Help me fix my experimental setup!
April 10, 2012 11:24 AM   Subscribe

What type of pump do I need to buy to remove air from an animal cage and deliver it at a controlled rate to an experimental setup?

Okay, so here's what I need to do... I have a semi-airtight cage in which I can place a small animal. I need to get air from that cage through plastic tubing to deliver to holes in the top of a petri dish. (The experiment is trying to determine whether parasites of these animals can find/distinguish between hosts by odor). The original plan for the experiment was that air would be delivered to the cage at between 7 and 15 liters per minute (as required by Animal Safety--this is the range considered appropriate to maintain healthy oxygen levels and such) from a compressed medical air tank, and that the flow of air in would be sufficient to push air out of the "out" tube and into the dish. Notably, the flow rate we need into the plates is very low.... 2-10 mls/minute is sufficient, and too high just dries out my plates and ruins the experiment.

This works, sort of, but it causes pressure buildup in the cage, and the outflow rate is incredibly variable. We've tried putting flow meters between the outflow tubing and the petri dish, but this seems to restrict the flow too much such that more air leaks out of the imperfectly airtight cage rather than forcing it's way through the constriction of the flow meter. We are also concerned that pressure buildup in the cage is unpleasant for the small animal in the cage (yes, I do research on animals, but I follow the rules and take very good care of them. I do not want them distressed by this experiment any more than necessary).

The next thing we have come up with to try is some kind of pump to pull air OUT of the cage at a controlled rate. We want to get rid of the need for a pressure buildup in the cage to force the air through the experimental system. We could allow most of the air pulled out of the cage to simply be released, only a very small amount of airflow needs to be channeled to my parasites in the dish. My problem is that I don't know what I'm looking for precisely in terms of the pump. I know what I want it to do... remove air at between 7 and 15 LPM, and then put air out in a way such that I have some reasonable way to channel the airflow to my plate. What I don't know is how to find this device I have in my head. Any suggestions?

Alternatively, if you have a better idea for how to run this whole system, I'm all ears. Keep in mind, however, that I am very constrained here by concerns for animal safety. Also, major changes to the design that substantially affect the experience of the animal, such as changing the container used for the cage to something more airtight, are things that would have to get cleared with the animal research oversight people, which can take months that I really don't want to wait, so those are options of last resort.

I'll be watching this thread closely, and I know this is a complicated question, so if you need anything clarified, please ask.
posted by keelyellenmarie to Science & Nature (4 answers total) 1 user marked this as a favorite
air would be delivered to the cage at between 7 and 15 liters per minute ... the flow rate we need into the plates is very low.... 2-10 mls/minute is sufficient, and too high just dries out my plates and ruins the experiment

I would try putting a split in the outflow from each cage, with a skinny branch to the petri dishes and thick branch to exhaust. Some sort of pinch valve on tubing to the petri dishes should help.
posted by exogenous at 11:32 AM on April 10, 2012 [1 favorite]

Aquarium supply places should have piston/diaphragm air pumps which operate in the range you are looking for. These are the larger scale pumps and so they are not cheap.

However, given your application, I think that would be overkill.

I think that exogenous is right and that you just need a better designed outflow manifold.
posted by selenized at 2:28 PM on April 10, 2012

If I did go the pump route, do you know of any pumps that I could get that would allow me to control the rate of airflow on the uptake end? I've found valves for managing outflow, but I haven't found an aquarium pump that has an adjustable pumping rate. It's quite likely though that I just don't know what I'm looking for.
posted by keelyellenmarie at 2:35 PM on April 10, 2012

When using pumps it is pretty common to have the pump run at a constant power and adjust the flowrate by adjusting a valve. As long as the pump is supplying more head than you require, you can just decrease to what you want by partially closing valves. This is of course much easier if you have a flowmeter.

If you have a valve on the outflow you don't really have to worry about the inflow. Just by simple mass balance you know that if your system is supplying a certain flow out then you must have that flow in (adjusting for the compressibility of air of course).

This would take a bit of tweaking but it shouldn't be too overly complicated: a pump, a valve, and a split.
posted by selenized at 3:21 PM on April 10, 2012

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