Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: Barometers



Subject: Barometers
Does anyone have suggestions on building barometers?  For obvious reasons, I have to avoid using Mercury.  What other fluids work well?

We once tried making such a barometer using a 40-foot water-filled pipe made of lengths of glass and rubber tubing from the chem lab. The tube extended from the roof of our four story school building to the ground below. We suggest that others try this technique also.

The major problem that we had was filling the tube and keep it from leaking near the bottom where the pressure was the greatest.

Herb Gottlieb from New York City
(Where vacuum oil is more expensive and less dense than water)

Herb or whoever it was that started this topic,

Get a long (40 feet or so) length of clear plastic tubing from a hardware store.   On the ground, hook one end  of the tube to a faucet and turn the water on.  Once you get the whole tube filled with water, fold over the other end and clamp it together so you get a nice seal.  Turn the water off, place the open end in a bucket of water and hoist the other end up in the air, preferably to a height greater than 32 feet. 

As an alternative to this (which, at least here in Wyoming in our limited facilities, can only be done outdoors - and I generally get to the topic of atmospheric pressure in the dead of winter when outdoor activities are unwise) I do the following activity.  This activity was inspired by one I saw at the University of Wyoming by Dr. Terry Deschler, a top-of-the-line atmospheric researcher type.  Anyway, I have this 2 m section of clear acrylic pipe that has a diameter of 12 inches.  One end of the thing is capped and sealed with a piece of Plexiglass.  The end cap has a gas fitting in the middle.  The pipe is placed in a large basin of water that is maybe 5 or 5 inches deep.  You have to have the pipe sitting on something so that it has a bit of clearance between the pipe bottom and the bottom of the basin.  A piece of rubber tubing is connected to the gas fitting at the top of the pipe.  A clamp of some type should be available on the tubing.  You then draw air out of the pipe with your lungs.  The water level in the pipe will rise a bit  - not a whole lot, though, unless you have better lungs than do I.  At any rate, once you've ran out of air with your feeble vacuum drawing efforts, you then hook the tubing up to a proper vacuum pump.  You then slowly draw a vacuum.  The water level will rise.  You will have to add water to the basin as the pipe fills up with water.  At the end, when the pipe is full, secure the pump and clamp the tubing; then disconnect the tubing from the pump.  You will have a six foot column of water in the pipe, held up by atmospheric pressure.  This is very impressive.  A huge mass of water that ought to come crashing and smashing out of the bottom of the pipe and inundating the entire class room.  But it don't.  This is the time when one can ask the question, how tall could the pipe be and still have atmospheric pressure hold the water in the pipe.  You can draw some force diagrams as well.

The problem then is what do you do with all this water?  Students perceive this as quite a challenge, picturing water gushing out all over the floor and this huge mess which the instructor is responsible for making.  This is for them an appealing mental picture . 

You siphon the water out of the basin into a sink or series of buckets (sink is best because there is a lot of water in the pipe so you would need a lot of buckets).  Release the clamp as needed to allow the water level to come down as the siphoning action takes place.  This is a good time to talk about siphoning as well.

Glenn (In Wyoming where it is darn hard to find a building that is over 30 feet tall)
-----------------------------------------------------------------
Physics Kahuna
Kahuna Physics Institute - on the flapping edge of physics research.