Science Buddies: "Ask an Expert"

Hello,

I am doing a science fair experiment to test the effects different low pressure levels have on the germination and growth rate of bean plants.

My plan is to construct sealed, air tight plexiglass containers, reduce the air pressure in them to different set levels (2 psi, 5 psi, 8 psi, and 14.7 psi as the control) and try to grow bean plants in them. I would record the results by measuring both germination rate and biomass. Also I would want a container large enough that I could have several pots of plants in them and do multiple trials at once.

The problem that I am running in to is finding a way to regulate the air pressure so that it stays at a constant level throughout the experiment. It is easy enough to get the pressure down to the correct levels by using, say, a vacuum pump, but, if I understand correctly, once the vacuum pump is turned off air would re-enter the environment. So what I need is some way to maintain the pressure and ensure that it will stay constant even after the vacuum pump, or whatever I use is turned off. There might be a very simple, easy solution to this problem, but this part of the experiment isn't my forte. Thank you in advance, any help is welcomed and feel free to ask questions if something doesn't make sense.

At the risk of suggesting the obvious: You have a sealed container with a tube connected to the pump. If you added a short section of something bendable, such as aquarium air hose, you could lower the pressure, bend the hose section double, then clamp it with any sort of clamp or strong binder clip. Then shut off the pump. Or if you have one, an air valve inline between the pump and the container. Pump, close the valve, then remove the pump.

If your container is truly airtight as you intend, there's no reason why the vacuum won't hold. You may end up having to recheck it periodically to make sure.

It seems like you've thought the experiment through pretty well. If you've ruled out a valve or clamped tubing, maybe we can come up with something else. Some vacuum pumps may close a valve when you shut them off - but in that model you'd need one dedicated pump for each of your setups for the duration of your trials.

Thank you very much. I knew there should have been a simple solution to this, I just a new set of eyes to look at it. I like the idea of the air valve inline, I was wondering if there is any type that you could recommend, or at least some things that I should look out for when deciding which type to purchase. Thank you very much, and again anything that you can suggest would be greatly appreciated.

I'd have to know more about the materials you plan to use (vacuum pump, hose diameters, thickness of the plexiglas and other details) to help you make up a shopping list. But - the idea is that you can either attach a hose to the tank, put a valve on the hose, and attach more hose to the pump OR you can screw the valve right into the plexiglas if it's thick enough and attach your hose that way. Whether the valve is really inline or mounted into the tank will determine whether you want screw threads or hose barbs. You may also want a vacuum gauge. Most of the inexpensive ones will be best screwed into a hole drilled into the cabinet.

So to answer your question requires planning the entire setup at the same time. I see a number of inexpensive vacuum gauges on Amazon (http://www.amazon.com/Winters-Economy-P ... B0087UB7FC seems promising) and McMaster-Carr (http://www.mcmaster.com) is a place I've bought many an item for stuff like this. I've also found several good articles on modifying a bicycle pump to turn it into a vacuum pump and then using aquarium air hose & valves. I guess it depends entirely on the quality of the hose and valves - but maybe the local pet supply store has something. I'd imagine brass ball valves work better then cheap plastic screw valves.

Building a larger tank will present more issues as the materials will flex. Keeping the tank sealed will be a challenge but I'm confident you'll work out those issues.

Howard

Thanks, you have been a great help. I look forward to moving on with the project. If I have anymore questions I will be sure to come here.