Constructions

Bottle Barometer
Making Cap Used in the Cloud Demo & in the Straws and Pumps Demo
Potential Next Generation Cap

Bottle Barometer

There are various designs of simple barometers but the most effective is as follows:

Materials:

• Wide mouth jar (e.g., mayonnaise jar)
• Piece of balloon
• Rubber band
• Piece of broom straw
• Tape
• Shoe box without cover
• Paperclip

1. Stretch the piece of balloon tightly over the mouth of the jar and secure with the rubber band.
2. Place the broom straw across the balloon so that one end is at the center and the other end sticks out well over the edge of the jar. Tape it in place.
3. Set the shoe box on end so that one edge of the open end is near the free end of the straw.
4. Place the paperclip on the edge of the box so that one part of it is even with the end of the straw.

Use:

This only works in rooms of fairly constant temperature since changes in temperature cause expansion and contraction of the air in the jar. If the temperature does not change, then the barometer will measure changes in air pressure as different weather systems pass by. If pressure increases relative to the pressure when the barometer was made, the balloon will be pushed down since the pressure inside the jar will be less than the pressure outside the jar. When the center of the balloon is depressed, the free end of the needle will go up. If pressure decreases relative to the pressure when the barometer was made, the balloon will bulge out since the pressure in the jar would be higher than the pressure outside the jar. The free end of the needle will go down.

Use the paperclip to mark the position of the end of the straw each day so that the change in pressure by the next day can be determined. Changes in pressure are much more important than the actual value. Increases in pressure are generally associated with approaching cloudy and even rainy conditions. The bigger the change in pressure, the bigger the change in weather is likely to be.

Making Cap Used in the Cloud Demo & in the Straws and Pumps Demo

The first instructions are for the original tried and true design. They do need to be refurbished periodically by replacing the electrical tape, particularly on the top. Tentative instructions on the potential next generation are given below.

Materials:

• Cap from a plastic soda bottle that has an easily removable inner liner
• Awl or other pointed object
• 1/4 inch drill bit
• pencil or chop stick
• About 2 inches of 1/4 inch outer diameter (OD) nylon tubing (available from hardware store or from McMaster-Carr (732-329-6666, catalog #5173K23, $.11fFt.)
• About 1 ½ inches of 1/4 inch inner diameter flexible tubing that fits tightly over the nylon tubing (e.g. from McMaster-Carr, catalog # 5233K56, $.14/ft
• electrical tape

1. With the awl bore a starter hole through the center of the cap including the liner from the top of the cap.
2. With the drill bit expand and clean out the hole from the top of the cap. The bit should push out the liner without expanding its hole.
3. Use the pencil or chop stick to widen the hole in the liner without cutting or tearing it. Push the stick in the same direction as the awl (i.e., from the part of the liner that touches the cap). Remove the stick.
4. Insert the nylon tubing into the liner in the same direction as above so that the end that was inserted extends out of the bottom of the liner about the width of the electrical tape.
5. Wrap the tubing and the part of the liner that overlaps it with a couple of tight winds of electrical tape.
6. Insert the untaped end of the nylon tube into the bottom of the cap and through the hole. Orient the liner so that it is centered.
7. Push on the bottom of the tubing and tightly wrap the tube right at the top of the cap with a couple of winds of electrical tape.
8. Place the piece of flexible tubing over the top end of the nylon tubing.

Potential Next Generation Cap

• Bottle cap
• Barbed tube fitting with threaded male end, 1/4 inch tube ID, 1/8 inch pipe size (e.g. from McMaster-Carr, nylon male nipple, catalog # 5372K111, $2.80/pack of 10)
• Probably an appropriate nut (not tried yet)
• About 1 ½ inches of ¼ inch inner diameter flexible tubing that fits tightly over the barbed end (as above)
• Awl
• 3/8 inch drill bit and low or variable speed drill

1. Tighten the cap onto a bottle.
2. Make a starter hole in the center of the cap with the awl.
3. Drill a 3/8 inch hole through the cap and liner. The hole in the cap should be clean. The liner should extend over the edge of the hole in the cap.
4. Screw the threaded end of the fitting into the whole from the top of the cap.
5. Place the flexible tubing on the barbed end of the fitting.