Selecting Vacuum Pump for Microfluidics Device

[kmichel1985]

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 let's 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?

 

[kmichel1985]

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.

 

[Mike_In_Plano]

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.

 

[kmichel1985]

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.

 

[Mike_In_Plano]

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.

An alternative approach is to simply ask around your facility.

When working in varying industries, I generally make friends with the facilities staff early. Most are great guys, and they always seem to know where little tidbits are being hidden away in storage. That, and any old timer that manages labs will usually have a hidden treasure trove.

Because people are by and large disrespectful of others property, custodians of the equipment are typically weary of strangers asking for favors. Hence, be very respectful when asking. Help them to understand what you need, when you would need it, where it would be used, and when it would be returned.

Best of Luck,

- Mike