Instructables
Picture of How to make a Plastic Bottle Barometer
This is an easy project and would make a great science experiment. Using a vacuum inside a plastic bottle with water in it, you can observe the changing air pressure with this simple barometer. Please note, this type of barometer only allows you to see if the air pressure is high or low.

Step 1: Supplies

Picture of Supplies
Items needed to make this barometer are:
1. A plastic soda or water bottle with lid, size not important!
2. Air Tubing, such as from an aquarium or oxygen use about 1 3/4 the length of the bottle.
3. A nail or other sharp instrument.
4. A rubber band
5. Low-Heat glue gun
6. Food Coloring (gel or liquid)
7. Water

Step 2: Making the Hole

Picture of Making the Hole
Using the nail, make a hole in the bottle slightly smaller then the outside diameter of the tubing at the base of the bottle. This is to ensure you get a tight seal. I actually heated the nail using my stove top to make this easier to do, and the heated nail allows for more accuracy.

Step 3: Inserting the Tubing

Picture of Inserting the Tubing
Insert the tubing until the tip is half way into the bottle, and should be resting on the bottom of the bottle. You may need the help of the cooled nail to insert the tubing. Remember, it must be a tight fit.

Step 4: Checking for leaks

Picture of Checking for leaks
Fill the bottle with enough water to come up about an inch on the bottle. If it leaks, dump the water out and apply a good size bead of low-heat glue around the tubing on the bottle. When the glue has cooled slightly, use your finger to push the glue into the area surrounding the tube to make a water-tight seal. Let cool, and check for leaks again. If still leaking, apply more glue.

Step 5: Attaching the Tubing to the top of the Bottle

Picture of Attaching the Tubing to the top of the Bottle
Using the rubber band, attach the tubing to the top of the bottle just below the threads, making sure you do not put pressure on the tubing.
 
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I was looking for something like this for my scout den thanks for posting it we build soon!!