I don't expect significant volumes of air to leave my device, maybe 5 microliters per hour from the water inside because small air bubbles don't appear too often. When a bubble flows through the water and gets trapped beneath the patch, I want the vacuum there to be able to suck it out. That's my only purpose in wanting a vacuum. If you have compressed air available, you may wish to purchase a simple air-powered vacuum generator. McMaster Carr sells these, and you could probably get one with fittings for about$100USD. They're not overly loud, especially compared to diaphragm and vane pumps. If the hiss is objectionable, place it in a box make of dense material (i.e. a shoe-shine box) or under an old coat.

Best of luck,

- Mike

PS, you can't make a bubble go away, if it doesn't want to. It's better to boil off any absorbed air and add the liquid to an evacuatted system.

Thanks Mike,

I found these pumps here, which I think you're referring to?

http://www.mcmaster.com/#vacuum-pumps/=fl8kek

I personally like pump C, at 0.5 cfm at 28" Hg. However, since I'll be running this thing for a long time I don't think any compressed air tank will last that long. On the same page, I also saw a hand squeezed vacuum pump with a pressure gauge. It only generates 25" Hg but it might be enough for me. How long would you think these can hold a vacuum for, until you need to squeeze the handle again?

For anyone else just reading this, let me simplify my system further to help you understand it. I just want to maintain a vacuum inside of my tubing. The tubing is connected to a thin membrane and the vacuum pump. Too robust a vacuum, and the membrane might tear with each cycle of the pump. I will not be needing to displace any significant amount of air. It's as though the tubing were simply attached to a solid wall.

Yes, if you don't have a ready source of air, then this isn't suitable.

The difficulty with using a vacuum vessel is that your liquid will out-gas and degrade the vacuum. If you introduce a cold trap or maintain the vessel in sub freezing conditions, this can trap the moisture that evaporates off.